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e meter/wheatstone bridge

Commander_Chaos

Oct 9, 2020
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My only experience with electronics is that I once built a distortion pedal from a book for my electric guitar. That was decades ago and it was totally spelled out for me so I didn't understand what I was actually doing.

I want to build an e-meter similar to the ones used by an organization that I won't name because it would be a PITA to have them contact me. This is strictly a recreational exercise. There's lots of schematics on the internet but all of them make big assumptions about the knowledge of the person reading them. Here's one:

https://www.cs.cmu.edu/~dst/E-Meter/koos-workingmeter.gif

To build it I'd need to know what the components are and where to source them. For instance, I know what a resistor is but when I look them up on line there's lots with the same value and I don't know what the difference is. I also see the electrodes marked "cans" but I don't see batteries. Yes, I'm that ignorant.

Any help would be appreciated.
 

ChosunOne

Jun 20, 2010
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I'll leave it to others better qualified to help you with the electronics, but I just wanted to say:
Congratulations on disconnecting from the organization that requires use of the "official", sanctioned, and poorly-made e-meter. I suspect they actually do make it within the organization itself, simply because it's probably the only organization in America where labor is cheaper than in China. Apparently they don't know how to reach you, or you'd still be getting tons of junk mail from them. I applaud your commitment to stay under their radar. Keep up the good work.
 

Nanren888

Nov 8, 2015
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Can't say I think such a circuit is much use for anything, but , ...
Battery marked "9 volt" near the left, above centre. Near a regulator which provides a regulated 5 volt to the ciruit from the 9 volt battery.
.
What method are you going to use to build it? I mean to support the components and connect them together?
vero board?
This will partly determine the types of components you want.
It's all low voltage, low power, so the components should be plentifully available.
Four presets to adjust and two potentiometers, presumably to be available to the user.
 

Commander_Chaos

Oct 9, 2020
4
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Oct 9, 2020
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I should have clarified that I was never a part of this organization, nor do I want to be. I also don't believe this device has any useful benefit. This is really just morbid curiosity about an extremely strange topic.
 

Commander_Chaos

Oct 9, 2020
4
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Oct 9, 2020
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I don't know what a Vero board is but when I made the guitar pedal I used a bread board. Is that the same thing?

For the potentiometers I'm assuming I need linear ones as opposed to the audio I've used rewiring guitars.

I think I'll go through the list, find components and post links here. Maybe I can get a thumbs up or down on whether they're right.
 

Harald Kapp

Moderator
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Nov 17, 2011
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I think I'll go through the list,
Good start.

For ease of building, use leaded resistors, 1/4 Watt, accuracy 5 % or better (but more expensive) like you can find e.g. here.
The pots in the list are standard linear potentiometers.
The presets are also a kind of potentiometer, but meant to be adjusted once, not continuously. Your more likely to find the under their modern label "trimmer potentiometer" like these here.
For the voltage regulator use an 78L05 or 7805 like one of these.
The electrodes marked "cans" that you are wondering about are simply metallic conductive "thingies" that you hold in your hands. You can use anything you please, e.g. empty tin cans, steel balls or whatever you like to hold - as long as it is conductive and you are able to connect the wires to them.
The mounting board from the list is what Nanren88 meant by Veroboard and possible the same "breadboard" you used in your previous build. Here's a video that shows how to create a circuit on a reusable breadboard and transfer it to a Veroboard (stripboard type). The stripboard type used has strips (hence the name) of copper traces running in parallel which makes it easy to connect components, but care has to be taken where to place components and the strips have to be interrupted in the right places to avoid short circuits. There is also a variant of Veroboard that has the contacts but no copper strips (traces we call them). That per se avoids short circuits and gives you more freedom in placing components but on the downside requires more wiring work as you need to make every connection with strands of wire.

Hope that helps.
 
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