In amateur radio practice, there is often a need to configure various conversion nodes of circuits, especially when it comes to inventive activity, when the circuit is born in the head. At such moments, the source of the control signal will come in handy.
I present to your attention square wave generator.
Specifications
Power supply: 10 ÷ 15 V DC.
Three generation modes:
1 - symmetric (meander), discrete switching of the ranges of the generated frequencies, smooth frequency control within the range;
2 - independent, discrete switching of the ranges of the generated frequencies, smooth separate adjustment of the pulse duration and the pause between pulses within the range;
3 - pulse-width modulation (PWM), discrete frequency selection with a range switch, smooth adjustment of the pulse duty cycle.
Two separate channels - direct and inverse.
Separate adjustment of the output signal level of the channels from 0 V to the value of the power supply voltage when connecting a high-resistance load, and to half the voltage of the power supply when connecting a load with an input impedance of 50 Ohm.
The channel output impedance is approximately 50 ohms.
Basic schemes
To build the generator, the basis is taken oscillator circuit on two logic inverters (Figure 1). Its principle of operation is based on periodic recharging of the capacitor. The moment of switching the state of the circuit is determined by the state of charge of the capacitor C1. The recharging process takes place through the resistor R1. The larger the capacitance C1 and the resistance R1, the longer the process of charging the capacitor takes place, and the longer the duration of the periods of switching the state of the circuit. And vice versa.
To build the generator circuit, the logic elements were taken microcircuit with four elements 2I-NOT - HEF4011BP... The basic circuit shown above produces a square wave at output Q of a fixed frequency and 50% duty cycle (square wave). To expand the capabilities of the device, it was decided to combine three different circuits in it, implemented on the same two logical inverters.
Square wave generator circuit
The meander generator circuit is shown in Figure 2-a. The time-setting capacitance of the circuit can vary from the value C1 to the total value of C1 and the capacitance connected by jumper P. This allows you to change the frequency range of the generated signal.
Resistor R1 allows you to smoothly change the charge (recharge) current of the capacitance. Resistor R2 is current-limiting to avoid overloading the output channel of the logic element DD1.1 in the case when the slider of the resistor R2 is in the uppermost position and its resistance is close to zero. Since the charge and recharge of the capacitor is carried out in one chain with unchanged parameters, the duration of the pulse and the pause between them are equal. Such a signal has a symmetrical rectangular shape and is called a square wave. By adjusting R1, only the frequency of the generated signal changes in a certain range, set by the time-setting capacitor.
Circuit of a rectangular pulse generator with separate adjustment of the pulse duration and pause
In Figure 2-b, the charge circuit and the overcharge circuit are separated by diodes VD1 and VD2. If a pulse is formed during the charging of the timing capacitor, its duration is characterized by the resistance of the VD1-R2-R1 chain. The duration of the pause between pulses during reverse overcharging of the capacitor is characterized by the resistance of the circuit R1-R3-VD2. So, by changing the position of the sliders of the resistors R2 and R3, you can smoothly separately set the pulse duration and the pause between them.
The frequency range of the generated signal, as in the first case, is switched by jumper P.
PWM generator circuit
The circuit in Figure 2-c has a similar separation of the circuits of the direct and reverse charge of the timing capacitor with the difference that the variable resistances are the arms of the variable resistor R2, which have an inverse dependence of the parameters in relation to each other. That is, with an increase in one arm of the resistor, the second decreases in direct proportion, and the total amount of their resistances is constant. Thus, by adjusting the ratio of the arms of the resistor R2, it is possible to smoothly change the ratio of the pulse duration to the duration of the pauses between them, and the time of the pulse repetition period will remain unchanged. This adjustment method allows the implementation of the pulse width modulation (PWM) function.
The frequency of the generated signal in this circuit is selected discretely by switching the jumper P. If necessary, you can use several jumpers P for summing large and small capacitance values, achieving a more accurate required signal generation frequency within the entire range.
Final generator circuit
Figure 3 shows generator circuit, in which all three circuits, considered in Figure 2, are implemented. The generator is based on two logical inverters on the elements DD1.1 and DD1.2. The choice of the frequency range (frequency in PWM mode) is carried out by switching the jumper P.
To assemble the desired version of the generator circuit, pin connectors are introduced, switched by parallel assemblies of jumpers, depicted by colored lines. Each jumper color corresponds to a different wiring diagram. Jumpers are realized by connecting pairs of contacts with wires from a ribbon cable of the FC-10P A connector. The pin connectors themselves are located in three groups of five pairs for easy switching. The jumper connector allows you to switch the generation mode.
Elements DD1.3 and DD1.4 act as inverting repeaters and serve to decouple the timing and output circuits of the generator to exclude their mutual influence. The inverted signal is taken from the DD1.3 output, and the main one from the DD1.4 output.
Resistors R5 and R6 are used to adjust the voltage level of the pulses of the corresponding channels. Transistors VT1 and VT2 are connected according to the emitter follower circuit to amplify the signals taken from the sliders of the resistors R5 and R6, respectively. Transistors VT3 and VT4 shunt the output circuits of their channels, pulling them to the supply minus. Their role is important when the generator signal is applied to a load with a capacitance, when a discharge of this capacitance is required during a no-current pause, such as when controlling field-effect transistors. Diodes VD5 and VD6 separate the base circuits of the shunt transistors from the generator output, eliminating the effect of the capacitive load on the operation of these transistors. Resistors R9 and R10 are needed to match the generator outputs with a 50 Ohm load resistance, as well as to limit the maximum current of the transistors of the channel output stages.
The VD3 diode protects the circuit from connecting the supply voltage of reverse polarity. The VD4 LED acts as a power indicator. Capacitor C21 partially smoothes ripple when powered from an unregulated source.
Features of the scheme
In order to reduce the dimensions of the device, SMD capacitors C1-C20 are used for the timing capacitor. With the smallest capacitor capacitance C1 = 68 pF, the generator generates a signal with a frequency of up to 17 ÷ 500 kHz. At intermediate capacitance values of 3.3 nF and 100 nF, the generator generates signals in the frequency ranges 360 ÷ 20,000 Hz and 6.25 ÷ 500 Hz, respectively. With the smallest capacitance C2 = 5.1 μF, a frequency is obtained in the range of 0.2-10 Hz. Thus, when using only four capacitors, it is possible to cover the frequency range from 0.2 Hz to 500 kHz. But at the same time, in the PWM mode, it will be possible to generate a signal of only four frequency values when using one jumper P. Therefore, to improve the characteristics of the generator, it was decided to introduce 20 capacitors of different capacities into the circuit with a uniform distribution of values over intervals. Additional accuracy of setting the frequency in PWM mode can be obtained by using several jumpers identical to P, which will allow you to adjust the frequency by connecting capacitors of lower values in comparison with the main additional one.
