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Monostable Multivibrator

### Introduction

In the previous experiment, you activated the R and S inputs to the latch circuit by turning a switch on, then off again. While this works, sometimes it would be nice to accomplish the same purpose with a single input action. That is, we'd like to be able to use a single input action to cause a digital pulse to be generated.

The circuit you'll demonstrate in this experiment is a variation in the bistable multivibrator circuit you have already seen in action. The difference is that we're going to modify the circuit so that once switched to the Set state, it will delay, then reset itself with no further intervention. This will give us the behavior we need.

For the circuit to behave this way, it needs to have one stable state (Reset), while the other state is not permanently stable. In practice, the Set state is quasi-stable in that it can be retained for a set period before the circuit reverts back to its stable state. In this experiment, we'll find out just how this can be accomplished.

### Schematic Diagram

As you can see in the schematic diagram to the right, the monostable multivibrator is very similar in design to the bistable multivibrator you have already demonstrated. The primary difference is the use of a capacitor (C in the schematic) as one of the cross-coupling elements. The resistor is still present (R in the schematic), but now connects the base of Q2 to +5 volts instead of to the collector of Q1.

Of course, the capacitor will take a certain amount of time to charge, but once it does so it will carry no current, and Q2 will be turned on by the current through its 15K base resistor. This in turn holds the Q output at logic 0. This output is also applied as before, holding Q1 off. Assuming the T (Trigger) input is also quiescent at logic 0, Q3 is also off and the circuit will remain indefinitely in this state.

At this point, C is charged to just about +5 volts (less VBE of Q2), with the Q1 collector connection being positive. The circuit will remain in this state until a logic 1 signal is applied to the T input.

When an input signal is received at T, Q3 turns on and pulls the left end of capacitor C down to ground. Since the capacitor voltage cannot change instantaneously, this forces the right end of C to -5 volts, immediately turning Q2 off. This in turn applies a logic 1 to Q1's input, turning Q1 on. At this point, the input to T can be discontinued; the Q output is logic 1 and Q1 will remain on.

Under these circumstances, the left end of C remains locked to ground through Q1's collector. But the right end gradually charges through R, Q2's base resistor, towards +5 volts. However, it never gets there; as soon as this voltage allows Q2's base to become forward biased, Q2 turns on and turns Q1 off again. This returns the circuit to its quiescent state.

Thus, this circuit cannot maintain a logic 1 output indefinitely; this is not really a stable state for this circuit. The circuit has only one stable state (Q = 0). It is therefore known as a monostable multivibrator.

The duration of the quasi-stable state (Q = 1) is determined by the two components R and C. Because the capacitor only charges to half the total range (from -5 volts to 0, while charging towards +5 volts), the duration of the output pulse is 0.693RC, where 0.693 is the natural logarithm of 2, R is in ohms, and C is in farads. For the component values shown here, the timing interval is 0.693 × 15,000 × 0.0001 = 1.04 seconds. So this circuit will produce a 1-second pulse each time it is triggered.

If the T input has already returned to logic 0, C will rapidly recharge through the 1K collector resistor and be ready for another input trigger signal. If T remains at logic 1, C will remain discharged until T drops again to logic 0. Then C will fully recharge in about 0.5 second and be ready for another trigger signal.

### Parts List

To construct and test the monostable multivibrator circuit on your breadboard, you will need the following experimental parts:

• (2) 1K, ¼-watt resistors (brown-black-red).
• (3) 15K, ¼-watt resistors (brown-green-orange).
• (3) NPN silicon switching transistors (2N3904, 2N4124, or similar).
• (1) 100 µf electrolytic capacitor.
• Black hookup wire.
• White hookup wire.
• Orange hookup wire.

### 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 assembly diagram and text below and install the parts as shown.

### Circuit Assembly

#### Starting the Assembly

You should have removed the experimental components from the previous experiments, once you completed it. Many of the components for this experiment will go in the same places, but there are enough differences that trying to modify the prior circuit might be too confusing.

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

You should have several 0.3" black jumpers left over from previous experiments. Locate one of these, or else prepare a new one in the fashion indicated by the pictorial, which you have used before. Install this jumper 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 shown to the right.

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

#### 0.3" Black Jumper

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

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

#### 0.3" White Jumper

Locate or prepare a 0.3" white jumper and install it as shown to the right.

Once more, 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) with its leads formed to a spacing of 0.5". You should have two of these left over from the previous experiment, but if necessary you can prepare a new resistor for this circuit. Install this resistor in 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 a spacing of 0.5". Install this resistor in the location shown to the right.

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

#### 1K, ¼-Watt Resistor

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

As before, 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 a spacing of 0.5". Install this resistor in the location shown to the right.

