2011-11-27
This little toy I've been meaning to build for many years. I finally got around to it today, and I already don't know how I lived without it. Build one, they are really useful.
I built Daniel Garcia's version of Charles' original circuit.
I substituted E12 preferred values and had to tweak the 33k resistor between the base of the last transistor and the +ve rail to set the quiescent current < 30 uA. I used BC549C and BC559C transistors - high gain is important, I haven't tried with vanilla 2N3904/6, you might need to burn more current if you use them. Having no off switch is really a nice feature, except when it touches something metallic on your desk or in your toolbox/bag.
Here's a quick video of me beeping out a hack "PCB" for the Day 4 Advent project (the sniffer/wavemeter).
I'll probably replace the sharpened bit of Copper wire with a bright Nickel or Gold plated test probe at some point. The Copper will corrode and become an unreliable contact eventually. I used a RCA connector to make replacing the tip easy. Another option is a small metal plate, especially for searching for mains wires.
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