The power supply of the circuit has some limitations. Despite the wide enough range of supply voltage of the microcircuit 3 ÷ 15 V, as experience has shown, when the supply voltage of the circuit is below 9 V, the generator does not start. Starting at 9 V is not stable. Therefore, it is recommended to use a 12 ÷ 15 V power supply.
With a supply voltage of 15 V, a 50 Ohm load connected to one channel of the generator and the maximum output signal level, the device consumes no more than 2.5 W of power. In this case, the bulk of the power is dissipated on the load and the matching output resistor R9 (R10).
It is not recommended to turn on the generator on a short-circuited load, since the output transistor in this case operates in the limit mode. This also applies to testing circuits with bipolar switches that do not have a limiting resistor base in the circuit. In such cases, it is recommended to lower the output signal level by at least half a turn of the resistor knob, and then add it as necessary.
In my case, to vary the frequency ranges of generation, I used the following series of capacitor ratings:
C1 - 68 pF;
C2 - 100 pF;
C3 - 220 pF;
C4 - 330 pF;
C5 - 680 pF;
C6 - 1 nF;
C7 - 2.2 nF;
C8 - 3.3 nF;
C9 - 9.1 nF;
C10 - 22 nF;
C11 - 33 nF;
C12 - 47 nF;
C13 - 82 nF;
C14 - 100 nF;
C15 - 220 nF;
C16 - 330 nF;
C17 - 510 nF;
C18 - 1 μF;
C19 - 2.4 μF;
C20 - 5.1 μF.
For any reason, you can apply denominations other than those indicated. The only limitation is that the minimum capacitance should not be less than 68 pF, otherwise the generator on this capacitance may simply not start, or start auto-generation in an unsaturated mode, in which the waveform is not rectangular, but a distorted rectangle tending to a sinusoid.
The ratings are highlighted in red, at which the entire range of generated frequencies is covered.
Photo gallery
Shown here is the laying of the jumper wires in the connector, the assembled connector and the ready-made jumper connector with cut conductors.
In these photos, the generator from different angles
And this is from the side of the seal. The quality of the tracks turned out to be just disgusting, so I had to pour so much tin.
And this is, in fact, the range switching jumper and the mode switching jumper. Slightly to the right are the sockets and pins that these jumpers commute.
Anyone can make a printed circuit board for the parts that are in stock. Who is interested in the seal of my version of the generator, you can download the archive from the link below. There is a seal in PDF page format, as well as in PCB format for P-CAD version not lower than 2010. The diagram is also in the archive, you can not try to save it from the page, just download the archive.
Pulse generators are used in many radio engineering devices (electronic counters, time relays), used when setting up digital technology. The frequency range of such generators can be from units of hertz to many megahertz. Here are given simple generator circuits, including those based on digital "logic" elements, which are widely used in more complex circuits as frequency-setting nodes, switches, sources of exemplary signals and sounds.
In fig. 1 shows a diagram of a generator that generates single rectangular pulses when the S1 button is pressed (that is, it is not an autogenerator, the diagrams of which are given below). An RS-trigger is assembled on logic gates DD1.1 and DD1.2, which prevents the penetration of bounce pulses of the button contacts to the scaler. In the position of the contacts of the S1 button shown in the diagram, output 1 will have a high level voltage, and output 2 will have a low level voltage; when the button is pressed, vice versa. This generator is convenient to use when checking the performance of various counters.
In fig. 2 shows a diagram of the simplest pulse generator on an electromagnetic relay. When power is applied, the capacitor C1 is charged through the resistor R1 and the relay is triggered, turning off the power source with contacts K 1.1. But the relay does not release immediately, since for some time a current will flow through its winding due to the energy accumulated by the capacitor C1. When the contacts K 1.1 are closed again, the capacitor will start charging again - the cycle repeats.
The switching frequency of the electromagnetic relay depends on its parameters, as well as the ratings of the capacitor C1 and the resistor R1. When using the RES-15 relay (passport RS4.591.004), switching occurs approximately once a second. Such a generator can be used, for example, to switch garlands on a Christmas tree, to obtain other lighting effects. Its disadvantage is the need to use a capacitor of significant capacity.
In fig. 3 shows a diagram of another generator based on an electromagnetic relay, the principle of operation of which is similar to the previous generator, but provides a pulse frequency of 1 Hz with a capacitor capacitance 10 times smaller. When power is applied, the capacitor C1 is charged through the resistor R1. After a while, the Zener diode VD1 will open and relay K1 will work. The capacitor will begin to discharge through the resistor R2 and the input resistance of the composite transistor VT1VT2. Soon, the relay will release and a new cycle of the generator will begin. Turning on transistors VT1 and VT2 according to the composite transistor scheme increases the input resistance of the stage. Relay K 1 can be the same as in the previous device. But you can use RES-9 (passport RS4.524.201) or any other relay that operates at a voltage of 15 ... 17 V and a current of 20 ... 50 mA.
In the pulse generator, the circuit of which is shown in Fig. 4, logic elements of the DD1 microcircuit and a field-effect transistor VT1 are used. When changing the ratings of the capacitor C1 and the resistors R2 and R3, pulses with a frequency of 0.1 Hz to 1 MHz are generated. Such a wide range was obtained thanks to the use of a field-effect transistor, which made it possible to use resistors R2 and R3 with a resistance of several megohms. Using these resistors, you can change the duty cycle: resistor R2 sets the duration of the high-level voltage at the generator output, and resistor R3 sets the duration of the low-level voltage. The maximum capacitance of the capacitor C1 depends on its own leakage current. In this case, it is 1 ... 2 μF. Resistors R2, R3 - 10 ... 15 MΩ. Transistor VT1 can be any of the KP302, KP303 series. Microcircuit - K155LA3, its power supply is 5V stabilized voltage. You can use CMOS microcircuits of the K561, K564, K176 series, the power supply of which lies within 3 ... 12 V, the pinout of such microcircuits is different and is shown at the end of the article.
If you have a CMOS microcircuit (series K176, K561), you can assemble a wide-range pulse generator without using a field-effect transistor. The diagram is shown in Fig. 5. For the convenience of setting the frequency, the capacitance of the timing circuit capacitor is changed by the switch S1. The frequency range generated by the generator is 1 ... 10,000 Hz. Microcircuit - K561LN2.
