555 bistable circuit


Martin McBride, 2020-06-03
Tags 555 timer bistable
Categories electronics analogue electronics analogue electronic circuits

Here we will look at a bistable circuit base on a 555. This circuit isn't that useful on its own, but it is a good way to understand how the 555 works.

The circuit

Here is the circuit. We have included the internal workings of the 555 (based on the main article) as well as the external circuitry:

The extra elements we have added to the circuit are:

  • A Set button on the Trigger pin. The Trigger is is normally pulled up to +Vcc, but the button takes it low.
  • A Reset button on the Threshold pin. The Threshold is is normally pulled down to 0V, but the button takes it high.
  • An Output LED that lights when the output pin goes high.
  • A Discharge LED that lights when the discharge pin is pulled low (see later).

There are a few extra things that you always need to do with a 555 circuit:

  • Pins 1 and 8 are power pins - the MUST be 0V and +Vcc respectively to power the chip. The circuit works with a single 5V to 12V supply.
  • The Control pin (5) can be used to vary the frequency of the oscillator using a control voltage. If you are not using it, it is good practice to connect it to ground via a small capacitor, to eliminate are noise effects. This isn't absolutely necessary but it doesn't do any harm.
  • If the Reset pin (4) is not being used, it should be tied to +Vcc (Reset is active low).

Inputs

The 555 inputs generally should not be left floating. This is particularly important for the Trigger and Threshold pins, which are connected to very sensitive voltage comparators. If you leave these inputs floating, the will almost certainly be affected by any noise in the circuit, and will probably cause the output to fluctuate randomly between high and low.

Operation

The operation of the 555 is as follows:

  • The Set button takes the Trigger pin to 0V when it is pressed. When the Trigger pin is taken below 1/3 of +Vcc, the Trigger comparator output will go high. This will set the flip flop. Setting the flip flop will cause Q to go high and ~Q to go low. Since the Output LED is connected to ~Q by an inverter, this means that the LED will come on when you press the Set button.
  • When you release the Set button, the flip flop remembers its current state and the LED stays on.
  • The Reset button takes the Threshold pin to +Vcc when it is pressed. When the Threshold pin is taken above 2/3 of +Vcc, the Threshold comparator output will go high. This will reset the flip flop. Resetting the flip flop will cause Q to go low and ~Q to go high. Again, since the Output LED is connected to ~Q by an inverter, this means that the LED will go off when you press the Reset button.
  • When you release the Reset button, the flip flop remembers its current state and the LED stays off.

You can alternately turn the Output LED on or off by pressing the Set and Reset buttons. The Output pin of the 555 provides the output of the oscillator.

The Discharge pin behaves differently. When the flip flop is in the off state, the transistor is turned on, which pulls the Discharge pin to 0V. This is intended to discharge the timing capacitor. The Discharge light will always be on when the Output light is off, and vice versa.

Here is a quick breadboard implementation of the circuit:

Copyright (c) Axlesoft Ltd 2020