Night gated lighting circuit

Shown is a night door-controlled lighting circuit. In this circuit, the switch SI is closed when the door is closed. According to the logic function of the NOR gate
, as long as one input terminal is high level , the output terminal is low level. During the day, when there is light, the phototransistor VT1 is in
a low-resistance state, and its emitter is extremely high level, so the NOR gate 1 outputs a low level, and the monostable circuit composed of NOR gates 2 and 3 will not be triggered to work.
VT2 is in the cut-off state, the relay Kl does not work, its contact Kl_I will not be closed, and the lighting lamp H will not light up.
    At night, VT1 presents a high-resistance state due to lack of light, and its emitter becomes low level. According to another logical function of the NOR gate circuit,
when all input terminals of the NOR gate are low level, the NOR gate output is high level, so when the gate is opened, the switch S is turned off, and both of the NOR gate 1 Both
input terminals are low level, and their output terminals output high level. This level triggers the single-state circuit to flip, or the output of gate 3 is high level, driving VT2
to conduct, and the relay Kl works. Click KJ. 1 pulls in, and the lighting lamp H is lit. After about 1 minute, the monostable circuit returns to its original state,
and NOR gate 3 outputs low level . VT2 is cut off and the light H goes out.