If you need high stability of the generated frequency, then such a generator can be made "quartz" - turn on the quartz resonator at the desired frequency. An example of a 4.3 MHz crystal oscillator is shown below:
In fig. 6 shows a diagram of a pulse generator with an adjustable duty cycle.
The duty cycle is the ratio of the pulse repetition period (T) to their duration (t):
The duty cycle of the high-level pulses at the output of the logic element DD1.3, the resistor R1 can vary from 1 to several thousand. In this case, the frequency of the pulses also changes slightly. Transistor VT1, operating in the key mode, amplifies the pulses in power.
The generator, the diagram of which is shown in the figure below, produces pulses of both rectangular and sawtooth shapes. The master generator is made on logical elements DD 1.1-DD1.3. A differentiating circuit is assembled on the capacitor C2 and the resistor R2, due to which short positive pulses (with a duration of about 1 μs) are formed at the output of the logic element DD1.5. An adjustable current stabilizer is made on the field-effect transistor VT2 and variable resistor R4. This current charges the capacitor C3, and the voltage across it increases linearly. At the moment a short positive pulse arrives at the base of the transistor VT1, the transistor VT1 opens, discharging the capacitor C3. In this way, a sawtooth voltage is formed on its plates. Resistor R4 regulates the capacitor charging current and, consequently, the slope of the sawtooth voltage rise and its amplitude. Capacitors C1 and C3 are selected based on the required pulse frequency. Microcircuit - K561LN2.
Digital microcircuits in generators are interchangeable in most cases and can be used in the same circuit as microcircuits with "AND-NOT" and "OR-NOT" elements, or simply inverters. A variant of such replacements is shown on the example of Figure 5, where a microcircuit with K561LN2 inverters was used. Exactly such a scheme with the preservation of all parameters can be assembled both on the K561LA7 and on the K561LE5 (or the K176, K564, K164 series), as shown below. It is only necessary to observe the pinout of the microcircuits, which in many cases even coincides.
555 - analog integrated circuit, universal timer - a device for the formation (generation) of single and repetitive pulses with stable time characteristics. It is used to build various generators, modulators, time relays, threshold devices and other components of electronic equipment. As examples of the use of a timer microcircuit, one can indicate the functions of restoring a digital signal distorted in communication lines, bounce filters, on-off regulators in automatic control systems, pulse power converters, pulse-width control devices, timers, etc.
In this article I will talk about building a generator on this microcircuit. As written above, we already know that the microcircuit generates repetitive pulses with stable time characteristics, and this is what we need.
Switching circuit in astable mode. The figure below shows this.
Since we have a pulse generator, we must know their approximate frequency. Which we calculate by the formula.
The values of R1 and R2 are substituted in Ohms, C - in farads, the frequency is obtained in Hertz.
The time between the beginning of each next impulse is called the period and is denoted by the letter t. It consists of the duration of the pulse itself - t1 and the interval between pulses - t2. t = t1 + t2.
Frequency and period are the opposite concepts and the relationship between them is as follows:
f = 1 / t.
t1 and t2 of course also can and should be calculated. Like this:
t1 = 0.693 (R1 + R2) C;
t2 = 0.693R2C;
We ended up with the theory like this that let's get down to practice.
Developed a simple scheme with all the details available.
I'll tell you about its features. As many have already understood, the S2 switch is used to switch the operating frequency. The KT805 transistor is used to amplify the signal (install on a small radiator). Resistor R4 is used to adjust the output signal current. The microcircuit itself serves as a generator. The duty cycle and frequency of the working pulses are changed by resistors R3 and R2. The diode serves to increase the duty cycle (it can be excluded altogether). There is also a shunt and an operation indicator, an LED with a built-in current limiter is used for it (you can use a regular LED by limiting the current with a 1 kΩ resistor). Actually, that's all, then I'll show you how a working device looks like.
Top view, the switches of the operating frequency are visible.
I attached a memo below.
These trimming resistors regulate the duty cycle and frequency (you can see their designation on the memo).
Side power switch and signal output.
List of radioelements
| Designation | Type of | Denomination | Quantity | Note | Shop | My notebook |
|---|---|---|---|---|---|---|
| IC1 | Programmable timer and oscillator | NE555 | 1 | Into notepad | ||
| T1 | Bipolar transistor | KT805A | 1 | Into notepad | ||
| D1 | Rectifier diode | 1N4148 | 1 | Into notepad | ||
| C1 | Capacitor | 1 nF | 1 | Into notepad | ||
| C2 | Capacitor | 100 nF | 1 | Into notepad | ||
| C3 | Capacitor | 1000 nF | 1 | Into notepad | ||
| C4 | Electrolytic capacitor | 100 uF | 1 | Into notepad | ||
| R1 | Resistor | 500 ohm | 1 |
Generators of rectangular pulses are widely used in radio engineering, television, automatic control systems and computer technology.
To obtain rectangular pulses with steep edges, devices are widely used, the principle of operation of which is based on the use of electronic amplifiers with positive feedback. These devices include the so-called relaxation generators - multivibrators, blocking generators. These generators can operate in one of the following modes: standby, self-oscillating, synchronization and frequency division.
In standby mode, the generator has one stable equilibrium state. An external trigger pulse causes the waiting generator to jump to a new state that is not stable. In this state, called quasi-equilibrium, or temporarily stable, relatively slow processes occur in the generator circuit, which ultimately lead to a reverse jump, after which a stable initial state is established. The duration of the quasi-equilibrium state, which determines the duration of the generated rectangular pulse, depends on the parameters of the generator circuit. The main requirements for waiting generators are the stability of the duration of the generated pulse and the stability of its initial state. Waiting generators are used primarily to obtain a certain time interval, the beginning and end of which are fixed, respectively, by the rise and fall of the generated rectangular pulse, as well as for pulse expansion, for dividing the pulse repetition rate and for other purposes.
In the self-oscillating mode, the generator has two quasi-equilibrium states and does not have a single stable state. In this mode, without any external influence, the generator sequentially jumps from one quasi-equilibrium state to another. In this case, pulses are generated, the amplitude, duration and repetition rate of which are mainly determined by the parameters of the generator. The main requirement for such generators is high stability of the self-oscillation frequency. Meanwhile, as a result of changes in supply voltages, replacement and aging of elements, the impact of other factors (temperature, humidity, interference, etc.), the stability of the frequency of self-oscillations of the generator is usually low.
In the synchronization or frequency division mode, the repetition rate of the generated pulses is determined by the frequency of the external synchronizing voltage (sinusoidal or pulse) supplied to the generator circuit. The pulse repetition rate is equal to or a multiple of the sync voltage frequency.
