Finger Simulator  - Page 1
Forum section: electronics / projects

author: WZOLL
author_rank: Regular Contributor
timestamp: July 20, 2017, 03:01:34 am
content: This sounds kind of counter productive and strange but I need something that can simulate the resistance of a moist finger. I have a circuit board that only works when I place my moistened thumb on a group of surface mount parts. I have compared all the connections with the schematic but I cannot figure out what is wrong and my finger is too big to see what it is touching. In an effort to find a way around the problem I decided to make a thumb simulator. The resistance of my thumb is ~47k. Any suggestions (other than actually fixing it)?

author: jbb
author_rank: Super Contributor
timestamp: July 20, 2017, 05:20:14 am
content: You could perhaps try a graphite-loaded putty to give the squishy slightly-conductive behaviour. (Can be made with powdered graphite & Blu-tack.) But it'sverymessy.I'm assuming you don't have any voltages > 50V or high energy in the circuit that you're sticking a thumb into?Or perhaps work out what's wrong with the circuit. Here are some things you might want to think about:Do you have any decoupling caps?Is everything soldered down? I've sometimes had 99.9% invisible cracks which take forever to find.Try poking around the area with a plastic stick.Try poking around the area with a conductive pin, touching one node at a time. I.e. make a smaller version of your thumb so you can see what's up.Try wrapping a little cling film / Glad Wrap over your thumb and having a poke around. This will stop DC current flow but might still give similar capacitive effects.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2017, 08:29:23 am
content: Resistance varies from 10s of kohm when wet and pressing hard, to a couple meg when dry.Capacitance varies from fractional pF (proximate) to a few hundred pF, with ESR in the 100 ohm to 10kohm range, depending on frequency(?), touch and condition.There's an inductive element as well, seeing as fingers and arms have length, but the impedance is usually high enough, and the body is lossy enough, not to need to worry about that too much, or reflected waves.Tim

author: Benta
author_rank: Super Contributor
timestamp: July 20, 2017, 04:04:43 pm
content: Not sure i finger "putty" or the like will help you.It's very likely that your own finger is supplying noise to the circuit, especially 60 Hz, and your fake finger won't simulate that.

author: WZOLL
author_rank: Regular Contributor
timestamp: July 20, 2017, 05:18:59 pm
content: I tried conductive foam but the resistance was too low. I also tried an art pencil on paper which was closer but not quite right.

author: amitchell
author_rank: Regular Contributor
timestamp: July 20, 2017, 09:32:00 pm
content: I think the obvious solution is to use the finger of an infant.

author: DaJMasta
author_rank: Super Contributor
timestamp: July 20, 2017, 10:47:49 pm
content: Does it happen with other things applying pressure? Maybe you've got a bad solder joint that needs a little pressure.Otherwise, you could take a small through hole cap (20-100pF range is probably where I'd start) and just sort of probe around with the legs, then you can try the same with a decently large resistor (maybe 100-200k?). The best solution would probably be to try and identify the failure mode, the susceptibility of parasitics and component choice of the design topology, and try to simulate through some extreme conditions, as it may give you a characteristic that you can adjust that will make it work in much broader conditions (and without a finger applied).For example, if it was a linear regulator based circuit... it's just a bad solder joint. If it's an oscillator of a few MHz, maybe the component choice is just a bit too low to be resonant where you want it using the non-ideal components chosen. If it's a multi GHz rf circuit, it could be a little bit of crud on the board or some missing shielding. Without understanding what the circuit is and how it's operating, it's going to be real tough to figure out exactly what is making it react to that finger.