Multivibrator

MULTIVIBRATOR AND ITS APPLICATIONS

This lesson will be dedicated to a quite important and highly demanded topic: multivibrators and their practical applications. If I tried to just list where and how astable (free-running) symmetrical and asymmetrical multivibrators are used, it would take a good number of book pages. There is perhaps no branch of radio engineering, electronics, automation, pulse, or computer technology where such generators are not applied. In this lesson, theoretical information about these devices will be given, and at the end, I will provide several examples of their practical use for your DIY projects.

Astable Multivibrator

Multivibrators are electronic devices that generate electrical oscillations with a shape close to a rectangular wave (square wave). The spectrum of oscillations generated by a multivibrator contains many harmonics — which are also electrical oscillations, but multiples of the fundamental frequency. This is reflected in its name: multi (many) and vibra (oscillate).

Let's look at the schematic shown in (Fig. 1, a). Recognize it? Yes, this is a diagram of a two-stage audio frequency (AF) transistor amplifier with headphone output. What will happen if the output of such an amplifier is connected to its input, as shown by the dashed line on the diagram? A positive feedback is created between them, and the amplifier will self-oscillate, becoming an audio frequency generator, and we will hear a low-pitch sound in the headphones. In receivers and amplifiers, this phenomenon is strictly fought against, but for automatic devices, it turns out to be very useful.

Fig. 1 A two-stage amplifier encompassed by positive feedback becomes a multivibrator.

Now look at (Fig. 1, b). Here you see the schematic of the same amplifier with positive feedback, just like in (Fig. 1, a), but its layout is slightly modified. This is exactly how schematics of astable (self-oscillating) multivibrators are usually drawn. Experimentation is arguably the best method for understanding the essence of how any electronic device works. You've seen this many times. So now, to better understand the operation of this universal automatic device, I suggest conducting an experiment with it. You can see the basic schematic of an astable multivibrator with all the values for its resistors and capacitors in (Fig. 2, a). Assemble it on a breadboard. The transistors must be low-frequency types (e.g., MP39 - MP42), since high-frequency transistors have a very low emitter junction breakdown voltage. The electrolytic capacitors C1 and C2 should be rated for a nominal voltage of 10 - 12 V (like K50-6, K50-3, or modern equivalents). The resistance of the resistors can differ from those indicated on the diagram by up to 50%. It is only important that the values of the load resistors R1, R4 and the base resistors R2, R3 are as identical as possible. Use a 9V battery or a power supply. Connect a milliammeter (PA) set to 10 - 15 mA in the collector circuit of either transistor, and connect a high-resistance DC voltmeter (PU) up to 10 V across the emitter-collector junction of the same transistor. After checking the wiring, paying special attention to the polarity of the electrolytic capacitors, connect the power source to the multivibrator. What do the measuring instruments show? The milliammeter shows the transistor's collector current sharply increasing to 8 - 10 mA and then just as sharply decreasing to almost zero. The voltmeter, conversely, shows the collector voltage dropping to almost zero, then rising to the power supply voltage. What do these measurements indicate? They show that the transistor in this arm of the multivibrator is operating in a switching mode. The highest collector current and simultaneously the lowest collector voltage correspond to the open (ON) state, while the lowest current and the highest collector voltage correspond to the closed (OFF) state of the transistor. The transistor in the second arm of the multivibrator works in exactly the same way, but, as they say, with a 180-degree phase shift: when one transistor is open, the other is closed. You can easily verify this by connecting a similar milliammeter to the collector circuit of the second transistor; the pointers of the measuring instruments will alternately deflect from the zero marks on their scales. Now, using a watch with a second hand, count how many times per minute the transistors switch from the open to the closed state. It will be about 15 - 20 times. This is the number of electrical oscillations generated by the multivibrator per minute. Consequently, the period of one oscillation is 3 - 4 seconds. Continuing to watch the milliammeter pointer, try to graph these oscillations. Plot the time intervals of the transistor being in the open and closed states on the horizontal axis, and the corresponding collector current on the vertical axis. You will get a graph similar to the one shown in Fig. 2, b.

