Photoelectric switch circuit diagram on AC power grid

As shown in the figure, the circuit will only operate when the light intensity is above 700lx. At this time, the phototransistor BPX25 and the transistor BFY52 trigger the current to the thyristor BTY91/800R. When there is light on the phototransistor, the trigger pulse always appears about 6 degrees after the starting point of each positive half cycle, and the arithmetic mean of the DC half-wave current flowing through the load is at most 16A.

The voltage regulator tube BZY88/C5 and the transistor T1 and their connecting resistance are 390 ohms, which are used to limit the voltage of the trigger circuit to no more than 6V. The diode BYX45/800R is used to block the negative half-cycle grid voltage, the resistor 33 ohms and the capacitor 0.1uF are used to protect the thyristor and prevent over-voltage.

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