Telephone Ringer

Posted on Feb 7, 2014

This circuit generates the sound of the electronic ringer found in modern telephones, supplanting the 2-bell electromechanical ringers used previously. The ringing pattern follows the United States convention of 2 seconds on, and 4 seconds off. During the `on` interval, the tone alternates between two selectable frequencies at a rate of 20Hz. Refe

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ring to the schematic, NAND Schmitt trigger U1A is configured as a relaxation oscillator, adjusted with RV1 for a 40Hz waveform at test point TP1. This waveform is divided by flip-flop U2A to provide a 20 Hz, 50-percent duty cycle square wave at U2A`s Q output, and a complimentary square wave at U2A`s Q-not output. When U2A`s Q output is high, the relaxation oscillator formed by U1B is enabled. When U2A`s Q-not output is high, the relaxation oscillator formed by U1C is enabled. NAND gate U1D provides an OR function for the two oscillator outputs, and drives flip-flop U2B, which divides the waveform by two to ensure a 50 per-cent duty cycle. The resulting waveform at U2B`s Q output is a square-wave that alternates between two audible frequencies at a 20Hz rate. The pitch of these frequencies are individually adjustable from approximately 500Hz to 2500Hz with RV2 and RV3, and may be selected for a suitable sound output. A frequency divider chain consisting of U4, U5 and U6 is driven from the 20Hz waveform at U2A`s Q-not output. U4 and U5 provide a frequency division of 40, resulting in a 0. 5Hz square wave at U5B pin 13. This square wave drives U6, which is configured as a divide-by-3. U6`s "0" output is a 0. 167Hz waveform which is at a high state for 2 seconds, and a low state for 4 seconds. Paralleled gates U3A and D, driven by U2B`s Q output, drive one side of the piezo audible transducer, as well as the inputs of two...

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