Off-hook signal detection circuit

Shown is the relay control circuit. There are many switches in the microwave oven. If the switch is turned on, it can provide power, and if the switch is turned off,
it can provide protection. There are many components in a microwave oven that need to be controlled, such as fans, light bulbs, food turntables, timers, etc.
    Figure 1 shows the off-hook signal detection circuit , which consists of photoelectric coupling PT2 and PT3. It is composed of resistor Rio, RJ2, capacitor c9 and
bidirectional diode VD2 connected in parallel with the telephone. This circuit is used to detect whether the phone is off-hook. The optocoupler is composed of a light-emitting diode and a phototransistor
. The phototransistor is turned on by the light of the light-emitting diode. When the telephone is off-hook, the DC circuit in the telephone is connected. Since
the two light-emitting diodes of the photoelectric couplers PT2 and PT3 are connected in reverse parallel, there must be a conductor. The light emitted by the turned-on light-emitting diode shines
on the corresponding photosensitive transistor, and the photosensitive transistor turns on. After being inverted by the driver IC3, HS1 (or HS2) outputs a low level, which indicates the off-hook
state. When the phone hangs up, because the DC loop in the phone is blocked, even if the external circuit has DC voltage to form a loop, the light-emitting diodes
of the photoelectric couplers PT2 and PT3 will not conduct at this time, and the corresponding phototransistors will not conduct. When it is on, HS1 or (HS2) outputs high level, which means on-hook status.