This is part 2 of a series of articles on the 555 timer. If you haven’t already, you may want to read part 1, 555 Timer Basics – Monostable Mode first, to see the pin layout and functions of each pin.

Bistable Mode of the 555 Timer

The 555 timer in bistable mode is also known as a flip-flop circuit. A flip-flop circuit alternates between two stable states, in this case the output of electrical current from the output pin. This is a fairly basic 555 timer circuit and unlike monostable mode and astable mode, it does not depend on a resistor and capacitor to set the timing of the circuit. In fact there is no timing in this circuit. There are only two stable states (on and off) controlled directly by the trigger pin and reset pin.

How Bistable Mode Works

555 Timer Bistable Mode

Consider two switches, a trigger switch and a reset switch, that control an LED as in the schematic below. In this circuit, when we press the trigger switch once, the LED turns on and stays on. When the reset switch is pressed, the LED turns off and stays off. When the trigger button is pressed, the voltage supplied to the trigger pin flows to ground, causing the trigger pin to go low. When the trigger pin goes low, the output is switched on. In monostable mode and astable mode, the output would stay on until the voltage at the threshold pin reaches or exceeds 2/3 Vcc. Since the threshold pin is wired to ground in this circuit, it never reaches 2/3 Vcc, and the output stays on indefinitely. If the reset button is now pressed, the voltage supplied to the reset pin flows to ground and the pin goes low. When the reset pin goes low, the output is switched off, until it the trigger pin receives another low pulse.

Watch a demonstration of the 555 timer in bistable mode here:

Example Bistable Mode Circuits

To observe the 555 timer in bistable mode, connect up a circuit like this:

555 Timer Bistable 3

R1: 10K Ohm

R2: 10K Ohm

R3: 470 Ohm

C1: 0.01 μF

S1: Reset Switch

S2: Trigger Switch

Now, press the trigger switch (S2) once, and the LED should turn on and stay on. Pressing the reset switch (S1) will turn the LED off. The Engineer’s Mini Notebook: Timer, Op Amp, and Optoelectronic Circuits & Projects might be a good resource for you if you are looking for interesting 555 timer circuits and OpAmp circuits. This book has about everything you need to know about the 555 timer.

Click here to go on to part 3, 555 Timer Basics – Astable Mode

Be sure to leave a comment below if you have any questions!

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