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Astable Multivibrator

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:



Constructing the Circuit

Select an area on your breadboard socket that is clear of other circuits. You'll need two adjacent sets of five bus contacts for this project. Then refer to the image and text below and install the parts as shown.



Circuit Assembly

Start assembly procedure
















Starting the Assembly

While it is possible to keep the monostable circuit in place and just modify it for astable operation, such a procedure would be more confusing in a pictorial assembly procedure such as this. Therefore, we suggest that you start with a cleared space on your breadboard socket, even though most of the parts you'll be using are the same ones you've used before.

Click on the `Start' button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart' button that will replace the `Start' button.

0.3" Black Jumper

Locate or prepare a 0.3" black jumper and install it in the location indicated in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Locate or prepare a second 0.3" black jumper and install it in the location indicated to the right.

Again, click on the image of the jumper you just installed to continue.

1K, ¼-Watt Resistor

Locate a 1K, ¼-watt resistor (brown-black-red). You should have two with their leads already formed to a spacing of 0.5". Install one on your breadboard socket, at the location indicated in the assembly diagram.

Click on the image of the resistor you just installed to continue.

15K, ¼-Watt Resistor

Locate a 15K, ¼-watt resistor (brown-green-orange) with its leads formed to 0.5" spacing. Install this resistor in the location indicated to the right.

Again, click on the image of the resistor you just installed to continue.

15K, ¼-Watt Resistor

Locate a second 15K, ¼-watt resistor (brown-green-orange) with its leads formed to 0.5" spacing. Install this resistor in the location indicated to the right.

As before, click on the image of the resistor you just installed to continue.

1K, ¼-Watt Resistor

Locate a second 1K, ¼-watt resistor (brown-black-red) with its leads already formed to a spacing of 0.5". Install this resistor in the location indicated in the assembly diagram.

Once more, click on the image of the resistor you just installed to continue.

NPN Switching Transistor

Locate a 2N3904 or 2N4124 NPN switching transistor and form its leads to fit 0.1" spacing. Install this transistor in the location shown to the right. Be careful to observe the orientation of the transistor; the emitter must be connected to the black grounding jumper.

Click on the image of the transistor you just installed to continue.

NPN Switching Transistor

Locate another 2N3904 or 2N4124 NPN switching transistor and form its leads to fit 0.1" spacing. Install this transistor in the location shown in the assembly diagram. Be careful to observe the orientation of the transistor.

Again, click on the image of the transistor you just installed to continue.

100µf Electrolytic Capacitor

You should have a 100µf capacitor left over from the previous experiment, with its leads clipped to ¼". Install this capacitor in the location indicated to the right. Be careful to orient the negative lead to the left as shown in the assembly diagram.

Click on the image of the capacitor you just installed to continue.

100µf Electrolytic Capacitor

Locate a second 100µf capacitor and clip its leads to a length of ¼" so it will fit snugly on your breadboard socket. Install this capacitor as shown to the right. Be careful to orient the negative lead to the rightt as shown in the assembly diagram.

Again, click on the image of the capacitor you just installed to continue.

1½" Green Jumper

Locate the 1½" green jumper you used in the last experiment, or else make a new one by taking a 1½" length of green hookup wire and removing ¼" of insulation from each end. Install this jumper as shown in the assembly diagram.

Click on the image of the jumper you just installed to continue.

1½" Green Jumper

Make another 1½" green jumper in the same manner, and install it in the location shown to the right.

Again, click on the image of the jumper you just installed to continue.

10" White Jumper

Locate a 10" white jumper, which you have used in previous experiments. Insert one end in the location shown in the assembly diagram. Connect the other end to the L0 input.

As before, click on the image of the jumper you just installed to continue.

10" White Jumper

Locate a second 10" white jumper and insert one end in the location shown to the right. Connect the other end to the L1 input.

One more time, click on the image of the jumper you just installed to continue.

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

Restart assembly procedure
Continue assembly procedure


Performing the Experiment

Turn on power to your experimental circuit, and observe LEDs L0 and L1. There are no inputs to this circuit, so there is no experimental action you can take. However, you can verify that this circuit continues to change states at a steady pace, and you can verify the time interval on each half of the cycle.

When you have made this determination, turn off the power to your experimental circuit and compare your results with the discussion below.



Discussion

You should have found that this circuit does indeed oscillate between its two states, neither of which is stable. That's why this circuit is known as an astable multivibrator. With these component values, each LED remained on for just about 1 second before the circuit switched states. This continued for as long as you left power on.

With smaller capacitors to increase the frequency of oscillation, this type of circuit is sometimes used as a clock generator for sequential digital circuits that don't need to operate at some precise frequency.

When you have completed this experiment, make sure power to your experimental circuit is turned off. Remove the experimental components from your breadboard socket and put them aside for the next experiment.


Prev: Monostable Multivibrator Next: Schmitt Trigger

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