The NAND Gate Bistable Multivibrator
The NAND gate bistable multivibrator is a very useful basic circuit, which can be set into one of two stable states by pressing a switch. The circuit stays in that state until the other switch is pressed. These are useful in burglar alarms for example. The circuit is shown below. The output from each NAND gate is an input for the other.
Initially suppose that P is 0 and Q is 1. Then input A has to be 1. If switch S is pressed (so that input A to the top NAND gate is 0) P and C become 1 and Q becomes 0
Releasing S and then pressing it again will not alter P or Q since the top NAND gate requires only one input of 0 to produce an output of 1. Changing P and Q can only be done by pressing R.
The truth table for the bistable circuit is shown here.
S | R | B | C | P | Q |
1 | 1 | 1 | 0 | 0 | 1 |
0 | 1 | 0 | 1 | 1 | 0 |
1 | 1 | 0 | 1 | 1 | 0 |
1 | 0 | 1 | 0 | 0 | 1 |