555 Timer Remembering Astable and Monostable Circuits

In GCSE Electronics exams, you may need to complete a circuit diagram of either an astable or a monostable circuit; therefore, you need to be familiar with their circuit configurations. One of the ways to understanding them is to compare the two circuit diagrams to see where the big differences are.

Firstly, notice how they sneakily swap over the discharge, trigger, and threshold pins in the component symbol for the astable circuit, and monostable circuit. Notice that in monostable mode, the threshold and discharge pins join, however in astable mode; the trigger and threshold pins join.

Trigger Pin

A monostable circuit usually has a trigger pin, which is active low; hence, a momentary connection to ground initiates the trigger function. Typically, a trigger condition occurs when the trigger input falls to Vcc/3.

A pull-up resistor P usually establishes the trigger pin at logic 1 state, and therefore, there is always a resistor from this pin to the positive rail to hold it in a high state. To initiate the trigger event, a non-locking switch is required between the trigger pin and ground. Therefore, if you were to press the switch, then the trigger pin becomes logic 0 initiating a trigger event.

Reset and Control Voltage Pins

The reset pin is active low; therefore, a momentary earth connection will initiate the reset function. When not used; it usually remains fixed to the positive voltage rail. The control voltage pin usually has a capacitor between it and the ground rail. This capacitor is usually around 10 nF and is the same in both modes.

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