Fig. 2 Schematic of a symmetrical multivibrator (a) and the current pulses it generates (b, c, d).

Therefore, it can be considered that a multivibrator generates electrical oscillations of a rectangular shape. In the multivibrator signal, regardless of which output it is taken from, you can distinguish current pulses and the pauses between them. The time interval from the moment one current (or voltage) pulse appears to the moment the next pulse of the same polarity appears is called the pulse repetition period T, and the time between pulses is the pause duration Tp. Multivibrators that generate pulses whose duration equals the pauses between them are called symmetrical. Consequently, the experimental multivibrator you built is symmetrical. Replace capacitors C1 and C2 with other capacitors of 10 - 15 µF. The multivibrator remains symmetrical, but the frequency of the oscillations it generates has increased 3 - 4 times — up to 60 - 80 per minute or, which is the same, approximately 1 Hz. The instrument pointers can barely keep up with the changes in currents and voltages. And what if capacitors C1 and C2 are replaced by film/ceramic capacitors of 0.01 - 0.05 µF? How will the instrument pointers behave now? Having deflected from zero, they stand still. Has the oscillation stopped? No! It's just that the oscillation frequency has increased to several hundred hertz. These are audio frequency oscillations, which DC instruments can no longer register. You can detect them using a frequency meter or headphones connected via a 0.01 - 0.05 µF capacitor to either output of the multivibrator, or by connecting them directly into the collector circuit instead of the load resistor. You will hear a low-pitch sound in the headphones. How does the multivibrator work? Let's return to the schematic in Fig. 2, a. When the power is turned on, the transistors of both arms open because a negative bias voltage is applied to their bases through resistors R2 and R3. Simultaneously, the coupling capacitors begin to charge: C1 — through the emitter junction of transistor V2 and resistor R1; C2 — through the emitter junction of transistor V1 and resistor R4. These capacitor charging circuits act as voltage dividers, creating increasingly negative voltages on the transistor bases (relative to the emitters), striving to open the transistors even more. Opening a transistor causes a decrease in the negative voltage on its collector, which decreases the negative voltage on the base of the other transistor, closing it. This process occurs instantly in both transistors, but only one of them closes — the one with a slightly higher positive base voltage, for example, due to differences in current transfer ratios ($h_{FE}$), or component tolerances. The second transistor remains open. However, these states are unstable because electrical processes in their circuits continue. Let's assume that shortly after turning on the power, transistor V2 is closed and transistor V1 is open. From this moment, capacitor C1 begins to discharge through the open transistor V1 (whose emitter-collector resistance is low at this time) and resistor R2. As capacitor C1 discharges, the positive voltage on the base of the closed transistor V2 decreases. As soon as the capacitor fully discharges and the voltage on the base of transistor V2 approaches zero, current appears in the collector circuit of this now-opening transistor, which acts through capacitor C2 on the base of transistor V1, lowering the negative voltage on it. As a result, the current through transistor V1 starts to decrease, while the current through V2 increases. This causes transistor V1 to close and V2 to open. Now, capacitor C2 will start to discharge, but through the open transistor V2 and resistor R3, ultimately leading to the opening of the first transistor and closing of the second, and so on. The transistors continuously interact, causing the multivibrator to generate electrical oscillations. The oscillation frequency depends on both the capacitance of the coupling capacitors (which you have already tested) and the resistance of the base resistors. Try replacing base resistors R2 and R3 with higher value resistors. The oscillation frequency will decrease. Conversely, if their resistance is lower, the frequency will increase. The approximate oscillation frequency of a symmetrical multivibrator can be calculated using this simplified formula: F = 700 / (R * C), where F is the frequency in Hertz, R is the base resistor value in kilo-ohms, and C is the coupling capacitor value in microfarads. Let's return to the original component values (Fig. 2, a). Replace capacitor C2 with a 2 - 3 µF capacitor, connect a milliammeter to the collector circuit of V2, and graph the current oscillations. Now, the current will appear in shorter pulses (Fig. 2, c). The pulse duration Th will be approximately as many times shorter than the pause as the capacitance of C2 was reduced. Now move the milliammeter to the collector circuit of V1. You will see current pulses whose duration is much longer than the pauses (Fig. 2, d). What happened? By reducing the capacitance of C2, you broke the symmetry of the multivibrator — it became asymmetrical. Therefore, the generated oscillations also became asymmetrical. From Output 1 of such a multivibrator, you can take short voltage pulses, and from Output 2 — long ones. Swap capacitors C1 and C2, and the outputs will reverse.

