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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Varieties of one circuit (asymmetric multivibrator). Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur In this article, we present several devices based on one circuit - an asymmetric multivibrator based on transistors of different conductivity. Using this circuit as a non-contact device, you can assemble a device with a flashing light bulb (see Fig. 1) and use it for various purposes. or ship models like a flashing lantern.
The load of an asymmetric multivibrator assembled on transistors T1, T2 is a light bulb L1. The pulse repetition frequency is determined by the value of the capacitance of the capacitor C1 and resistors R1, R2. Resistor R1 limits the maximum flash frequency, and resistor R2 can smoothly change their frequency. It is necessary to start work from the maximum frequency, which corresponds to the upper position of the resistor R2 engine according to the diagram. Please note that the device is powered by a 3336L battery, which gives 3,5 V under load, and the L1 light bulb is used for a voltage of only 2,5 V. Will it burn out? No! The duration of its glow is very short, and the thread does not have time to overheat. If the transistors have a high gain, then instead of a 2.5 V x 0.068 A light bulb, you can use a 3.5 V x 0.16 A light bulb. MP1-MP35 type transistors are suitable as transistor T38, and MP2-MP39 type T42. If you install a loudspeaker in the same circuit instead of a light bulb, you will get another device - an electronic metronome. It is used in teaching music, for timing during physical experiments and for photo printing. If you change the circuit a little - reduce the capacitance of the capacitor C1 and introduce a resistor R3, then the duration of the generator pulse will increase. The sound will intensify (Fig. 2).
This device can act as a house bell, a model horn or a children's pedal car. (In the latter case, the voltage must be increased to 9 V.) And it can also be used to teach Morse code. Only then, instead of the Kn1 button, you need to put a telegraph key. The tone of the sound is selected by the capacitor C1 and the resistor R2. The larger R3, the louder the sound of the generator. However, if its value is more than one kiloohm, then oscillations in the generator may not occur. The generator uses the same transistors as in the previous circuit, and headphones or a head with a coil resistance of 5 to 65 ohms are used as a loudspeaker. An asymmetric multivibrator based on transistors of different conductivity has an interesting property: during operation, both transistors are either open or locked at the same time. The current drawn by the disabled transistors is very small. This allows you to create economical indicators of changes in non-electrical quantities, such as indicators of humidity. A schematic diagram of such an indicator is shown in Figure 3.
As you can see from the diagram, the generator is constantly connected to the power source, but does not work, since both transistors are locked. Reduces current consumption and resistor R4. A humidity sensor is connected to sockets G1, G2 - two thin tinned wires 1,5 cm long. They are sewn to the fabric at a distance of 3-5 mm from each other. The resistance of the dry sensor is high. When wet, it falls off. The transistors open, the generator starts to work To reduce the volume, it is necessary to reduce the supply voltage or the value of the resistor R3. Such a moisture indicator can be used in the care of newborns. If you expand the circuit a little, then the humidity indicator will give a light signal simultaneously with the sound signal - the L1 light will start to light up. In this case, as can be seen from the diagram (Fig. 4), two asymmetric multivibrators are installed in the generator on transistors of different conductivity. One is assembled on transistors T1, T2 and is controlled by a humidity sensor connected to sockets G1, G2. The load of this multivibrator is lamp L1. The voltage from the collector T2 controls the operation of the second multivibrator, assembled on transistors T3, T4. It works as an audio frequency generator, and loudspeaker Gr1 is turned on at its output. If there is no need for an audible signal, then the second multivibrator can be disabled.
The transistors, lamp and loudspeaker in this moisture indicator are the same as in previous devices. Interesting devices can be built using the dependence of the frequency of an asymmetric multivibrator on transistors of different conductivity on the base current of the transistor T1. For example, a generator that imitates the sound of a siren. Such a device can be installed on the ambulance model, fire engine, rescue boat. The schematic diagram of the device is shown in Figure 5.
In the initial position, the Kn1 button is open. The transistors are off. The generator is not working. When the button is closed through resistor R4, capacitor C2 is charged. The transistors open and the multivibrator starts to work. As the capacitor C2 charges, the base current of the transistor T1 increases and the frequency of the multivibrator increases. When the button is opened, everything is repeated in reverse order. The sound of a siren is simulated when the button is periodically closed and opened. The rate of rise and fall of sound is selected by resistor R4 and capacitor C2. The tone of the siren is set by the resistor R3, and the volume of the sound is set by the selection of the resistor R5. Transistors and loudspeaker are selected the same as in previous devices. Given that transistors of different conductivity are used in this multivibrator, you can use it as a device for testing transistors by replacing. A schematic diagram of such a device is shown in Figure 6. The sound generator circuit is taken as a basis, but a light pulse generator can be used with equal success.
Initially, by closing the Kn1 button, check the operability of the device. Depending on the type of conductivity, connect the transistor under test to sockets G1 - G3 or G4-G6. In this case, use the switch P1 or P2. If there is a sound in the loudspeaker when the button is pressed, then the transistor is working. As switches P1 and P2, you can take toggle switches with two contacts for switching. The figure shows the switches in the "Control" position. The device is powered by a 3336L battery. Based on the same multivibrator, you can build a fairly simple generator to test receivers and amplifiers. Its schematic diagram is shown in Figure 7. Its difference from the sound generator is that instead of a loudspeaker, a 7-step voltage level regulator is included at the output of the multivibrator.
Author: E.Tarasov
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Comments on the article: Michael Not bad. Especially for beginners. I have known this for 40 years, but I downloaded it so as not to rummage if necessary. It would be great if you adapted the circuits to a more modern element base. It is not difficult, but for beginners it is a problem. Thank you. a guest Article useful for beginners [up] Alexander the site is excellent. it was created just for us - beginner radio amateurs. the choice of articles is huge. [up] Valery Semyonov Between the collector T1 and the base T2 it is mandatory to include a limiting resistor with a minimum rating equal to the supply voltage divided by the maximum allowable base current T2. Otherwise, the transition B-E T2 will burn out, and then the same fate will befall the K-E T1. Bogdan Worried about the polarity on the capacitor C1 (Fig. 1);) Andrei But what about the operation of this multivibrator on the winding of a transformer? Forgot this option? You can also install a field unit instead of T2 with a slight change in the circuit. I just don’t know whether after this the design will remain a classic asymmetrical multivibrator.
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