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.
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.