As shown in the figure below, it is an automatic delay lighting switch circuit, which has certain practicability. When leaving the room at night and turning off the lights, if the light switch is not at the door, it is very inconvenient to go to the door in the dark after turning off the lights. However, if you use the circuit shown in the figure below, you can delay for a period of time after turning off the light, so that you have enough time to leave the room and avoid the pain of darkness.

In the circuit, a and B are respectively connected at both ends of the original switch. When the switch s in the circuit is closed, the positive half cycle of AC is added to the thyristor control pole through D6, R2, R1 and D1 to trigger thyristor conduction; the negative half cycle of AC is also added to the thyristor control pole through D4, R2, R1 and D1 to trigger thyristor conduction. After the thyristor is turned on, it is equivalent to short circuiting two points c and D. therefore, two points a and B are also closed through the diode and the turned on thyristor. At this time, the lighting lamp housing. After the switch S is disconnected, the thyristor still has a trigger current and remains on because the capacitor C1 is discharged through the thyristor control electrode, R1 and D1. As the discharge current decreases gradually, after a period of time, the thyristor is cut off, and the lamp goes out. According to the parameter values shown in the figure, the circuit delay time is about 40 ~ 50 seconds.

Component selection: thyristor with working current of 1a and withstand voltage of 400V can be selected; The rectifier diode of ln4004 can be selected for D1, D3 ~ D6. The withstand voltage of CL is 630V and the capacity is 35 μ F electrolytic capacitor.

Note: after the circuit is correctly assembled, if the light does not light after closing the s switch, the resistance of R1 can be appropriately reduced.

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