Detecting Mobile Phone Transmissions With a Sound Card

Anyone who had a cheap set of computer speakers in the early 2000s has heard it – the rhythmic dit-da-dit-dit of a GSM phone pinging a cell tower once an hour or so. [153armstrong] has a write up on how to capture this on your computer.

It’s incredibly simple to do – simply plug in a set of headphone to the sound card’s microphone jack, leave a mobile phone nearby, hit record, and wait. The headphone wire acts as an antenna, and when the phone transmits, it induces a current in the wire, which is picked up by the soundcard.

[153armstrong] notes that their setup only seems to pick up signals from 2G phones, likely using GSM. It doesn’t seem to pick up anything from 3G or 4G phones. We’d wager this is due to the difference in the way different cellular technologies transmit – let us know what you think in the comments.

This system is useful as a way to detect a transmitting phone at close range, however due to the limited bandwidth of a computer soundcard, it is in no way capable of actually decoding the transmissions. As far as other experiments go, why not use your soundcard to detect lightning?

Filed under: phone hacks

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