Bistable or Flip Flop Mode 555 Timer – Switching alternate LEDs

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. You can switch the output from one state to the other at will. Or you can add circuitry that can send signals to change the output in response to other ininfluences.

555 timer integrated circuit IC pin layout
555 timer integrated circuit IC pin layout

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.
Bistable mode 555 timer flip flop circuit schematic diagram by electronzap
Bistable mode 555 timer flip flop circuit schematic diagram by electronzap

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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 might oppose calling this a flip flop circuit.

A high output will be close to the positive supply voltage. It will probably be slightly less than 4V if you are using an NE555. The LED that is between the output and ground will light up based on that voltage difference across it, and the value of 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 and the value of it’s protective resistor.

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