Circuit provides message on disabled phone line

Kevin Kelley, BAE Systems, Greenlawn, NY -- EDN, 1/20/2000

Phone companies often disconnect a misbehaving phone line from a complainant's residence for troubleshooting purposes. With the problem between the residence and the central office, the residence is left with a dead phone line and no visible repairman while the line is under repair. 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. A Radio Shack key-chain voice-memo recorder (part number 63-945) or a similar device provides solid-state voice-message storage and playback in a small package and also powers the phone line with its internal 6V batteries. You open and modify the recorder to bring four signals out to the external circuit: Battery (+), Battery (-), Speaker (+ or -), and the Play button contact. You can disconnect the internal speaker to save power.

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. The transistor provides a high ac impedance between C₁ and the battery, allowing audio-signal transfer to the line, and provides a low dc resistance to maximize the low battery voltage to the phones. The transistor current turns the optocoupler on, and the voltage on Pin 5 drops to near 0V. This negative edge generates a low pulse into the Play contact, as if you had pressed the Play button. The message plays once in its entirety every time a receiver goes off-hook. C₂ prevents any clicks at the end of the message from restarting the sequence if the receiver goes on-hook before the message ends. (DI #2472)