Circuit provides message on disabled phone line

Posted on Nov 20, 2012

The circuit in Figure 1 adapts a small key-chain voice recorder to the Tip and Ring lines of a phone line that has been disconnected from the central office. The purpose is to play a prerecorded message into any phone on the line when its receiver goes off-hook. With all phones on-hook, phone-line current is near zero, keeping the optocoupler off and its transistor open with the voltage at Pin 5 at the battery voltage. The Tip and Ring lines are at 6 and 0V, respectively, to power the phones on the line (Most phones operate on as little as 3V.) Battery drain in this condition is minimal. When a receiver goes off-hook, the line impedance drops, and several milliamps flow through the saturated transistor.

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