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Introduction
Now that we have seen the bistable multivibrator and then modified it to form a monostable multivibrator, the next question is, Can we modify it further to use capacitor coupling on both sides? And what would happen if we did?
Well, of course we can do this; the further question would be, Do we want to? And that depends on what happens when we build the circuit this way.
Since the use of one capacitor prevents the circuit from remaining stable in one of its two possible states, it seems likely that with both sides coupled this way the circuit will be unable to remain stable in either state. That is in fact the case, and in this experiment we will construct and demonstrate this circuit.
Schematic Diagram
As shown in the schematic diagram here, the astable multivibrator simply extends the modification that converted the bistable multivibrator to a monostable version of the circuit. Now, both transistors are coupled to each other through capacitors. Whichever transistor is off at any moment cannot remain off indefinitely; its base will become forward biased as that capacitor charges towards +5 volts. Once that happens, that transistor will turn on, thereby turning the other one off.
If we pick a moment when Q1 has just turned off and Q2 is on, then the left end of C2 is at -5 volts. This negative voltage decreases as C2 charges through R2 towards +5 volts. However, the moment C2 charges enough to provide forward bias to the base of Q1, Q1 turns on and the 5 volt drop in Q1's collector voltage is coupled through C1 to the base of Q2. This turns Q2 off at once. As we saw in the previous experiment, the time that Q1 remains on and Q2 remains off is 0.693RC, which for the component values shown here is about 1 second.
Now Q2 is held off while C1 charges through R1, until Q2's base becomes forward biased. At that point the transistors switch states again and the whole thing starts over. There is no stable state where the circuit can come to rest, so this circuit is known as an astable multivibrator.
The time Q2 remains off is set by R1 and C1, just as the time Q1 remains off is set by R2 and C2. For our circuit, the components are of the same values on each side, so the timing will be the same on each half of the cycle. This is not required; the two halves of the circuit can have totally different time intervals. They actually operate independently of each other, even though they work together.
Since this particular circuit will spend about 1 second on each half cycle, the total cycle time, or period, is about 2 seconds. The operating frequency of the circuit is the reciprocal of the period, or 0.5 Hz.
Parts List
To construct and test the astable multivibrator circuit on your breadboard, you will need the following experimental parts:
- (2) 1K, ¼-watt resistors (brown-black-red).
- (2) 15K, ¼-watt resistors (brown-green-orange).
- (2) 100µf, 15 volt electrolytic capacitors (higher voltage rating is OK).
- (2) NPN silicon switching transistors (2N3904, 2N4124, or similar).
- Black hookup wire.
- White hookup wire.
- Green hookup wire.
