1. Circuit working principle
The circuit is shown in the figure below. When the telephone is on hook, there is a voltage of 45 ~ 60V between telephone lines L1 and L2. Due to the action of capacitors C1 and C2, the delay circuit composed of integrated circuit IC1 and other components cannot be triggered. When the phone calls in, the ringing signals from L1 and L2 are in alternating state. They are coupled by capacitors C1 and C2, rectified by voltage doubling of diodes VD1 and VD2, filtered by capacitor C3 and applied into a forward DC voltage to the base of triode VT1, resulting in saturated conduction of VT1. At this time, pin (2) of IC1 changes from high level to low level, and the output end of pin (3) of IC1 changes from low level to high level, so as to pull in relay K1, When the normally open contact K1 is connected, the radio or tape recorder inserted in the power socket CZ can play the radio broadcast program or tape content. If the photosensitive triode vt2 has high resistance when it is dark, the relay K2 is pulled in through the action of integrated circuit IC2, its normally open contact K2 is also connected, and the electric lamp HL is on. After IC1 is triggered, the power supply charges capacitor C5 through resistor R2. When the voltage at both ends of C5 rises to 2 / 3 of the power supply, IC1 resets, its output terminal (3) pin changes to low level, and the radio or recorder and electric lamp are switched on and off at the same time. When the phone calls again, the above process will be repeated over and over again.
2. Selection of electronic components
IC1 can use time base integrated circuits of NE555 and LM555. IC2 adopts TWH8778 integrated circuit produced by Guangdong Dahua electronics factory. VT1 adopts NPN triode of 9013 and 8050. 3du31-33 photosensitive triode is used for vt2. 1N4001 diodes are used for VD1 and VD2, 1N4004 diodes are used for VD3 and VD4, and 1N4007 diodes are used for vd5, vd6, vd7 and vd8. Relays KL and K2 are jrx-13f type, and their coil working voltage is 9V. T1 adopts 8W, 220V / 9V power transformer. There are no special requirements for other resistance capacitance elements, which can be selected according to the marked parameters in the figure.
3. Installation and commissioning
After the circuit is installed correctly, it can be powered on for debugging. First replace the resistance R2 with a resistance of 100k Ω, integrate L1 and L2 into the telephone line, and let others dial your phone. At this time, the relay K1 should be pulled in, and K1 will be released after about 30s. Then adjust the resistance value of the adjustable resistor RP, so that when the light reaches VT2, the relay K2 is in the release state. When the light is weak, K2 attracts.
Finally, replace R2 with a resistance of 10m Ω. The delay time can be changed by changing the values of resistance R2 and capacitance C5. When installing the photosensitive triode vt2, avoid direct light to it to prevent interference.
When in use, insert the power plug of the recorder into the power socket CZ, turn on the power switch, adjust the volume, and tune to a radio station that ends late. Or insert the power plug of the tape recorder into the power socket CZ, put in a 60min tape, press the playback key, and adjust the volume.