Chip Recorder Customizes Phone Ringer



Category: Telephone, cellular phone and intercom
Manufacture: Maxim Integrated Products
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Description:
Maxim > App Notes > AUDIO CIRCUITS Keywords: Chip Recorder Customizes Phone Ringer
APPLICATION NOTE 1187
Chip Recorder Customizes Phone Ringer
Companies usually purchase one type of telephone for all employees. But this means that every ringer has the same sound and when one phone rings, a dozen people pause and look at their telephones. Equally annoying is the absence of a visual message indicator, so anyone who suspects they may have a message must lift the handset and listen for a special message tone. By placing a circuit in series with the telephone (Figure 1), you can customize a phone ringer without modifying the phone. The heart of the ringer is a chip-recorder IC (U5) capable of playing up to 10 seconds of telephonequality recorded sound. A high-efficiency step-down converter (U1) allows the circuitry to operate from a supply voltage of 5V to 14V.
For larger image Figure 1. This circuit plays up to 10 seconds of recorded sound in place of the ring from a telephone. It also indicates when someone has called. An opto-coupler and associated front-end circuitry monitor the line, sensing when the line receives a highvoltage ringer signal (Figure 2). Zener diodes D4 and D5 prevent the on-hook voltage from activating the optocoupler. Comparator U3 latches LED D6 "on" when a call is received, and a pushbutton switch (SW1) clears the comparator.
Figure 2. The ringer signal of a typical telephone exceeds 200VP-P (vertical scale is 50V/div). The circuit shown in Figure 3 records up to 10 seconds of sound in the chip recorder's proprietary multi-level EEPROM. A switch-mode, Class-D audio amplifier (U6) maintains high efficiency while delivering adequate power to an 8 speaker (even a PC speaker will work).

 
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