A generator of periodically repeating rectangular relaxation-type pulses is called a multivibrator.
The multivibrator circuit can be implemented both on discrete elements and in integral design.
Discrete multivibrator. In such a multivibrator, two amplifying stages are used, covered by feedback. One feedback path is formed by a capacitor and a resistor 
and the other is 
and 
(fig. 6.16).
states and provides the generation of periodically repeating pulses, the shape of which is close to rectangular.
In a multivibrator, both transistors can be in active mode for a very short time, since as a result of the action of positive feedback, the circuit jumps into a state where one transistor is open and the other is closed.
Let us assume for definiteness that at the moment of time 
transistor VT1
open and saturated, and the transistor VT2
closed (fig. 6.17). Capacitor 
due to the current flowing in the circuit in previous moments of time, it is charged to a certain voltage. The polarity of this voltage is such that to the base of the transistor VT2
a negative voltage is applied to the emitter and VT2
closed. Since one transistor is closed, and the other is open and saturated, the self-excitation condition is not satisfied in the circuit, since the amplification factors of the stages 
.
In this state, two processes take place in the circuit. One process is associated with the flow of a capacitor recharge current 
from the power supply through the resistor circuit 
- open transistor VT1
.The second process is due to the charge of the capacitor 
through a resistor 
and the base circuit of the transistor VT1
, as a result, the voltage at the collector of the transistor VT2
increases (fig. 6.17). Since the resistor included in the base circuit of the transistor has a higher resistance than the collector resistor ( 
), the charging time of the capacitor 
less capacitor recharge time 
.
|
|
Capacitor charging process |
exponential with time constant 
... Therefore, the charging time of the capacitor 
, as well as the rise time of the collector voltage 
, i.e., the duration of the leading edge of the pulse 
... During this time, the capacitor 
charging with voltage 
.Due to overcharging of the capacitor 
base voltage 
transistor VT2
growing, but so far 
transistor VT2
closed, and the transistor VT1
open because its base turns out to be connected to the positive pole of the power supply through a resistor 
.
Basic 
and collector 
transistor voltage VT1
at the same time do not change. This state of the circuit is called quasi-stable.
At a moment in time 
as the capacitor overcharges, the voltage across the base of the transistor VT2
reaches the opening voltage and the transistor VT2
goes into active operating mode, for which 
... On opening VT2
collector current increases 
and accordingly decreases 
... Decrease 
causes a decrease in the base current of the transistor VT1
, which, in turn, leads to a decrease in the collector current 
... Decrease in current 
accompanied by an increase in the base current of the transistor VT2
since the current flowing through the resistor 
, branches off to the base of the transistor VT2
and 
.
After the transistor VT1
will exit the saturation mode, the self-excitation condition is fulfilled in the circuit: 
... In this case, the process of switching the circuit proceeds like an avalanche and ends when the transistor VT2
goes into saturation mode, and the transistor VT1
- in cut-off mode.
In the future, a practically discharged capacitor 
(
) is charged from the power source through the resistor circuit 
- the basic circuit of the open transistor VT2
exponentially with time constant 
... As a result, over time 
there is an increase in the voltage across the capacitor 
before 
and the front of the collector voltage is formed 
transistor VT1
.
The closed state of the transistor VT1
ensured by the fact that initially charged to voltage 
capacitor 
through an open transistor VT2
connected to the base-emitter gap of the transistor VT1
, which maintains a negative voltage at its base. Over time, the blocking voltage at the base changes as the capacitor 
recharged through the circuit resistor 
- open transistor VT2
... At a moment in time 
transistor voltage VT1
reaches the value 
and it opens.
In the circuit, the self-excitation condition is again satisfied and a regenerative process develops, as a result of which the transistor VT1
goes into saturation mode, and VT2
closes. Capacitor 
turns out to be charged to voltage 
, and the capacitor 
almost discharged ( 
). This corresponds to the moment in time 
, from which the consideration of the processes in the diagram began. This completes the full cycle of operation of the multivibrator, since in the future the processes in the circuit are repeated.
As follows from the timing diagram (Fig. 6.17), in a multivibrator, periodically repeating rectangular pulses can be removed from the collectors of both transistors. In the case where the load is connected to the collector of the transistor VT2
, pulse duration 
is determined by the process of overcharging the capacitor 
, and the pause duration 
- the process of overcharging the capacitor 
.
Capacitor overcharge circuit 
contains one reactive element, therefore, where 
;
;.
Thus, .
Recharge process 
ends at time 
, when 
... Therefore, the duration of the positive pulse of the collector voltage of the transistor VT2
is defined by the formula:
.
In the case when the multivibrator is made on germanium transistors, the formula is simplified, since 
.
Capacitor recharge process 
which determines the length of the pause 
between pulses of the collector voltage of the transistor VT2
, proceeds in the same equivalent circuit and under the same conditions as the process of recharging the capacitor 
, only with a different time constant: 
... Therefore, the formula for calculating 
is similar to the formula for calculating 
:
.
Usually, in a multivibrator, the pulse duration and pause duration are adjusted by changing the resistance of the resistors. 
and 
.
The durations of the edges depend on the opening time of the transistors and are determined by the charging time of the capacitor through the collector resistor of the same arm 
... When calculating the multivibrator, it is necessary to fulfill the saturation condition of the open transistor 
... For transistor VT2
excluding current 
overcharge capacitor 
current 
... Therefore, for the transistor VT1
saturation condition 
, and for the transistor VT2
-
.
Generated pulse frequency 
... The main obstacle to increasing the pulse generation frequency is the long rise time of the pulses. A decrease in the pulse rise time due to a decrease in the resistance of the collector resistors can lead to a non-fulfillment of the saturation condition.
With a high degree of saturation in the considered multivibrator circuit, cases are possible when, after switching on, both transistors are saturated and there are no oscillations. This corresponds to a rigid self-excitation regime. To prevent this, one should choose the operating mode of an open transistor near the saturation limit in order to maintain a sufficient gain in the feedback circuit, and also use special multivibrator circuits.
If the pulse width 
equal to duration 
, which is usually achieved at, then such a multivibrator is called symmetric.
The duration of the front of the pulses generated by the multivibrator can be significantly reduced if diodes are additionally introduced into the circuit (Fig. 6.18).
When, for example, a transistor turns off VT2
and the collector voltage begins to increase, then to the diode VD2
a reverse voltage is applied, it closes and thereby disconnects the charging capacitor 
from the collector of the transistor VT2
... As a result, the charging current of the capacitor 
no longer flows through the resistor 
, and through the resistor 
... Therefore, the duration of the leading edge of the collector voltage pulse 
is now determined only by the closing process of the transistor VT2
... The diode works in a similar way. VD1
when charging the capacitor 
.
