Can I use a contact microphone to detect a fan running / not running inside an electronic product?

Question

I'm involved in thermal testing of an electronic product which contains a small centrifugal fan. The product is entirely sealed and no signals can be transmitted through the case when it's operating.

We need to be able to (passively) verify that the fan turns on when the temperature goes above 30 Celsius, and then goes off again when it falls below 20 Celsius. The only variable we have control over is the temperature (using a laboratory grade digitally controlled oven).

When outside the oven the fan is just barely audible if the room is quiet. My hope was that I could bond a musician's contact microphone to the metal case exterior right near the fan and somehow see it's signal change each time the fan turns on and off.

When in the oven, the sound of the fan is drowned out by the oven's ventilation system.

I have been looking into contact microphones and it appears that they are mostly immune to air-borne sounds (e.g. the oven's noises), and are mostly sensitive to physically coupled vibrations (the fan noise, through the thin aluminium case).

I was thinking of using an oscilloscope to read the signal from the contact mic, but I will need some sort of amplifier I think. What kind of amplifier would be suitable?

I don't need to listen to the sounds directly, just viewing the waveform would be enough I think.

Is this feasible?

I'm thinking of using something like a TE Connectivity "CM-01B" (Datasheet) although it's a bit expensive. This might be overkill, but it does say it's good for use as a stethoscope, which sounds promising. I could run it off a few AA cells on short wires and have the scope probe connected near to it.

Further info:

The fan draws very little current compared to the overall system so I can't simply watch the total current draw change. The system draws current in rapid and unpredictable ways, the fan's current would be way down in the noise.

Answer 1

If your microphone is at the typical microphone levers (-60dBu to -40dBu, ie 1 to 8 mV), then you need a good scope to measure it properly (and no long non shielded cables). If you want to work with an entry level scope, then you will need an amplifier (excepted if your microphone has higher output levels than -40dBu).

The "tricky" part will be to extract the signal from the fan from the rest of the noise.

The first thing I would try is to use the FFT mode of your scope, and see if you get a signal at the frequency of the rotation of the fan and/or the frequency of the PWM used to drive it when the fan turns on.

If you don't have an FFT function on your scope, or if the signal is still lost in noise, then you might try to add a pass-band filter around the frequency of the fan/pwm in order to remove all other signals : this way, you can probably see the amplitude of the sine directly on the scope