Electrician night work flash indicator circuit

 The circuit of flashing warning light for night operation is shown in the figure.
cl, VD, island and Zener diode VS1 form a capacitor step-down,
half-wave rectification, filtering and voltage stabilizing circuit. The 555 time base circuit and photosensitive
transistor VT form a light-controlled oscillator. The light intensity is high during the day , and the resistance
between the collector and emitter of VT is very small. The internal discharge tube of 555 is discharged, so the circuit does not oscillate. Its output pin ③ is low level, the bidirectional thyristor VTH is turned off, and the warning light EL does not light up. As night falls, the light intensity is weak. As the resistance of VT increases , the voltage of pin ④ of 555 also increases, the 555 circuit oscillates, and pin ③ of 555 outputs the oscillation frequency, triggering VTH to turn on, causing EL - to shine brightly. Changing the resistance of RP can change the flashing frequency of the light.