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Telephone recorder circuit for security to avoid wiretapping

Most of the phone market, with intrusive security anti-theft device that detects when the voltage drops outside, the strong anti-theft audio output signal to interference thieves, it is not normal call. Although the thieves can not normally talk, but often still be able to dial a phone number from outside, the phone still need to pay the owner, and afterwards did not have details. In this, the author to introduce a recording made easy style security, were not easy to sleep in the theft investigation, the system automatically recorded the dial tone and content of the conversation later, this recording can be treated to one of the dial tone restore the phone number, plus the conversation with the complaint as a basis for compensation.
This paper described the principle recording phone anti-theft devices see photos. When you hear someone outside when the voltage is lower than the theft of 27V, VD1 not conduct, V1 end, V2 ~ V4 conduction (Click image to connect, V4 turns on, c, e will be lower than the saturation voltage between poles 0 ? 1V), pocket tape recorder was electric start recording at the same time, the audio signal on the line through the R1, C1 into the tape recording input or microphone inputs. When the thieves hanging outside the line after the resumption of 48V DC, VD1 remains on, so V2 ~ V4 and the cut-off in the standby state, the recorder to stop recording. K to detect whether there is recording the switch, when you press the playback button, close the K, you can check the recording situation. In addition, although the VD1 end their phone call, but the D5 ~ D8 and are conducting optocoupler P521, P521 V2 base will remain zero potential, so the audio part of the circuit does not work, if it felt necessary, will be closed that K recordable.

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Telephone recorder circuit for security to avoid wiretapping