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Practical Circuit 2: Bistable Multivibrator
The Bistable multivibrator or flip-flop has the ability of operating in two stable states (HIGH or LOW). When a ‘trigger’ is applied on any of its two inputs, the output of the Bistable multivibrator indefinitely remains either HIGH or LOW depending on which of it’s two inputs was triggered; only changing state when a different trigger input is triggered. This behaviour is in contrast with the Monostable Multivibrator which has one stable state or the Astable Multivibrator which oscillates from HIGH to LOW with no stable state. This behaviour of the Bistable multivibator makes it find use in circuits as memory storage devices like Random Access Memories (RAM). The 555 timer IC can easily be connected to function as a Bistable Multivibrator. The circuit diagram below shows a 555 timer operating in Bistable mode.
Bill Of Materials
Bill of materials required to build the Bistable Multivibrator circuit shown below.
| Component | Label | Quantity | Buy |
|---|---|---|---|
| Breadboard | – | 1 |  ₦1600 View Add to Cart |
| 470Ω Resistor | R1 | 1 |  ₦10 View Add to Cart |
| 10KΩ Resistor | R2, R3, R4, R5 | 2 | ₦40 View Add to Cart |
| 0.1uF Capacitor | C1, C2, C3 | 3 |  ₦30 View Add to Cart |
| Red LED | D1 | 1 |  ₦10 View Add to Cart |
| 9V Battery with connector | BAT 1 | 1 | |
| Tactile Pushbutton Switch | S1, S2 | 2 |  ₦60 View Add to Cart |
| 555 timer IC | U1 | 1 | |
| Connecting Wires | – | – |
Circuit Diagram of a Bistable Multivibrator using a 555 timer IC
The 555 Bistable Multivibrator circuit assembled on a breadboard
The circuit can easily be tested on a breadboard. Follow the illustration above to assemble and test out the Bistable Multivibrator circuit on your breadboard.
Circuit operation is as follows: When the circuit is powered on, the output (Pin3) state of the Bistable Multivibrator is unknown; it can be either HIGH or LOW. Let’s say the output is HIGH at power on. When pushbutton switch S1 is pressed, Capacitor C3 is charged through R5. This causes a momentary voltage drop on pin 4 of the 555 thus resetting the circuit and setting pin 3 output LOW. The output continue to maintain this state until S2 pushbutton is pressed. C2 charges through R2, dropping the voltage on pin 2 LOW (lower than 1/3 of the supply voltage). This causes the output of the 555 to go HIGH, lighting up the LED. The output remains HIGH indefinitely except S1 is pressed; at which point, the output changes state to a LOW.
