This is a simple but fun one. I'm using a La Crosse weather radio. Set to AM, go to quiet frequency, then you have to get the radio within at least 3 or less inches to any kind of electrical device and you hear a very loud and ugly static noise, occasionally what sounds like some kind of clear modulating tones on some devices. One of my LCD's had a weird signature that you'd be able to pick out among my others. I tested turning off my iPad and seeing if it was still on (I think it still is but you can't really tell), and I tried it on my phone charger. When you unplug the charger, for 5 seconds or so I guess a cap is still discharging and the noise continues then promptly shuts off.
The primary use case I see for something like this would be to check that your laptop/cellphone is really off and not transmitting after turning off/removing battery (so security use cases). Of course you could get unlucky if a malware has some pre-programmed random times to transmit and you miss it. Just put your device in a static bag and metal box (padded w/ foam for audio muffling) if that's a problem for you.
Got some pics of the radio internals, this is a very hacker friendly device (but quality could be a bit better). I did spot a 1N4148 diode up around the light, as well as some transistors, and a 100K resistor (10K's are other places), that's the AM detector circuit I suspect.
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| What the front looks like |
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| When you first open case |
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| Flipping over the PCB |
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