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 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. Unlike the monostable mode and astable modes, bistable mode doesn’t need 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
I’ll demonstrate the bistable mode of the 555 timer with an LED and push buttons connected to the trigger pin and reset pin. Pressing the trigger button once will make the LED turn on and stay on. Pressing the reset button will make the LED turn off and stay off.
Pressing the trigger button allows current to flow from Vcc to ground, which causes the voltage at the trigger pin to drop. As we saw in the monostable mode article, when the trigger pin is at a low voltage, the output gets switched on and the LED lights up. The output stays on until the voltage at the threshold pin goes above 2/3 Vcc. Since the threshold pin is wired to ground in this circuit, it never reaches 2/3 Vcc, so the output stays on indefinitely.
Now if the reset button is pressed, the voltage at the reset pin flows to ground and the pin goes low. When the reset pin goes low, the output is switched off.
You can see a demonstration of the 555 timer in bistable mode here:
Example Bistable Mode Circuit
To observe the 555 timer in bistable mode, wire up a circuit like this:
- R1: 10K Ohm
- R2: 10K Ohm
- R3: 470 Ohm
- C1: 0.01 μF
- S1: Reset Button
- S2: Trigger Button
Now, press the trigger button (S2) once, and the LED should turn on and stay on. Pressing the reset button (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
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I have to operate ic555 in bistable mode via capacitor for triggering and switch for reset..plz help me…. Give anyone ur email id i will send u a circuit and tell u how i have to operate. Then i want ur suggestions… Plz… My email id : valadharmesh68@gmail.com
..thank you..
Ahh, the 555 timer. My first acid etched, electronics board, college project. My unorthodox design was one of few that worked
I used 555 as bistable, but when i gave low to pin 2, output came high. And when i gave high to pin 2, output came low. No signal was provided to pin 6. Whats the problem?
Hi, that is normal. When the trigger (pin 2) pin goes low, the output is switched on. The output will stay on until the threshold pin reaches 2/3 Vcc, or the reset pin goes low.
I am facing an issue, If the power (+Vcc) is turned OFF when the Output is HIGH and Turned ON the Power, (Power Recycle) then the Output goes to HIGH automatically. My requirements calls for the Output to go LOW in such condition. Please advise how to achieve this?
The cap is not needed to be utilized in this arrangement.
The cap is there to set duration in stable mode and freq in unstable mode.
Removing it to create a latching circuit does not change the operation in any way.
I need the LED to be ON fir more time..how will I achieve that??
I wonder if a circuit in bistable mode with 2 buttons and a BLINKING LED is possible?
I built this for a engineering project at school and idea what I did but the 555 blew up in my face.
Ha ha ha oh no… Did you have it connected properly? What voltage were you using?