Exhaust fan automatic control circuit (3)

Circuit working principle: When the switch S is switched to the "1" point, the exhaust fan can be turned on manually;
when the switch S is switched to the "2" point, the exhaust fan is in the starting working state. The 220 V alternating current
is stepped down by transformer T and diode VD. Half-wave rectifier, capacitor C. After filtering, the power supply
voltage Vcc-3V is obtained. The diodes VD2 and VDi play a voltage stabilizing role, and the voltage drop is 1.4 V, then
UA =1,6 V. Resistor R. Connected in series with the thermistor Ro, then UB=1.6R/(R. +
Ro), Ro uses a positive temperature coefficient thermistor . Normally "<1.4 V, the diode
VD. and the triode VT are cut off, and the photocoupler composed of LED and photoresistor R3 does not
work. At this time, the bidirectional thyristor VS is cut off, and the exhaust fan motor does not rotate.