5_V_DIAL_TONE_CIRCUIT

Published:2009/6/22 22:45:00 Author:May | From:SeekIC

This circuit uses inexpensive, common components to generate a precise dial tone for phone applications (see the figure). U1 (an Intel 82C54 timer-counter) generates 350- and 440-Hz square waves that are filtered by R1/C1 and R3/C2, and mixed together by resistors R2 and R4.An operational amplifier configured as a 395-Hz, Sallen-Key, second-order bandpass filter (halfway between 350 and 440 Hz) removes unwanted signal harmonics. Almost any timer-counter can be used as the signal source, so long as it produces roughly square-wave outputs.



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