Monostable Multivibrator (One-Shot)

This type of multivibrator generates current (or voltage) pulses only when triggering signals from another source are applied to its input. To turn an astable multivibrator into a monostable (waiting) multivibrator, you need to do the following: remove capacitor C2, and instead connect a 10 - 15 kOhm resistor (R3 in Fig. 3) between the collector of V2 and the base of V1. Between the base of V1 and the ground, connect a 1.5V battery (G1) in series with a 4.7 - 5.1 kOhm resistor (R5), ensuring the positive pole connects to the base. Add an input capacitor C2 (1000 - 5000 pF) to the base circuit of V1. The initial state of transistor V1 in this multivibrator is closed (OFF), and V2 is open (ON). Check if this is true: the voltage on the closed transistor's collector should be close to the supply voltage, and on the open one — not exceed 0.2 - 0.3 V. If you briefly apply a negative pulse to the input (Uin), the milliammeter in V1's collector circuit will instantly jump to 8 - 10 mA, freeze for a moment, and return to zero, waiting for the next signal. This is a single current pulse generated by the multivibrator. What is the principle of operation? In a monostable multivibrator, the coupling between the collector of V2 and the base of V1 is not capacitive, but resistive — through R3. Transistor V1 is reliably kept closed by the positive voltage from G1. This state is very stable. However, when a negative voltage pulse arrives at the base of V1, the transistors enter an unstable state. V1 opens, and the voltage drop on its collector closes V2 through capacitor C1. They stay in this state until C1 discharges. Once discharged, V2 opens again, closing V1, and the circuit returns to its stable waiting mode. Thus, a monostable multivibrator has one stable and one unstable state, generating a single rectangular pulse whose duration depends on the value of C1.

Fig. 3 Experimental monostable multivibrator.

Multivibrator in Generators and Electronic Switches

Electronic Bell. A multivibrator can be used for an apartment doorbell, replacing the standard electromechanical one. It can be assembled according to the schematic shown in (Fig. 4). Transistors V1 and V2 work in a symmetrical astable multivibrator generating at about 1000 Hz, while V3 acts as a power amplifier. The amplified signal is converted into sound by the dynamic speaker B1. It consumes very little power and a 9V battery will last for months.

Fig. 4 Electronic bell based on a multivibrator.

Electronic Switch. This device (Fig. 5) can be used to alternate two strings of Christmas tree lights powered by AC mains. The switch circuit itself can be powered by a 9-12V DC source.

Fig. 5 Electronic switch based on a multivibrator.

The switch schematic is very similar to the electronic bell. However, the capacitance of C1 and C2 is much larger, making it generate oscillations at about 0.4 Hz. The load of the power amplifier (V3) is an electromagnetic relay K1. When V2 opens, it drives V3 into saturation, activating the relay. When V2 closes, the relay releases. You can change the switching speed by swapping C1 and C2 for different values.

Metronome. A metronome is a specific clock that allows you to count equal time intervals by sound signals with fraction-of-a-second accuracy. It is used in music training or for learning Morse code. The schematic is shown in (Fig. 6).

Fig. 6 Metronome based on a multivibrator.

This is an asymmetrical multivibrator using complementary transistors (V1 is NPN, V2 is PNP), which reduces the component count. The pulse repetition rate can be adjusted by potentiometer R1 from roughly 20 to 300 pulses per minute.

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