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

#### 15K, ¼-Watt Resistor

Locate one more 15K, ¼-watt resistor (brown-green-orange) with its leads formed to a spacing of 0.5". Install this resistor as 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.

#### NPN Switching Transistor

Locate one more 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.

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

#### 100µf Electrolytic Capacitor

Locate a 100µf, 10 volt or higher electrolytic capacitor with radial leads (both leads extending from the same end of the capacitor as shown in the pictorial here). The lead spacing is almost certain to be 0.1", so clip the leads to a length of ¼ to allow this component to fit snugly on your breadboard socket.

Note that one lead is clearly marked as the negative lead. Make sure that this marked lead is oriented to the left as you install this capacitor in the location indicated to the right.

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

#### 1½" Green Jumper

Cut a 1½" length of green hookup wire and remove ¼" of insulation from each end. Then install this jumper as shown in the assembly diagram to the right.

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

#### 10" White Jumper

Locate one of the 10" white jumpers you have used in past experiments, and connect one end to the location shown to the right. Connect the other end to the input to L0.

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

#### 6" Orange Jumper

Locate one of the 6" orange jumpers you have used in previous experiments. Connect one end of this jumper to the location shown to the right. Connect the other end to S0.

Once more, 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.

### Performing the Experiment

Set S0 to produce a logic 0 output, and then turn on power to your experimental circuit. What response do you observe from L0?

While observing L0, move S0 to the logic 1 position, then immediately back to logic 0. Pause for a few seconds, then repeat this action. What is L0's response to this input signal?

Now move S0 to logic 1 and leave it there. What effect does this have on the behavior of L0? Move S0 to logic 0, count 5 seconds, and repeat this sequence. Does L0 behave any differently with a long-term logic 1 input than it did with a much more brief logic 1 input?

Finally, Move S0 to logic 1 and wait for L0's response to be completed. Then move S0 to logic 0 and immediately back to logic 1 again. What effect does this have on the behavior of L0? Is this behavior consistent and repeatable?

### Discussion

When you first turned power on, capacitor C was fully discharged. Since its voltage cannot change instantaneously, it had the effect of holding Q2 off initially. Therefore L0 turned on briefly when you first applied power to this circuit. However, the capacitor rapidly charged enough to allow Q2 to turn on, whereupon L0 turned off. Capacitor C then continued to charge through Q1's 1K collector resistor and Q2's emitter-base junction until C was charged to nearly 5 volts. Once this had occurred, the circuit was ready to be triggered.

When you applied a logic 1 to the T input using S0, the collector voltage of Q1 and Q3 dropped immediately to nearly ground potential. However, capacitor C still has a 5 volt charge on it. Since its left end has been essentially grounded, its right end is now at a voltage of nearly -5 volts. This immediately turns Q2 off, which in turn turns Q1 on. L0 turned on, and remained on when you returned S0 to its logic 0 position.

The time that the circuit remained in this state was set by the values of capacitor C and resistor R. C charges through R towards +5 volts, a process that normally should require 5 times the R × C time constant. However, once the capacitor is charged sufficiently to allow Q2's base to become forward biased, Q2 will turn on. This will turn Q1 off again, and will also turn L0 off. This only required about 69% of one time constant (ln 2 = 0.693...). Thus, you should have found that L0 turned on for about 1 second each time you applied a logic 1 via S0. The exact timing interval depends on the component tolerances of R and C.

When you turned S0 on and left it on, you noticed no difference in circuit behavior. Q2 still turned on at the end of the same timing interval, turning L0 off. The difference was that Q3 remained on, thus preventing capacitor C from charging. However, when you set S0 to logic 0, C rapidly recharged so that setting S0 to logic 1 after a delay caused the same behavior as before.

When you set S0 to logic 0 and then immediately back to logic 1, you were trying to retrigger the circuit before C had time to recharge. However, unless you were very fast, you probably didn't notice any variation. C recharges through the 1K collector resistor for Q1 and Q3, so it recharges 15 times as fast as it charges while L0 is on. This is long enough to prevent contact bounce in the switch from being a problem, but not long enough to exercise manual control of the circuit timing.

Basically, each time you triggered this circuit, L0 remained on for about 1 second, then turned off. You could not extend this interval, nor could you shorten it. Thus, this circuit generates a single, fixed-duration output pulse each time it is triggered, and then waits for the next trigger pulse.

Since the duration of the pulse produced by this circuit (the "pulse width") is controlled by R and C, we can set the pulse width by modifying these components. Of course, R must have a value suitable for the correct biasing of Q2. Therefore, the value of C is selected to get the pulse width as close as possible to the desired value, and then R is adjusted to "fine tune" the pulse width. If exact timing is critical, a potentiometer is used for R.

When you have completed this experiment, make sure power to your experimental circuit is turned off. Remove all experimental components from your breadboard socket and set them aside for use in later experiments.

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