This 100-percent TUP & TUN circuit measures the peak value of an AC voltage up to 6 volts in amplitude, regardless of the waveform or if the peak occurs in the positive or negative excursion of the signal.
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To allow over-the-weekend construction the project is built using the cheapest and commonest parts possible—just BC5xx transistors and 20 or so passive components, all through-hole and off the shelf, or ex junkbox.