That's a tachometer. A lo pass filter, a comparator, a oneshot, a capacitor, and a meter on a trimpot. Every zero crossing, the comparator fires the oneshot, the cap integrates the pulses, and the meter shows the freq. Cal it with sine waves.... no harmonics.
Re: inexpensive analog audio pitch (frequency?) meter or schematic / plans for one?
Dec 31, 2007
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Or a $2 micro averaging the DCT, FFT or XYZ (forgot the name of that spectral analyser).
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