trigger, 555 as a

The IC 555 is normally used as a timer. The time delay for these applications is obtained by charging and discharging a capacitor between two well-defined voltage limits. For this purpose the IC has two voltage-sensitive inputs, which can be used in a trigger circuit. The input voltage is applied to pin 2; pin 5 receives a reference voltage from potentiometer Pl. With this poten- tiometer the trigger threshold can be set between practically O V and half the supply voltage. The input impedance at point 2 is about 1 M.