Posted on Jun 3, 2012

Q1 and Q2 are used to form the basis of an interface circuit for attaching a cassette recorder to the phone line. The circuit does not require a power supply because operating power is drawn from the telephone line itself. The incoming signal is fed across a bridge-rectifier circuit, consisting of diodes D1 through D4. When the phone is on hook, the voltage at the output of the bridge at the Rl/R3 junction is near 48 V. That voltage is fed across a voltage divider consisting of R1 and R2.

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The voltage at the junction formed by R1 and R2 is fed to the gate of Q1, turning it on. That pulls the drain of Q1low. Since the gate of Q2 is connected to the drain of Q1, the bias applied to the gate of Q2 is low, holding it in the OFF state. When the answering machine responds to a call or a phone is taken off hook, the voltage across the phone lines drops below 10 V, causing Q1 to tum off. At that point, the voltage at Q1 "s drain rises, turning Q2 on. The remote input of the cassette is connected to Q2"s drain and source through S1, and a miniature plug is connected to the remote input jack. Switch S1 must be in a position so that the positive lead of the recorder"s remote input connects, through switch position 1, to Q2"s drain and the negative input to Q2"s source. Switch S1 provides a convenient way to reverse the circuit"s trigger output without having to unsolder and resolder leads. The phone"s audio is coupled through C 1, C2, and T1 to the microphone input of the cassette recorder.

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