Moving coil meter with audio signal

[Tom41]

One time when I was young I had an old-style moving coil meter, that I think was set up to measure voltages. I decided to try putting an audio signal from a cassette player into it. I was expecting the needle to bounce around like the level meters on audio equipment, but something quite unexpected happened. The needle barely moved up the scale, but started to vibrate reproducing the audio like a miniature loudspeaker! Putting my ear on the meter I could clearly hear the audio from the cassette.

Any idea why this happened? Perhaps I would have been better with an AC voltmeter?

[T30M31]

Re: Moving coil meter with audio signal

The meter movement is very much like a speaker. The big difference being that the coil in a speaker is connected to the diaphragm and in the meter it is connected to the needle. From what you described the moving coil meter was most likely set up to measure DC and because of this the audio which is AC would only cause the needle to vibrate which resulted in the sound output from the meter movement. An AC voltmeter might have behaved the way you were expecting if it had a frequency response that extended into the audio range.

[budm]

Re: Moving coil meter with audio signal

You will need to convert your AC (audio, audio will have to be rectified for the meter to show the average DC or if it has cap, then it can show the peak readings) into DC for the meter movement, look at the terminals of the meter, it should have +/- label or red dot (for +) on one of the terminal. If the has needle at rest siting at the center of the scale, the the needle will move left or right depends on the polarity of the signal. Just think of it as a motor with magnet as stator and the coil as the rotor.
http://en.wikipedia.org/wiki/Galvanometer

http://en.wikipedia.org/wiki/VU_meter

[Longbow]

Re: Moving coil meter with audio signal

No matter what the dial face on the meter says, all meters of this type are ammeters, and specifically d.c. ammeters. Once the internal resistance of the meter is known, the value of the meter shunt is calculated to give a full scale reading appropriate for a specific application. Have a look at any device with simple metering systems. Power supply output meters look at the voltage drop across a low value shunt resistor. For a.c. measurement, a rectifier circuit must be used, and the meter face fabricated to read the equivalent RMS, average or peak values.