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I am trying to build a scientology type e-meter which is based on a Wheatstone bridge with a quad opamp. It is designed to measure the electrical resistance of the skin of a person holding the electrodes (labelled PC) on the diagram. ( i.e. the changes in resistance unbalance the bridge ). It has 2 x pots for basic adjustment to set the needle in the centre of the meter dial.

My electronic experience and knowledge is very limited. I got the schematic off the internet and have attempted a board layout and have built one from this. It is not working properly as I am getting full power on the meter and no adjustment is working.

Perhaps someone who is interested could look over my plan to see if there are any gross errors in my interpretation of the schematic? Any suggestions would be appreciated.

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originally posted here: http://stephenbennington.comoj.com/meter


Thank you for your detailed response to my question. I will endeavour to add the information you are missing. I cannot take any responsibility for the design - it is not mine but by a man called Ralph Hilton who has worked professionally on the original meters and has endeavoured to make improvements which he has built and tested and made publicly available. The schematic drawing is his and the coloured drawing below is my attempt at a board plan.

I am just trying to build one with limited experience and unfortunately he is not longer available for consultation. Please don't be hard on me - I have no electronics training and I am learning as I go.

This is the original web page where I got the design from. It lists all the parts and has a VERY rough sketch of a board plan - perhaps this will explain more than I can:

http://www.cs.cmu.edu/~dst/E-Meter/Ralph_Hilton/bmeter.htm

The OpAmp suggested is a LM324. I have include the + and - inputs on my drawing as I understand is correct for this type. I too was puzzled why there is only one shown for amp 3 on the schematic but there are three connections on the board plan - the output and minus to two of the pot terminals. This is a grey area and maybe where the problem lies.

I look forward to your responses.

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  • \$\begingroup\$ The term you are searching for here is "Galvanic Skin Response" or "ElectroDermal Response". That should get you a lot of references from the medical literature about what you're trying to do. \$\endgroup\$ – Connor Wolf Mar 22 '13 at 21:08
  • \$\begingroup\$ There is no way this will ever work with an LM324, and a 4.5V supply. That op amp needs so much head room, you'll have about 1V to work with between the rails. \$\endgroup\$ – Matt Young Mar 23 '13 at 17:51
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  1. Put component designators on all your parts. They are missing from the opamps.
  2. The + and - inputs to the opamps aren't labeled, so we can't tell if you got them right, and in some cases what you even intended.
  3. You don't need all those gain stages. The fourth opamp is completely unnecessary.
  4. The third opamp only has a single input shown.
  5. The LED will clamp the supply to the LED forward voltage when on. This is unlikely what you intended. R9 is probably meant to be in series with the LED only, not the whole circuit.
  6. The opamps need to be powered.
  7. There is no bypass cap accross the opamp power, and it would be a good idea to put a bulk cap accross the power too.
  8. The opamps aren't labeled, so we can't tell if it would work. Plenty of opamps would not work in this circuit.

All in all, I think your general concept is OK after a little tweaking. However, you need to be much more careful with the schematic, especially when you are asking others to look at it. You'd never hand in a homework assignment like that, so don't dump it on us here either. We deserve at least the same respect you'd show a teacher or professor.

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    \$\begingroup\$ Hand-clap - Olin's quite correct and I'd add that I can tell from the breadbaord picture you added power supplies to the op-amp so why all the laziness in showing it on the circuit diagram. The circuit diagram is far more important than the implementation at least initially. This nearly got downvoted by me but I'm kind and believe you'll do better and address the points above. \$\endgroup\$ – Andy aka Mar 22 '13 at 20:23
  • \$\begingroup\$ @Andy: I thought about downvoting too, but then decided to give him a chance to clean things up since other parts of the question seemed reasonable. I notice someone did downvote. I can see their point. \$\endgroup\$ – Olin Lathrop Mar 22 '13 at 22:02
  • \$\begingroup\$ personally i think a down-voter should come clean with a comment but that's probably just me. \$\endgroup\$ – Andy aka Mar 22 '13 at 22:09
  • \$\begingroup\$ @Andy: I agree in general, but in this case there is a obvious legitimate reason for downvoting. The OP should not be wondering why the downvote, any more than he would wonder why the C- grade from handing in homework like that. \$\endgroup\$ – Olin Lathrop Mar 22 '13 at 22:11
  • \$\begingroup\$ @Olin - Olin & Andy, I appreciate your comments and have added more information to my post. \$\endgroup\$ – Stephen Bennington Mar 23 '13 at 17:51

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