Photoelectric-acoustic conversion circuitb

When using the phone at night, it will be very convenient if there is light illumination. The photoelectric control phone light circuit introduced in this example solves this problem.
The working principle is as follows: As shown in Figure 9-29, this circuit can achieve the following functions:
    1) Dial lighting: Lift the microphone and the light will turn on, which can be used as a dial lighting.
    2) Reminder to hang up: After making a call and hanging up, if the light does not go out, it means the microphone is not in place, and you can be reminded to hang up correctly.
    3) Burglary call warning and outside line disconnection reminder: When there is no call at home and the lights are on, you can pick up the microphone and listen to see if there is any sound. If there is sound,
it means someone is stealing your phone line; if there is no sound, then Maybe the external line is disconnected.
    4) Incoming call reminder: The old man is deaf and cannot hear the ringing sound. When he sees the phone light flashing. You will know that someone is calling.
    Ll and L2 are connected to the telephone outside line of the telephone junction box (connected in parallel with the telephone). Due to the connections of VD1 to VD4, Ll and L2 do not need to be separated into
positive and negative poles. In the on-hook state, the feed voltage is about 48V, and the current is limited by RJ, causing vs to breakdown. The light-emitting diode in the optocoupler IC is turned on,
the phototransistor is saturated, the thyristor is blocked due to no trigger voltage, and the light HL does not light up.
    When a call comes, the ringing current of about 25 Hz/90V is rectified into pulsating DC by VD1~VD4, which causes the optocoupler lc to continuously conduct and
cut off , and the thyristor VTH to continuously conduct and block, and the light bulb emits Flashing light. When off-hook. The feed voltage with high internal resistance immediately drops below 12V,
vs cannot break down, the optocoupler IC is cut off, its @ pin voltage rises, the gate VTH is triggered and turned on, and the light bulb HL lights up. In order to extend
the service life of the lamp and save power, VTH uses a unidirectional thyristor. In this way, the voltage at both ends of the bulb HL is only 115V when it is lit.
  When the bulb is not on, the circuit consumes almost no power.