Use NE555 to make automatic telephone lighting

The circuit for making automatic telephone lighting using NE555 is shown in Figure 3-12. It mainly consists of a photoelectric coupler, a negative pulse
generator , a monostable trigger, a light control circuit, a thyristor switch circuit and a power supply circuit. Partially composed.
    Terminals Xl and
The negative pulse generating tube VT is in a cut-off state, its collector is at high level, the monostable trigger A2 composed of NE555 is in a stable state, pin 3 of the output terminal outputs low level, the thyristor vs is turned off, and the lighting H does not light up.     When a call comes in from the outside or the owner picks up the phone and dials, current flows through the telephone line, and the built-in luminous tube of Al lights up and the corresponding photosensitive tube is turned on, that is, pin 4 of Al outputs a high level, and VT obtains the base current. is turned on, its collector outputs a low level, that is, a negative pulse is input to pin 2 of the monostable trigger A2, A2 is triggered to flip, and the bit is set to enter a transient state, and pin 3 of the output terminal outputs a high level, vs. The gate gets the trigger current and turns on, and the lamp H is lit. If no one picks up the phone after ringing or hangs up after making a call, pins 1 and 2 of Al will lose the voice current, the built-in light-emitting tube will go out, and the corresponding photosensitive tube will return to the cut-off state, and VT will also cut off. At this time, the positive power supply Then cl can be charged through R3. After about 1 minute, the level of pin 6 of A2 rises to 2/3 VDD. The transient ends and the flip-flop returns to the stable state. Pin 3 outputs low level. vs loses the triggering voltage, when the AC power crosses zero, it turns off and the light H goes out.     SB is the button to turn on the light. When you need to turn on the light, you just need to press SB to artificially input a low level to pin 2 of A2, forcing A2 to flip into a transient state, which will cause the lamp H to light up for 1 minute.     RO and RP form a light control circuit. During the day, RG is exposed to indoor natural light and has low resistance. Therefore, even if , although the built-in photosensitive tube of Al is turned on, the resistance of RP is much greater than that of RG. Pin 4 of Al can still remain low, and VT remains in the cut-off state, so the circuit will not be triggered to work. Only when night falls and the lights are not turned on indoors, the RO has a high resistance due to no light exposure, and the resistance value is much greater than RP. When a call comes in and the photosensitive tube built in A1 is turned on, This will enable pin 4 of A1 to output a high level , prompting the circuit to flip over. However, the manual key switch SB is not controlled by light. In any case, the light can be turned on as long as the SB is pressed.     The diodes VDI~VD4 are connected to form a bridge circuit. The purpose is to facilitate wiring without distinguishing between the positive and negative poles when connecting the terminals XI and X2 to the telephone line. VD5, VD6, C4 and c5 form a simple capacitor step-down half-wave rectification and voltage stabilizing circuit to provide 12V DC working voltage for the whole machine.