Although in such a scheme the rise time is significantly reduced, the charging time of the capacitors, which limits the pulse duty cycle, practically does not change. Time constants 
and 
cannot be reduced by reducing 
... Resistor 
in the open state of the transistor through an open diode is connected in parallel with the resistor 
. As a result, for 
the power consumption of the circuit increases.
Integrated circuit multivibrator(Fig. 6.19). The simplest circuit contains two inverting logic gates LE1 and LE2, two timing chains 
and 
and diodes VD1
,
VD2
.
|
|
|
Let us assume that at the moment of time Input voltage LE2 as the capacitor charges |
|
(fig. 6.20) voltage 
, a 
... If the current through the capacitor 
does not leak, then the voltage on it 
, and at the input of the element LE1
... The capacitor charging current flows in the circuit 
from LE1 through a resistor 
.
decreases, but for now 
,LE2 is at zero output.
At a moment in time 
and at the exit LE2
... As a result, at the entrance LE1 through capacitor 
which is charged to voltage 
, voltage is applied and LE1 goes to zero state 
... Since the voltage at the output LE1 decreased, then the capacitor 
starts to discharge. As a result, on the resistor 
a voltage of negative polarity will appear, the diode will open VD2
and capacitor 
quickly discharges to voltage 
... After the end of this process, the voltage at the input LE2
.
At the same time, the process of charging the capacitor takes place in the circuit 
and over time the voltage at the input LE1 decreases. When at a point in time 
voltage 
,
,
... The processes begin to repeat themselves. The capacitor is charged again 
, and the capacitor 
discharged through an open diode VD1
... Since the resistance of an open diode is much less than the resistance of the resistors 
, and 
, capacitor discharge 
and 
happens faster than their charge.
Input voltage LE1 in time interval 
is determined by the process of charging the capacitor 
:, where 
;
- the output resistance of the logic element in the state of unity; 
;
, where 
... When 
, the formation of a pulse at the output of the element ends LE2, therefore, the pulse duration
.
The duration of the pause between pulses (time interval from 
before 
) is determined by the process of charging the capacitor 
, therefore
.
The duration of the front of the generated pulses is determined by the switching time of the logic elements.
In the timing diagram (Fig. 6.20), the amplitude of the output pulses does not change: 
, since its construction did not take into account the output impedance of the logic element. Taking into account the finiteness of this output resistance, the amplitude of the pulses will change.
The disadvantage of the considered simplest multivibrator circuit based on logic elements is a hard self-excitation mode and the associated possible absence of an oscillatory mode of operation. This drawback of the circuit can be eliminated if an additional logical element AND is introduced (Fig. 6.21).
|
|
When the multivibrator generates pulses, then the output LE3
, insofar as 
... However, due to the hard self-excitation mode, such a case is possible when, when the power supply voltage is turned on, due to the low rate of voltage rise, the capacitor charge current 
and 
turns out to be small. In this case, the voltage drop across the resistors 
and 
may be less than the threshold 
and both elements ( LE1 and LE2) will be in a state when the voltages at their outputs 
... With this combination of input signals at the output of the element LE3 there will be tension 
which through the resistor 
is fed to the input of the element LE2... Because 
, then LE2 is set to zero and the circuit begins to generate pulses.
To build generators of rectangular pulses, along with discrete elements and LEs in integral design, operational amplifiers are used.
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Operational amplifier multivibrator has two feedback circuits (Fig. 6.22). The feedback loop of the non-inverting input is formed by two resistors ( |
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and 
) and, therefore, 
... Feedback on the inverting input is formed by a chain 
,
therefore the voltage at the inverting input is 
depends not only on the voltage at the output of the amplifier, but is also a function of time, since 
.
|
Let us consider the processes taking place in the multivibrator starting from the moment of time |
|
(Fig. 6.23) when the output voltage is positive ( 
). In this case, the capacitor 
as a result of the processes that took place in the previous moments of time, it is charged in such a way that a negative voltage is applied to the inverting input.
Positive voltage applied to non-inverting input 
... Voltage 
remains constant, and the voltage at the inverting input 
increases over time, tending to the level 
, since the circuit is in the process of overcharging the capacitor 
.
However, while 
, the state of the amplifier determines the voltage at the non-inverting input and the output remains at the level 
.
At a moment in time 
the voltages at the inputs of the operational amplifier become equal: 
... Further slight increase 
leads to the fact that the differential (difference) voltage at the inverting input of the amplifier 
turns out to be positive, so the output voltage drops sharply and becomes negative 
... Since the voltage at the output of the operational amplifier has changed polarity, the capacitor 
in the future, it is recharged and the voltage on it, as well as the voltage at the inverting input, tend to 
.
At a moment in time 
again 
and then the differential (difference) voltage at the input of the amplifier 
becomes negative. Since it acts at the inverting input, the voltage at the amplifier output abruptly again takes on the value 
... The voltage at the non-inverting input also changes abruptly 
... Capacitor 
which by the time 
charged to a negative voltage, recharges again and the voltage at the inverting input increases, tending to 
... Since in this case 
, then the voltage at the amplifier output is kept constant. As follows from the timing diagram (Fig. 6.23), at the time 
the full cycle of operation of the circuit ends and in the future the processes in it are repeated. Thus, at the output of the circuit, periodically repeating rectangular pulses are generated, the amplitude of which at 
is equal to 
... Pulse duration (time interval 
) is determined by the capacitor recharge time 
exponentially from 
before 
with time constant 
, where 
Is the output impedance of the operational amplifier. Since during pause (interval 
) the overcharging of the capacitor occurs under exactly the same conditions as during the formation of pulses, then 
... Hence, the circuit works like a balanced multivibrator.
occurs with a constant time 
... With negative output voltage ( 
) diode is open VD2
and the time constant of the capacitor recharge 
defining the duration of the pause, 
.
A waiting multivibrator or one-shot has one stable state and provides the generation of rectangular pulses when short triggering pulses are applied to the input of the circuit.
Discrete Single Vibrator consists of two amplifying stages, covered with positive feedback (Fig. 6.25).
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One branch of the feedback, as in the multivibrator, is formed by a capacitor |
and resistor 
; the other is a resistor 
connected to the common emitter circuit of both transistors. Due to this inclusion of the resistor 
base-emitter voltagetransistor VT1
depends on the collector current of the transistor VT2
... This circuit is called an emitter coupled single shot. The parameters of the circuit are calculated in such a way that in the initial state, in the absence of input pulses, the transistor VT2
was open and full, and VT1
was in cutoff mode. This state of the circuit, which is stable, is provided when the following conditions are met: 
.
