I want to make a mobile call/sms detector

[sherial]

I have always been noticing one thing that whenever I put my mobile near my pc speaker....when the phone rings(incoming calls actually),speakers generate noise.That means these pc speakers detects the frequencies and return an oscillating voltage...I tried to detect incoming call with a wire that i put on my mobile but i found nothing detectable in my multimeter(neither ac nor dc).I built a transistor based amp but it didnt return anything hopeful.then i built a non inverting amp but that also failed...now i want to know why my speaker is detecting that and my long wire is not.Thnx for reading my post.

 

[dougy83]

It's very easy to detect the mobile phone transmissions. A simple coil of wire (maybe 10 turns) connected between the base and emitter of an NPN transistor will e.g. allow the transistor to turn on an LED whenever the phone rings.

EDIT: at least it was very easy for an old GSM phone on OPTUS network back when I tried...

 

[WTP Pepper]

I did just this for a work project when I wanted to detect when a GSM modem was making a call.
I wound 10 turns of 20SWG wire around a pencil and placed an LED in series with a PIN diode HP 5082-3188 (very fast switching diode to rectify the GSM high frequency and I had some in my parts box) across the coil.
Placed within 20cm to the aerial and it works a treat.

In the UK it used to be a fashion where you could buy such a device and place it around the aerial in the 90's. Cost about 1UKP from the market. Now most phones have the aerial integrated in the phone casing so it's now not physically possible.

 

[Diver300]

Most accidental cellphone detection comes from non-linearities in the circuit. Although the voltages that you can pick up are quite large, they are very high frequencies (900 or 1800 MHz in Europe, 1900 MHz in the US) so very small capacitances will short them out.

You should start with a high speed diode, or with a transistor that has a transition frequency (ft in the datasheets) that is at least 1 GHz if you want to get any gain at all at GSM frequencies.

The buzz that you get on the speakers is because the GSM phones only transmit in 58 μs bursts, 216 times a second. The amplifiers behave differently during the burst, and so the repeat rate of the burst is clearly audible.

 

[Mr RB]

The amplifier in the PC speaker has high gain and a wire to the PC probably a few feet long. You can make a similar RF/EM energy detector with a LM386 amp and a single FET front end to give high gain, and some type of wire antenna connected to its input. You should get a few circuit from googling "LM386 high gain amp", "LM386 microphone amp", "LM386 big ear" etc.