5V Dial Tone Circuit Circuit

Posted on Jul 24, 2012

This circuit uses inexpensive, common components to generate a precise dial tone for phone applications (see the figure). Ul (an Intel 82C54 timer-counter) generates 350 and 440Hz square waves that are filtered by Rl/Cl and R,JC2, 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.

5V Dial Tone Circuit Circuit
Click here to download the full size of the above Circuit.

Almost any timer-counter can be used as the signal source, so long as it produces roughly square-wave outputs.

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