Let us assume that the one-shot is in a steady state. Then the currents and voltages in the circuit will be constant. Transistor base VT2
through a resistor 
connected to the positive pole of the power supply, which in principle ensures the on state of the transistor. To calculate the collector 
and basic 
currents, we have the system of equations
.
Having determined from here the currents 
and 
, the saturation condition is written in the form:
.
Considering that 
and 
, the resulting expression is greatly simplified: 
.
On a resistor 
due to the flow of currents 
,
voltage drop is generated 
... As a result, the potential difference between the base and emitter of the transistor VT1
defined by the expression:
If the schema satisfies the condition 
then transistor VT1
closed. Capacitor 
at the same time it is charged to voltage. The polarity of the voltage across the capacitor is shown in Fig. 6.25.
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Let us assume that at the moment of time |
|
(Fig. 6.26) a pulse arrives at the input of the circuit, the amplitude of which is sufficient to open the transistor VT1
... As a result, the process of opening the transistor begins in the circuit. VT1
accompanied by an increase in collector current 
and decreasing the collector voltage 
.
When the transistor VT1
opens, capacitor 
turns out to be connected to the base-emitter region of the transistor VT2
in such a way that the base potential becomes negative and the transistor VT2
enters cutoff mode. The process of switching the circuit is of an avalanche-like nature, since at this time the self-excitation condition is fulfilled in the circuit. The switching time of the circuit is determined by the duration of the processes of turning on the transistor VT1
and turn off the transistor VT2
and is fractions of a microsecond.
When closing the transistor VT2
through a resistor 
collector and base currents cease to flow VT2
... The resulting transistor VT1
remains open even after the end of the input pulse. At this time on the resistor 
voltage drops 
.
The state of the circuit when the transistor VT1
open and VT2
closed, is quasi-stable. Capacitor 
through a resistor 
, open transistor VT1
and resistor 
turns out to be connected to the power source in such a way that the voltage across it has opposite polarity. A capacitor recharge current flows in the circuit 
, and the voltage across it, and therefore on the base of the transistor VT2
tends to a positive level.
Voltage change 
is exponential: where 
... Initial voltage at the base of the transistor VT2
determined by the voltage to which the capacitor is initially charged 
and the residual voltage across the open transistor:
The limit value of the voltage to which the voltage at the base of the transistor tends VT2 , .
It is taken into account here that through the resistor 
not only the capacitor recharge current flows 
but also the current 
open transistor VT1
... Hence, .
At a moment in time 
voltage 
reaches firing voltage 
and transistor VT2
opens. The emerging collector current 
creates an additional voltage drop across the resistor 
, which leads to a decrease in voltage 
... This causes a decrease in the base 
and collector 
currents and the corresponding increase in voltage 
... Positive increment in the collector voltage of the transistor VT1
through capacitor 
transmitted to the base circuit of the transistor VT2
and contributes to an even greater increase in its collector current 
... In the circuit, a regenerative process develops again, ending in the fact that the transistor VT1
closes and the transistor VT2
goes into saturation mode. This completes the pulse generation process. The pulse duration is determined by putting 
:
.
After the end of the pulse in the circuit, the process of charging the capacitor takes place 
through a circuit consisting of resistors 
,
and the emitter circuit of the open transistor VT2
... At the initial moment, the base current 
transistor VT2
is equal to the sum of the capacitor charge currents 
: current 
limited by the resistance of the resistor 
, and the current flowing through the resistor 
... As the capacitor charges 
current 
the current of the base of the transistor decreases and accordingly decreases VT2
, tending to the stationary value determined by the resistor 
... As a result, at the moment of opening the transistor VT2
voltage drop across a resistor 
turns out to be greater than the stationary value, which leads to an increase in the negative voltage at the base of the transistor VT1
... When the voltage across the capacitor reaches the value 
the circuit returns to its original state. The duration of the process of recharging the capacitor 
, which is called the stage of recovery, is determined by the ratio.
The minimum pulse repetition period of the one-shot 
, and the maximum frequency 
... If the interval between input pulses is less 
, then the capacitor 
will not have time to recharge and this will lead to a change in the duration of the generated pulses.
The amplitude of the generated pulses is determined by the voltage difference across the collector of the transistor VT2 in the closed and open states.
A monovibrator can be implemented on the basis of a multivibrator, if one branch of the feedback is made not capacitive, but resistor and a voltage source is introduced 
(fig. 6.27). Such a scheme is called a single-shot with collector-base connections.
To the base of the transistor VT2
negative voltage is applied and it is closed. Capacitor 
charged to voltage 
... In the case of germanium transistors 
.
Capacitor 
, playing the role of a boost capacitor, is charged to voltage 
... This state of the circuit is stable.
When applied to the base of the transistor VT2 unlocking pulse (Fig. 6.28) in the circuit, the processes of opening the transistor begin to flow VT2 and closing the transistor VT1 .
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In this case, the condition of self-excitation is fulfilled, a regenerative process develops and the circuit passes into a quasi-stable state. Transistor VT1
turns out to be in a closed state, since due to the charge on the capacitor |
|
a negative voltage is applied to its base. Transistor VT2
remains in the open state even after the end of the input signal, since the collector potential of the transistor VT1
when it was closed, it increased, and accordingly the voltage at the base increased VT2
.
When the circuit is switched, the front of the output pulse is formed, which is usually removed from the collector of the transistor. VT1
... In the future, the process of recharging the capacitor takes place in the circuit. 
.The voltage on it 
and, consequently, the voltage at the base 
transistor VT1
changes exponentially 
,where 
.
When at a point in time 
base voltage reaches 
, transistor VT1
opens, the voltage on its collector 
the transistor decreases and turns off VT2
... In this case, a cutoff of the output pulse is formed. The pulse duration is obtained if we put 
:
.
Because 
, then . Slice duration 
.
Subsequently, the capacitor charging current flows in the circuit 
through a resistor 
and the base circuit of the open transistor VT1
... The duration of this process, which determines the recovery time of the circuit, 
.
The amplitude of the output pulses in such a one-shot circuit is practically equal to the voltage of the power supply.
One-vibrator on logical elements... To implement a one-shot on logic gates, NAND gates are usually used. The structural diagram of such a one-shot includes two elements ( LE1 and LE2) and timing chain 
(fig. 6.29). Inputs LE2 combined and it works like an inverter. Output LE2 connected to one of the inputs LE1, and a control signal is applied to its other input.
