5V Dial Tone Circuit Circuit

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 - schematic

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

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