The output of a 555 timer wired in bistable mode stays in one of 2 states until you force it to change to the other state.
In the circuit shown below:
- Low input (pressing switch) to pin 2 (trigger) sets the output high. You can release the switch and the output will stay high.
- Low input (pressing switch) to pin 4 (reset) sets the output low. You can release the switch and the output stays low.
- Bistable means that there are two (bi) stable states. The output will stay either high or low (it’s 2 possible states) until forced to change.
- Pin 4 overpowers pin 2. If you give both of them a low input, then pin 4 will hold the output low.
This circuit is sometimes called a flip flop, but there’s other flip flop circuitry out there that works a bit differently. So some people refuse to call this a flip flop circuit.
A high output will be close to the positive supply voltage. It will probably slightly less than 4V if you are using a NE555. The LED that is between the output and ground will light up based on that voltage difference across it and it’s protective resistor.
A low output will be really close to 0v (ground). The LED between +5V and the low output will light up based on that voltage difference across it.
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