I built a fair number of these, starting in the '70s shortly after Radio Shack started carrying the NE555 timer. I usually used the Micronta 50uA panel meters for the readout. I'd watch for project boxes on sale in Radio Shack and surplus electronics stores. I used the dry transfer rub-on lettering, or sometimes I'd make the front panel out of PCB and etch lettering into it.

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I didn't set out to sell them. I'd show them to someone, they'd ask where I bought it, call me a liar for saying I designed and built it... then ask me how much to buy it from me. So I'd sell it and build another.

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In the mid '80s, I started building them to use a DMM as the display. On the lowest scale, 1mV = 1pF, so for a meter on 200.0mV, 0.1pF resolution. I built an ugly prototype with parts I had on hand, then started building nicer ones and selling them.

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I still have the ugly prototype, and a few years ago I found the notebook with the schematic for it. Hackaday.io is having a 555 timer contest, I thought I might as well document this there and enter it.

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I found a resistor or two in the ugly prototype has apparently drifted, or the selector switch may be dirty. So I'll be cleaning it up and may be replacing some resistors, then I think I will make a surface mount version and do it as a kit for our local makerspace. I think I'll add traces so it can be built for several purposes, including liquid level, touch switch, remote sensing on 2 wires, etc.

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Anyway, here it is if you are interested:

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I thought it might be interesting to see if I can push this to be stable enough to be useful to a 0.01pF/10fF resolution. Without a microcontroller. For that, I think I'll add a CD4060 with crystal.

I drew a clearer schematic and a block diagram with explanation. I welcome comnents.

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Steve Greenfield AE7HD

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Steve Greenfield AE7HD