RSS Feed (xml)
Introduction
We've looked at a number of gates, constructed using different technologies. Now it's time to combine gates into more interesting circuits, and see what can be done with such combinations.
As a start, we'll take two basic RTL inverters and cross-couple them, so that each has its input connected to the output of the other. At first thought this may seem pointless, since such a circuit would seem to be locked forever into one condition, or state. One transistor would be turned on while the other would be turned off, and each would continue to enforce the state of the other.
In fact, this is exactly what we want. Either transistor can be on while the other is off, and the circuit will retain its state until it is changed by an external signal or power is turned off. Thus, this circuit represents the simplest possible binary memory. In this experiment, you will construct and demonstrate such a circuit.
Schematic Diagram
As shown here, the circuit we're looking at really is nothing more than two basic inverters, each taking its input from the other's output. If, when power is first applied, Q1 turns on, its output will be a logic 0. This will be applied to Q2's input resistor, keeping Q2 turned off so that its output will be a logic 1. This logic 1 will be applied back to Q1's input resistor, keeping Q1 turned on and holding the entire circuit locked into this stable state.
On the other hand, if Q1 stays off at power-up, it will apply a logic 1 to Q2's input, thus turning Q2 on. The resulting logic 0 output from Q2 will in turn hold Q1 off. The circuit will then remain in this stable state indefinitely.
Because this circuit has two possible logical states, it is known technically as a multivibrator. Because it has two possible stable states, it is a bistable multivibrator. It is also the most basic possible binary latch circuit. In the next few experiments we'll look at ways to expand this circuit and modify its behavior. But first, we'll examine the operation of this basic circuit.
Parts List
To construct and test the bistable 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) 2N3904 or 2N4124 NPN silicon transistors.
- Black hookup wire or existing jumpers.
- White hookup wire or existing jumpers.
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
Starting the Assembly
Select an area on your breadboard socket that is clear of all other circuits. As shown in the assembly diagram to the right, we have selected the right-hand end of the breadboard to construct our experimental circuit.
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.Locate a 0.3" black jumper left over from previous experiments, or else prepare a new 0.3" black jumper using the same method you have used before. Insert this jumper into you breadboard socket as shown to the right
Performing the Experiment
Turn on power to your experimental circuit, and note the states of L0 and L1. What does this tell you about the states of the transistors in your experimental circuit?
Turn off power and wait for 30 seconds. Then turn power back on. Now what are the states of L0 and L1? Repeat this action several more times. Do L0 and L1 always indicate the same states at power-up? Or do they come up at random?
Take the free end of the grounding black jumper wire you prepared for this experiment, and touch it briefly to the collector lead (farthest away from the other transistor) of the transistor whose LED indicator is off. Does this cause any change in the circuit?
Now, touch the end of the black jumper to the transistor collector whose LED is currently turned on. What effect does this have on your experimental circuit?
Repeat these two actions several times, until you are confident that you clearly understand what this circuit is doing, and how it reacts to your touches with the black jumper.
When you have made your determinations, turn off the power to your experimental circuit and compare your results with the discussion below.
Discussion
When you first turned on power, either L0 or L1 turned on and stayed on. In nearly every case, turning power off long enough to discharge the reservoir capacitor and then turning it back on again made no difference; the two transistors are not identical and one turned on faster than the other. That transistor kept its LED off, while the other transistor remained turned off and therefore turned its connected LED on to indicate a logic one at that collector.
Grounding the collector of the transistor whose LED was off had no effect on the circuit. That transistor was already turned on, so its collector was already essentially grounded. Grounding it harder won't change anything. However, when you grounded the collector of the transistor whose LED was currently on had the effect of turning it off and turning the other LED on instead. Removing the ground made no difference; the circuit had already changed state and was completely stable in its new state.
With the experimental circuit now in the opposite state, the same basic rules still apply to its behavior. Grounding the collector of the transistor whose LED was currently off had no effect, while grounding the collector of the transistor whose LED was on caused the circuit two switch back to its previous state. Both states are completely stable, meaning that only the application of an external signal (your grounding jumper) can cause this circuit to change states. It will not spontaneously changes states so long as the circuit is powered.
When you have completed this experiment, make sure power to your experimental circuit is turned off. Do not remove any of your experimental components; you will use this circuit in your next experiment.
