on/off with a single push button

elektorjuly/august 1983 65 o n/off with a single push button A very useful function expressed in a 1 simple circuit that is suitable for many applications. Basically the out- put of the Schmitt trigger Nl changes (toggles) when the switch is closed momentarily. This toggle s function is achieved in such a simple circuit by the fact that the inputs of the trigger are held be- tween the switching threshold levels. If we assume that the output logic Nl = IC1 0 level (Q) of the trigger is at logic 1, capacitor Cl will charge via R1. When switch Si is closed the input of the trigger will now be taken to logic 1 (because the capacitor is fully charged) and the Q output will of course become logic "0". The capacitor will now discharge but not completely because the closed switch will hold the level to that existing at the wiper of Pl. However this drop in voltage at the input of the gate will not cause its output to change state again because the input level will stil) be above the lower...