Electronics Components: The 555 in Bistable (Flip-Flop) Mode

A flip-flop is an electronic circuit that alternates between two output states. In a flip-flop, a short pulse on the trigger causes the output to go high and stay high, even after the trigger pulse ends. The output stays high until a reset pulse is received, at which time the output goes low.

This type of circuit is called bistable because the circuit has two stable states: high and low. The circuit stays low until it's triggered. Then, it stays high until it's reset. This type of circuit is used extensively in computers and other digital circuits.

For computer applications, the 555 is a poor choice for use as a flip-flop. That’s because its output doesn’t change fast enough in response to trigger or reset pulses in computer circuits that are driven by high-speed clock pulses. For computer applications, better flip-flop chips are readily available.

That being said, the 555 is often used in bistable mode for noncomputer applications where high-speed response isn’t necessary. For example, imagine a simple robot that drives itself forward until it bumps into something in front of it, and then drives backward until it bumps into something behind it.

The robot would be equipped with contact switches on the front and rear connected to the trigger and reset inputs of a 555 in bistable mode. The robot’s drive motor would be connected to the output such that when the output is low, the motor runs forward, and when the output is high, the motor runs backward. Then, the bistable 555 would cause the robot to drive back and forth between two obstacles.

This is a schematic for a 555 used in bistable mode. As you can see, this circuit doesn't require a capacitor. That’s because in bistable mode, the 555 isn't used as a timer. The highs and lows of the output signal are controlled by the trigger and reset inputs, not by the charging and discharging of a capacitor.

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Both the trigger (pin 2) and the reset (pin 4) inputs are connected to Vcc through a 10 kΩ resistor. When the set switch is depressed, pin 2 is shorted to ground. This causes the voltage to bypass pin 2, resulting in a momentary low pulse, which triggers the 555. Once triggered, the output pin goes high.

In astable or monostable mode, the output pin would remain high until the voltage at the threshold pin (pin 6) reaches two-thirds of the supply voltage. However, because pin 6 isn't connected to anything in this circuit, no voltage is ever present on pin 6. Thus, the threshold is never reached, so the output remains high indefinitely until the 555 is reset by a low pulse on the reset pin (pin 4).

The reset input (pin 4) is connected to Vcc in the same manner as the trigger input. As a result, when the reset switch is pressed, pin 4 is short circuited to ground, creating a low pulse, which resets the 555 and brings the output back to high.

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