Experiment 2 - The Bistable Multivibrator

In the last experiment you saw how logic gates can be used to store a value. But what does it mean, a flip-flop (latch) is a "bistable multivibrator that can assume two stable states"? Here you find out.

Consider the circuit diagram containing two NOR gates. You realize it has two outputs Q and Q', which can take two states...

Circuit diagram bistable multivibrator.

Q Q' 1 0 0 1 Possible states of Q and Q'.

A bistable multivibrator switches back and forth between the two states (hence the part bi-). Has the multivibrator found a result, it remains stable. Only by using the inputs R and S you switch the multivibrator. This behaviour applies to all latches and flip-flops.

Circuit bistable multivibrator. (Enlarge)

With the circuit shown, it becomes more evident how it works... When you connect the battery, either Q or Q' lights up. This is determined by chance. After that you can switch between the two results by using R and S. Try this out to understand its behaviour. This is already the first latch you built.