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I'd like to build a device that can detect a weak electrical connection, likely in the domain of tens of megaohms up through low gigaohms, so that a circle of people holding hands can trigger the continuity signal. There is a commercially available product that does this: the Energy Stick.

After searching, I've found no useful guidance on first steps towards building a device like this. I suspect it is designed to measure tiny currents; the back of my envelope says that if it sends out a test voltage of 5V through a 1 gigaohm circle of humans in series, only 5 nA will flow, which ain't a lot. I don't know how to measure such a low current, nor do I know how to discern it from the noise which I imagine would exceed that signal.

(An idea: perhaps sending an AC test voltage, it could also be detecting an AC current in that same phase on its other pole, and reject noise that way? I don't know if this is heading in the right direction or irrelevant.)

Any/all advice appreciated.

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Years ago I had to design the electronics for a pH sensor, which have an impedance between 1GHom and 10Gohm, and your problem is the same, how to sense such small signals. The secret to success is to use a very low bias current operational amplifier and isolate it. As a start I suggest to take a look over the circuit note CN-0326, they explain all much better than I the problem and how to solve it, and take a look for their very low bias current OP amps, especially the LTC6268, I used it for my design and works quite well.

Also, there is a book named "Low-Level Measurements Handbook, Precision DC Current, Voltage, and Resistance Measurements" that will help you to understand how to deal with low measurements from the connection point of view (also important here, grounding and stuff).

After that, I suggest you come back and ask again if you have more doubts. Measure low signal is a big topic in electronics and you need to get some background of the key points to make the correct questions and we able to help you properly. And doesn't matter if you don't understand much, that's why we are here.

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What it comes int my mind is injecting a known signal and then compare what you get back. Like sending voltage pulses, measuring current, acquiring data and computing. Now how would you compare two signals and know that it is about the same? By using correletion. Which shape of signal gives best results? So called PRBS signal - Pseudo Random Binary Signal.

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for a $7.99 product that runs off coin cells (or some other invisible source) and uses no current when idle (as evident by the "try me" tag, there's really only one circuit they could use.

schematic

simulate this circuit – Schematic created using CircuitLab

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There is nothing special in this task.

The resistance of the human body ranges from 3000 to 100 000 Ohms (sometimes more). Any transistor switch will handle this task and will turn on everything you want. You can also use field-effect transistor or CMOS, or operational amplifier and measure resistance up to about 10^15 Ohms. For example, you can check this INA116 - input bias currents of 3 fA (3E-15 A)!

About this Energy Stick toy. There were a lot of schematics like this in 80s for the beginners in electronics, try to search Theremin, skin sensor switch and similar.

For example, you can try this schematics from here: enter image description here Sensors - two conductor strips 10x100 mm, but you can also try just bare wires.

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