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Can I add a pot? Answered

Hi. I have a metal detector and it can be quite loud when it finds a target. Unfortunately, there is no volume adjustment on it. I don't have a lot of experience wiring pots into circuits so I wanted to ask before I try something and damage my detector: Can I wire a pot inline with the speaker? And if so, would I ground it to the negative side of the battery pack or would I ground it elsewhere? Thanks in advance!



2 years ago

Maybe. If you can find the signal path before it's amplified.

If you're working directly with the (low impedance) signal connected to a speaker, it probably requires an attenuator (rheostat) rather than a volume control. I'd expect the driving power on a small device to be somewhere around a watt (give or take 150%). A rheostat is electrically the same as a volume control, but can handle more wattage.

I suggest you look up the "Cracker Box Amp" from Make mag, it uses a 25 ohm, 25 watt rheostat (which once was available from Radio Shack, but no more, I think), for a 386 chip amp that outputs about a watt.

But you can make one from fixed resistors, clearly the little 1/8 watt resistors aren't gonna cut it with a one watt output. But 2W to 5W resistors are probably fine.

For resistor values, use the speaker impedance as a guide -- an 8 ohm speaker won't require resistance values much more than that (which is why the rheostat in the above project is only 25 ohms).

As to why a std volume control (POT) probably won't work, they are usually 1/4 to 1/2 watt. If you can find one that's rated for 2 watts, that might work. But it's gotta be a very low resistance value to work with a low output impedance. Even a 1K pot will barely have any useful range. Again -- it's why the above rheostat is 25 ohms.


2 years ago

If a cheap detector does not even have a headphone socket it might not be worth trusting it ;)
Having said that: Add a headphone socket if missing and use a headset with volume control in the line.
You can still get them from quite cheap to quite expensive for the use with TV's.
You wire it parallel to the speaker and the switch in the socket is wired into the positive speaker wire.
When you plug the headset in the speaker is off and you can adjust the volume on the headset.
If that is too loud stiil then you can add a resistor as mentioned by Jack.
The positive side effect is that you get much longer battery and usage time with headsets compared to the built in speaker.
My Minelabs might be a better model but has the same gain with a headset.
About 5 hours with speaker, over 9 hours with headset on medium volume.

Jack A Lopez
Jack A Lopez

2 years ago

Someone was asking the answers forum about this one, just the other day, here:


Typically, putting a resistor in series with the speaker is the trick for making a too-loud electronic gizmo be more quiet.

The tricky part to this trick, is figuring out the approximate size of the resistor needed, which is maybe why you want to use a potentiometer; a.k.a. pot; i.e. a variable resistor.

However the range over which a pot is adjustable is not huge, like zero to infinity ohms. It is more like 0.1 to 1.0 multiplied by the full value; e.g. a 100 ohm pot is practically adjustable from like 10 ohms to 100 ohms.

Anyway, starting out, it is hard to guess how much series resistance will make the sound as quiet as you want. So you start out with some actual resistors, not pots, powers of 10, like:

1 ohm, 10, 100, 1000,

and listen to what those sound like.

In the answer I wrote to the question I linked to above, I think I recommended a resistor about the same size as the impedance of the speaker, or perhaps one about ten times the impedance of the speaker .

Regarding your question of where to put it, like, on the left terminal, or the right one, or between the speaker and ground ( if the speaker originally has one of its terminals connected to ground)... The answer to all of these is that it does not matter exactly where you put the resistor. All that matters is that you wire it in series with the speaker. Also resistors do not have polarity, so it is impossible to put a resistor in a circuit "backwards".

Regarding the question of damage the existing speaker driver circuit, I would expect putting a resistor in series with the speaker, increasing the impedance it sees, will result in it driving less current, and less power. In other words, I expect the driver circuit to be underloaded, doing less work than it was before, and therefore running a little cooler, and happier, than before.