The switch described has two times ‘, ranges that are selected with a push-button. In the prototype, the ranges were 5 minutes and 20 minutes, but these can be altered easily. Pressing the switch three times in succession switches off the load (here, a lamp).
When S1 is pressed, 1C3 is enabled via D1 and NAND gate IC2b; at the same time, capacitors C3 and C4 are charged. When the switch is released, IC3 gets a clock pulse from 1C2a. via R2, so that its QA output goes high. This results in the relay being energized by T2 SO that its contact closes and the lamp comes on.
At the same time, the, QA output switches on transistor T1, so that R4 is short-circuited. Capacitor C4 is then discharged via R3. After the potential across C4 has dropped to the lower trigger level of 1C2 b, which takes about 5 minutes, IC3 is reset and the relay switches off the lamp.
When S 1 is pressed twice in succession, the counter gets two clock pulses and its QB output goes high. Transistor T1 then remains off and C4 is discharged via R3+R4, which takes about 20 minutes. As an indication that the longer time has been selected, immediately after the second press on S1. the lamp goes out briefly. This is effected by 1C2d, which given a pulse by differentiating network R9-C5 when QB of IC3 goes high.
When S1 is pressed three times in succession, both QA and QB of IC3 go high, which results in the output of IC2b becoming 0 and C4 being discharged rapidly. The lamp is the switched off instantly.