This example introduces a delay switch for controlling the ventilator of a bathroom. When the control button is pressed, it can be automatically powered off after a few minutes of power on. The switch can also be used to control lights in corridors, corridors, and the like.
Circuit Operation Principle The ventilation fan delay switch circuit consists of a trigger switch circuit and a delay control circuit, as shown in the figure.

The trigger switch circuit is composed of a thyristor VT, a control button S, rectifier diodes VD1, VD2, a capacitor C1, resistors Rl, R2, R4 and a æ°– indicator light HL.
The delay control circuit is composed of a time base integrated circuit IC, transistors V1, V2, diodes VD3, VD4, resistors R3, R5, R6, a potentiometer RP and capacitors C2, C3.
When S is not pressed, the VT is in an off state, the illumination lamp EL does not emit light, and the ventilation fan motor M does not operate. When S is pressed, the AC 220V voltage is rectified by EL step-down and VDl-VD3 (VD1 is turned on during the negative half cycle of the alternating current, and VD2 and VD3 are turned on during the positive half cycle of the alternating current), and the capacitors C1 and C2 are charged. After Cl is fully charged, the gates of R2 and VT are discharged, so that VT is triggered and turned on, EL is lit, and M is energized.
At the same time, the timer consisting of lC and peripheral components works, but the voltage across the C3 can not be abrupt, the Z pin and pin 6 of lC are low level, the 3 pin outputs low level, and Vl is at the cutoff. The state has no effect on the trigger switch circuit. After releasing S, the charging voltage on C2 maintains the power supply to the IC, and also charges C3 through RP and R5, so that the voltage of the 2 pin and 6 pin of the IC rises continuously, when the voltage rises to a certain value (about 10 min) The circuit in the IC is flipped, and the 3 pin changes from low level to high level, so that Vl is turned on, Cl is quickly discharged through Vl, VT is turned off when the alternating current crosses zero, EL is extinguished, and the IC stops working after the end of C2 discharge.
HL is extinguished when VT is on, and illuminates when VT is off.
Adjusting the resistance of the RP or changing the capacity of C3 can change the delay time of the delay switch.
The components and parts choose Rl, R3, R5 and R6 to select 1/4W carbon membrane resistor or metal membrane resistor for use; R2 and R4 select 1-2W metal membrane resistor for use.
The RP uses a solid potentiometer.
Cl-C3 selects aluminum electrolytic capacitors with a withstand voltage of l6V.
VDl-VD4 uses 1N4004 or 1N40 silicon rectifier diodes.
Vl and V2 select S9013 or 3DG6, 3DG2O1 type silicon NPN transistor.
VT selects TLC336A (3A, 600V) bidirectional crystal tube.
lC selects NE555 type time base integrated circuit.
S selects the move (normally open) button.
HL selects the common æ°– indicator.