3-meter fencing test box - help with ohmmeter circuit

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Joined Feb 13, 2019

I did some searching and did find some projects related to what I'm doing, but that aren't exactly what I would like to do.

I want to build a test box for fencing equipment, which will allow me to test for conductivity and resistance in the items I am testing. In particular, I have body cords, which consist of a set of three wires, each wire should be less than 2 ohms between the endpoints, and I have masks and jackets made with conductive material that should be less than 5 ohms between any two measurement points.

I would like the box to have three analog meters, where each meter will show the conductivity and resistivity of one of the three wires in a body cord, and then use one of the meters to test the resistivity of the mask/jacket conductive material.

Why three analog meters? The last national fencing competition I went to as a volunteer required just over two hours for ten people to handle the initial onslaught of fencers having their gear checked. We want to be able to plug a cord in to jacks, see that they have conductivity, see their resistivity and then we wiggle the cords to check for wire breaks and loose connections. We also use a metal test weight that gets dragged over the jacket and mask to check for conductivity and resistivity on those items.

Having three meters allows us to see the state of all three body cord lines at the same time. Having analog meters allows us to quickly see wire breaks or conductivity issues by watching how much the needle jumps as we test.

So basically I need three ohmmeter circuits where, ideally, the half-scale deflection on the meters represents about 5 ohms. The circuits should hopefully only require a couple of AA batteries or maybe a 9V battery. I'm looking to keep the battery number and size down to keep the weight of the whole test box down. It would also be helpful for the circuits to have pots to zero the meters prior to use.

I am a software engineering guy whose last circuits class was mumble, mumble years ago. :rolleyes: I am familiar with Ohm's and Kirchhoff's laws and the basic series ohmmeter circuit. What I'm having trouble wrapping my head around is how to modify that basic circuit to add in the zeroing pot and, more frustratingly for me, make the half-scale deflection represent 5 ohms with sane values for the circuit resistors and an off-the-shelf ammeter (I don't have any real requirements for the meters except I'd like them to be easily obtainable and have a relatively small footprint to keep the testing box size down).

I want to do this for several reasons. One, it's a learning experience. I want to understand the circuits and how they work and I want the experience of building my own box (I learn best by doing). Two, having my own box (as opposed to borrowing one), will open up more opportunities to work fencing tournaments which will allow me to eventually be hired to do so rather than working as a volunteer. Third, if the plans for this end up being archived here, I believe there will be many folks like me at fencing clubs all over the place who would love to have their own box to help out their clubs.

So to start, let's assume a 3 volt battery source and a 1 mA meter. I know I need a limiting resistor to protect the meter, so Ohm's law gives the necessary resistance for full scale deflection as 3 V / 0.001 A = 3000 Ohms (minus something for the meter's internal resistance, but I'm ignoring that for now to simplify the problem). With this circuit (see attached image), from Ohm's law, we know that the half scale deflection represents 6000 Ohms.

What I want is half scale deflection to represent 5 Ohms. How can I get that? Do I just need to add the right resistor network in there? What does that look like? Do I also need to change the voltage and/or meter rating? What values would work best?

Bonus points for throwing in a variable resistor for zeroing the meter. :)

Thank you.