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In order for the circuit to be in a stable state, to the control input LE1 voltage must be applied |
|
(fig. 6.30). Under this condition LE2 is in state "1", and LE1- in the "0" state. Any other combination of element states is not persistent. In this state, the circuit on the resistor 
there is some voltage drop due to current LE2 flowing in
its input circuit. The circuit generates a rectangular pulse with a short-term decrease (time 
) input voltage 
... At a time interval equal to 
(not shown in Fig. 6.29), at the output LE1 the tension will increase. This voltage surge across the capacitor 
passed to the input LE2... Element LE2 switches to state "0". Thus, at input 1 LE1 at intervals 
tension starts to act 
and this element will remain in the state of one, even if even after the time has elapsed 
voltage 
will again become equal to logical "1". For normal operation of the circuit, it is necessary that the duration of the input pulse 
.
As the capacitor charges 
output current LE1 decreases. Accordingly, the voltage drop decreases by 
:
... At the same time, the voltage increases slightly 
aiming for stress 
which when switching LE1 to state "1" was less 
due to the voltage drop across the output impedance LE1... This state of the circuit is temporarily stable.
At a moment in time 
voltage 
reaches the threshold 
and element LE2 switches to state "1". To entrance 1 LE1 signal is given 
and it switches to the log state. "0". In this case, the capacitor 
, which in the time interval from 
before 
charged, starts to discharge through the output resistance LE1 and diode VD1
... After the time has elapsed 
, determined by the capacitor discharge process 
, the circuit returns to its original state.
Thus, at the output LE2 a rectangular pulse is generated. Its duration, depending on the time of decrease 
before 
, is determined by the relation 
, where 
- output impedance LE1 in state "1". The recovery time of the circuit, where 
- output impedance LE1 in state "0"; 
- internal resistance of the diode in the open state.
and the voltage at the inverting input is low: 
, where 
voltage drop across the diode in the open state. The voltage at the non-inverting input is also constant: 
and since 
, then a constant voltage is maintained at the output 
.
When served at a point in time 
input pulse with positive polarity amplitude 
the voltage at the non-inverting input becomes greater than the voltage at the inverting input and the output voltage suddenly becomes equal to 
... In this case, the voltage at the non-inverting input also increases abruptly up to 
... Simultaneously diode VD closes, condenser 
begins to charge and a positive voltage rises at the inverting input (Fig. 6.32). Bye 
the output voltage remains 
... At a moment in time 
at 
the polarity of the output voltage changes and the voltage at the non-inverting input takes its initial value, and the voltage 
begins to decrease as the capacitor discharges 
.
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When Pulse duration, determined by the exponential process of charging a capacitor |
|
reaches the value 
, diode opens VD, and at this the process of changing the voltage at the inverting input stops. The circuit is in a steady state.
with time constant 
from stress 
before 
, is equal to 
.
Because 
, then 
.
The recovery time of the circuit is determined by the duration of the capacitor discharge process 
from 
before 
and taking into account the accepted assumptions 
.
Generators based on operational amplifiers provide the formation of pulses with an amplitude of up to tens of volts; the rise time depends on the bandwidth of the operational amplifier and can be fractions of a microsecond.
A blocking generator is a relaxation-type pulse generator in the form of a single-stage amplifier with positive feedback created by a transformer. The blocking generator can operate in standby and self-oscillating modes.
Standby mode blocking-generator. When operating in standby mode, the circuit has one stable state and generates rectangular pulses when trigger pulses are received at the input. The steady state of a blocking generator based on a germanium transistor is achieved by including a bias source in the base circuit. When using a silicon transistor, a bias source is not required, since the transistor is closed at zero voltage at the base (Figure 6.33).
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Positive feedback in the circuit is manifested in the fact that with an increase in the current in the primary (collector) winding of the transformer, i.e., the collector current of the transistor ( |
), a voltage of such polarity is induced in the secondary (base) winding that the base potential increases. And, conversely, for 
base voltage decreases. Such a connection is realized by the appropriate connection of the beginning of the transformer windings (in Fig. 6.33, shown by dots).
In most cases, the transformer has a third (load) winding to which the load is connected 
.
The voltages on the transformer windings and the currents flowing in them are interconnected as follows: 
,
,
,
where 
,
- transformation ratios; 
- the number of turns of the primary, secondary and load windings, respectively.
The duration of the process of turning on the transistor is so small that during this time the magnetizing current practically does not increase ( 
). Therefore, the equation of currents when analyzing the transient process of turning on the transistor is simplified: 
.
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When served at a point in time |
|
to the base of the firing pulse transistor 
(fig.6.34) there is an increase in current 
, the transistor goes into active mode and collector current appears 
... Increase in collector current by 
leads to an increase in voltage on the primary winding of the transformer 
, the subsequent growth of the reduced
base current 
and the actual current flowing in the base circuit of the transistor, 
.
Thus, the initial change in base current 
as a result of the processes occurring in the circuit, leads to a further change in this current 
, and if 
, then the process of changing currents and voltages is of an avalanche nature. Therefore, the condition for self-excitation of the blocking generator: 
.
In the absence of load ( 
) this condition is simplified: 
... Because 
, then the self-excitation condition in the blocking generator is satisfied quite easily.
The process of opening the transistor, accompanied by the formation of the pulse front, ends when it goes into saturation mode. In this case, the self-excitation condition ceases to be fulfilled and the pulse top is then formed. Since the transistor is saturated: 
, then a voltage is applied to the primary winding of the transformer 
and the reduced base current 
as well as load current 
, turn out to be constant. The magnetizing current during the formation of the pulse top can be determined from the equation 
, whence for zero initial conditions we obtain 
.
Thus, the magnetizing current in the blocking generator, when the transistor is saturated, increases in time according to a linear law. In accordance with the equation of currents, the collector current of the transistor also increases linearly 
.
Over time, the saturation of the transistor decreases as the base current remains constant. 
, and the collector current increases. At some point in time, the collector current increases so much that the transistor switches from saturation mode to active mode and again the self-excitation condition of the blocking generator begins to be fulfilled. Obviously, the duration of the pulse top 
is determined by the time during which the transistor is in saturation mode. The saturation mode boundary corresponds to the condition 
... Hence, 
.
From here we get the formula for calculating the duration of the pulse top:
.
Magnetizing current 
during the formation of the top of the pulse, it increases and at the moment of the end of this process, i.e., at 
, reaches the value 
.
