Analog meter modification question

[cevonk]

I just bought a Harbor Freight 9 Function metal detector. It seems to work fairly well at discrimination, and it has okay range in air tests (I haven't had a chance to do more than that with it.) The analog meter on it goes from zero reading to pegged completely with no in between. In other words, it's not much use as an indicator of degree of intensity (of whatever it is measuring.) It has two wires (red and black) that carry power to the meter. Would it be possible to insert a potentiometer in the negative (black) side of the meter to make it a useful indicator? I don't know if that would affect anything else the circuitry is doing -- if the meter is an isolated circuit that only does metering, or if it's part of a more complex circuit that would be affected by a change in resistance in that circuit.

My skill level is low. I know very little about electronics. I can do basic AC and DC wiring, and I can solder, and I can use the basic functions on a multimeter. I know I could wire in a variable resistor (potentiometer) in the line, but I don't know if that would be a useful thing or a harmful thing to try to do.

Thanks!

[dbanner]

Don't waste your time, that meter is absolutely useless and is only there to add some cosmetic enhancement to the stupid box.
That particular model (9function) is a very good metal detector with excellent tone discrimination and good depth. The stupid meter is absolutely useless.
You can add a small resistance in series with the red wire to reduce the amount of deflection, but it doesn't really matter because it gives the operator absolutely no useful information.

[cevonk]

Thanks for the reply!

I'm doing some air testing on it, and find that it does indeed do a good job at discrimination and has (to my inexperienced understanding) decent detection range for a variety of metals.

I probably won't try adding the potentiometer, based on your information. But, why would you put the resistance on the red line? My line of thinking was that reducing the current flow on the incoming (negative) side would dampen the activity of the needle. If the resistance is on the return (positive) side of the circuit, it doesn't seem like it would affect the needle's action at all. (Or am I misunderstanding current flow?)

Edit: The meter does have some range of deflection when the target is at the very boundary of detectability...but once you get a hair inside that boundary, the needle pegs itself. This is what made me think that the meter could be desensitized by using a resistor.

[waltr]

I probably won't try adding the potentiometer, based on your information. But, why would you put the resistance on the red line? My line of thinking was that reducing the current flow on the incoming (negative) side would dampen the activity of the needle. If the resistance is on the return (positive) side of the circuit, it doesn't seem like it would affect the needle's action at all. (Or am I misunderstanding current flow?)

A resistor in series with the meter is in series and doesn't matter on which side it is on.
The meter moves with current flowing, a series R reduces the current.

[cevonk]

Thanks! That makes sense.