How to Use the Radio to Record Phone Calls Automatically

The following figure shows an additional circuit that can be used to automatically record telephone calls using various radio recorders. The control principle is to use the different voltage levels of the telephone line when on-hook or off-hook, and use the additional circuit to identify and control the total power supply of the radio recorder.

When the phone is on-hook, the voltage at both ends of the incoming line is about 48V (true current). After this voltage is transformed by the D1-D4 pole pieces, the DC voltage generated at points C and D is much greater than the regulated voltage value of DW1 + DW2 (24V). DW1 and DW2 are reversely broken down, T1 is turned on, T2 and T3 are turned off, relay j does not work, and the recorder is in a stopped state without AC power supply. When the user picks up the phone, the voltage at both ends of the incoming line of the phone drops to about 8V. At this time, the voltage between points C and D also drops below 8V, DW1 and DW2 are turned off, T1 is turned off, T2 and T3 are turned on, the relay absorbs the platform, and the power of the recorder is turned on, and the recorder starts recording. After the phone is hung up, the recorder stops working again.

R1, C1, R2, and C2 are audio signal coupling and matching networks. When in use, the recorder is in recording state. Insert the plug into the MIC jack of the recorder, and insert the power plug of the recorder into the power socket of the recorder in this circuit to record the phone call.