Since the voltage of the power source is applied to the primary winding of the pulse transformer during the formation of the top of the pulse 
, then the amplitude of the pulse on the load 
.
When the transistor switches to active mode, the collector current decreases 
... A voltage is induced in the secondary winding, leading to a decrease in base voltage and current, which in turn causes a further decrease in collector current. A regenerative process develops in the circuit, as a result of which the transistor goes into cut-off mode and a pulse cut is formed.
The avalanche-like process of closing the transistor has such a short duration that the magnetizing current 
during this time practically does not change and remains equal 
... Therefore, by the time the transistor turns off in the inductance 
stored energy 
... This energy is only dissipated in the load. 
, since the collector and base circuits of the closed transistor turn out to be open. In this case, the magnetizing current decreases exponentially: 
, where 
- time constant. Flowing through a resistor 
current creates a reverse voltage surge across it, the amplitude of which is 
, which is also accompanied by a voltage surge at the base and collector of the closed transistor 
... Using the previously found relation for 
, we get:
,
.
The process of dissipation of the energy stored in a pulse transformer, which determines the recovery time of the circuit 
, ends after a time interval 
, after which the circuit returns to its original state. Additional surge in collector voltage 
can be significant. Therefore, in the blocking generator circuit, measures are taken to reduce the value 
, for which a damping circuit consisting of a diode is included in parallel with the load or in the primary winding VD1
and resistor 
whose resistance 
(fig. 6.33). When the pulse is formed, the diode is closed, since a voltage of reverse polarity is applied to it, and the damping circuit does not affect the processes in the circuit. When, when the transistor is closed, a voltage surge occurs in the primary winding, then a forward voltage is applied to the diode, it opens and current flows through the resistor 
... Because 
, then the surge in collector voltage 
and reverse voltage surge on 
decrease significantly. However, this increases the recovery time: 
.
A resistor is not always included in series with the diode. 
, and then the amplitude of the burst turns out to be minimal, but its duration increases.
impulses. Let us consider the processes taking place in the circuit starting from the moment of time 
when the voltage across the capacitor 
reaches the value 
and the transistor will open (fig. 6.36).
Since the voltage on the secondary (base) winding during the formation of the peak of the pulse remains constant 
, then as the capacitor charges, the base current decreases exponentially 
, where 
- resistance of the base - emitter region of the saturated transistor; 
- time constant.
In accordance with the equation of currents, the collector current of the transistor is determined by the expression 
.
From the above relations it follows that in the self-oscillating blocking generator during the formation of the pulse top, both the base and collector currents change. As you can see, the base current decreases over time. The collector current, in principle, can increase and decrease. It all depends on the relationship between the first two terms of the last expression. But even if the collector current decreases, it is slower than the base current. Therefore, with a decrease in the base current of the transistor, there comes a point in time 
when the transistor leaves the saturation mode and the process of forming the pulse top ends. Thus, the duration of the pulse top is determined by the relation 
... Then it is possible to write down the equation of currents for the moment of the end of the formation of the top of the pulse:
.
After some transformations, we have 
... The resulting transcendental equation can be simplified under the condition 
... Using the exponential series expansion and limiting ourselves to the first two terms 
, we obtain the formula for calculating the duration of the pulse top 
, where 
.
During the formation of the top of the pulse due to the flow of the base current of the transistor, the voltage across the capacitor 
changes and by the time the transistor closes, it becomes equal to 
... Substituting in this expression the value 
and integrating, we get:
.
When the transistor enters the active mode of operation, the self-excitation condition starts again, and an avalanche-like process of its closing occurs in the circuit. As in the waiting blocking generator, after the transistor is closed, the energy stored in the transformer dissipates, accompanied by the appearance of surges in the collector and base voltages. After the end of this process, the transistor continues to be in the closed state due to the fact that the negative voltage of the charged capacitor is applied to the base. 
... This voltage does not remain constant, since in the closed state of the transistor through the capacitor 
and resistor 
overcharge current flows from the power supply 
... Therefore, as the capacitor overcharges 
the voltage at the base of the transistor increases exponentially 
, where 
.
When base voltage reaches 
, the transistor opens and the pulse shaping process begins again. Thus, the duration of the pause 
, determined by the time the transistor is in the off state, can be calculated by putting 
... Then we get 
For a blocking generator based on a germanium transistor, the obtained formula is simplified, since 
.
Blocking generators have a high efficiency, since in the pause between pulses, the current from the power source is practically not consumed. Compared to multivibrators and monovibrators, they allow you to get a greater duty cycle and shorter pulse duration. An important advantage of blocking generators is the ability to receive pulses whose amplitude is greater than the power supply voltage. For this, it is enough that the transformation ratio of the third (load) winding 
... In a blocking generator, in the presence of several load windings, it is possible to carry out galvanic isolation between the loads and receive pulses of different polarity.
The blocking generator circuit is not implemented in an integral design due to the presence of a pulse transformer.
Somehow they asked me to make a simple flashing light to control the relay or flash a low-power light. Assembling the simplest multivibrator, whether symmetrical or not symmetrical, is somehow trivial, and the circuit is unstable and not entirely reliable, despite the fact that it should work at a voltage of 24 volts in a truck, and even not too large in size.
Scheme
After searching the network for the circuit, I decided to include the popular NE555N microcircuit using the datasheet. Precision timer, the cost of which is very low - about 10 rubles per microcircuit in a dip case! But since our load is not entirely weak, and large currents may be required relative to the timer supply, then we need some kind of key that will be controlled by the timer itself.
You can take an ordinary transistor, but it will heat up due to large losses due to large drops at the junctions - therefore, I took a high-voltage field-effect transistor for several amperes of current, such a switch with a current of even 2 amperes does not need a radiator at all.
The 555 timer itself has limitations in the supply voltage - about 18 volts, although even at 15 it can safely fly out, so we assemble a chain of a limiting resistor and a zener diode with a filtering capacitor at the power input!
A regulator has been introduced into the circuit so that it is possible to rotate the regulator knob to change the frequency of pulses of a light bulb or relay actuation. If adjustment is not required, you can adjust the frequency to the desired one, measure the resistance and then solder the finished one. In the above, there are 2 regulators at once, which change the duty cycle (the ratio of the on-to-off state of the output). If a 1: 1 ratio is required, remove everything except one variable resistor.
Video
Some of the elements are made in dip cases, some in smd - for compactness and better layout as a whole. The pulse generator circuit started working almost immediately after switching on, it remains only to adjust to the desired frequency. It is advisable to fill the board with hot melt glue or put it in a plastic case, so that car owners would not guess to screw it directly to the case or put it on something metal.