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555 Bistable Multivibrator 2012-08-29

The most common 555 configurations are that of a Astable and Monostable Multivibrator. This shows how a 555 may be used as a Bistable Multivibrator. While a 555 has a Set/Reset flip flop built in (another type of Bistable Multivibrator) this circuit uses the concept of hysteresis to accomplish the same thing.

If you use the red wire shown on the illustration as a toggle the LEDs will flip states, and stay that way until the circuit is toggled again. It will work over the entire power supply voltage range of the 555, which is 4.5V to 15V. A CMOS 555 will also work well for this circuit, although it may have trouble driving the LEDs directly.





Because R3 and R4 creates a voltage that is exactly in the middle of the dead zone of the Schmitt Trigger hysteresis the output of the 555 is stable. It will hold the last state it was set in indefinitely while there is power. The capacitor C1 is at the same voltage as the output of the timer. When the toggle button is pushed the capacitor will put the same voltage on the input, causing the 555 (an inverter), to flip states. The capacitor will quickly charge or discharge to the voltage level that the network of R3, R4, R5 now presents. Since R5 is X10 larger than R3 and R4 this voltage will still be in the dead zone of the Schmitt Trigger and the output of the 555 is stable in its new state. When the toggle button is released the capacitor will again charge or discharge to its new voltage. The capacitor is being used as a memory to compliment the 555. It also makes a fairly convenient debounce for the button.

This circuit concept will work for all inverting Schmitt Triggers, though R5 may have to be increased to keep the transitions in the dead zone of the hysteresis.



Parts List
  • 9V Battery and Clip
  • Battery Clip (Radio Shack catalog # 270-325)
  • Mini Hook Clips (soldered to Battery Clip, Radio Shack catalog # 270-372)
  • One 555 timer IC (Radio Shack catalog # 276-1723)
  • D1 - Red light-emitting diode (Radio Shack catalog # 276-041 or equivalent)
  • D2 - Green light-emitting diode (Radio Shack catalog # 276-022 or equivalent)
  • R1,R2 - 1 KΩ Resistors 5% ¼W
  • R3,R4 – 10 KΩ Resistors 5% ¼W
  • R5 – 100 KΩ Resistors 5% ¼W
  • C1 - 1 µF Tantalum Capacitor (Radio Shack catalog 272-1025 or equivalent)
  • 2 inch (5 cm) piece of wire with the ends stripped and folded in half, red wire shown in Illustration.

Note: This article will also go into the textbook Lessons in Electric Circuits.
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