Companies usually purchase one type of telephone for all employees, which is understandable, especially if they obtain a discount for buying in quantity. One ringer sound for everyone can pose a problem, though, if the sound of a ringing phone makes a dozen people pause and look at their telephones. Equally annoying is the absence of a visual message indicator. Obliging everyone who suspects they may have a message to lift the handset and listen for a special message tone is far from ideal. By placing a circuit in series with the telephone, you can customize a phone ringer without modifying the phone (Figure 1). The heart of the ringer is a chip-recorder IC, IC₅, which can play as much as 10 seconds of telephone-quality recorded sound. A high-efficiency stepdown converter, IC₁, allows the circuitry to operate from a supply voltage of 5 to 14V.

An optocoupler and associated front-end circuitry monitor the line, sensing when the line receives a high-voltage ringer signal (Figure 2). Zener diodes D₁ and D₂ prevent the on-hook voltage from activating the optocoupler. Comparator IC₃ latches LED D₃ on when a call is received, and a pushbutton switch, S₁, clears the comparator. The circuit shown in Figure 3 records as much as 10 seconds of sound in the chip recorder's proprietary multilevel EEPROM. A switch-mode, Class D audio amplifier, IC₆, maintains high efficiency and delivers adequate power to an 8Ω speaker; even a PC speaker will work.

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