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Phone Catcher
Is your phone really off after you turn it off, or is it still
sending out information about you? This project will
detect transmissions from your cell phone.




Photo 1. The LED lights and stays lit until reset to
indicate that the telephone has transmitted a signal.


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Overview
I have had and used other devices that detect radio signals for years, so I have seen my telephone emitting signals at times other than those at which I was using it to talk with somebody.  This Cell Phone Catcher lights an LED indicator light and whenever the phone transmits, so even the briefest of transmission can be seen. The LED stays on until press the RESET button.

Using this Cell Phone Catcher as shown in
Photo 1, I have noticed the old original iPhone not only transmitting to the nearest cell when making a call, but also when the telephone is switched on for the day's use or switched off, upon entering Airplane Mode and upon leaving Airplane Mode,  sending and receiving a Text Message, and occasionally the LED comes on for no apparent reason. I suspect that when the light comes on for no obvious reason, that the phone has been "pinged" by the cell to see if it is still where the phone company thinks it is. One thing that I suspected but had not confirmed until making this Cell Phone Catcher is that when I use the phone to connect with the internet through the phone system, after I quit the web browser

To use the Cell Phone Catcher, I place it on the small loop antenna and wait for the LED to come on. While waiting for the phone to transmit, the current drawn by the circuit is only about 2.5 microamps, which is insignificant to to the AA cells that I us for power, so I did not even bother with an on/off switch.

The Circuit



   Schematic 1.   

The main parts of the circuit are shown in Schematic 1 are a small loop antenna with it output connected to a sensitive two transistor detector/ latch that then drives an LED, a 3 volt battery, and a reset button.

The two transistor latch is made with an 2N3904 NPN transistor and a 2N3906 PNP transistor connected in a way make makes the pair operate like a Silicon Controlled Rectifier, except it all happens in two pieces of silicon instead of one piece, which results in a higher voltage across the device. Both transistors drive the other one on so that once they start conducting, they circuit latches on.

Current induced in the looop antenna is fed to the bases of both transistors. When the current is sufficient (when the average rectified current is approximately 5 microamps, the transistor pair latches on. Q2, the 2N3904 sinks current through the LED and its current limiting resistor. Resistor R2 provides up to 1.8 milliamps to keep the pair of transistors latched.

The LED is red so that it doesn't not require very much voltage across it to operate, which allows the circuit to work at lower voltages as the battery runs down.

The reset button shorts out the base drive to Q2, which results in the pair of transistor "unlatching" so the LED goes off.

The detector (Q1 and Q2) is not tuned, although the sensitivity decreases as frequency increases, frequency dependency is most likely to come from the antenna. Using a very small loop should keep the bandwidth broad.




Click to see and hear the circuit description




Photo 2. The circuit uses just a few easy-to-obtain parts. I used 1/8 watt resistors but 1/4 or 1/2 watt resistors, if you have them, should work well too.

The small handful of parts cost me very little. As you can see from Photo 2 there aren't many parts to put together. A circuit board is not needed, but using some sort of circuit board, even a piece of cardboard with holes to pass the component leads, would make the circuit more rugged.

When interconnecting the components, just keep the component leads as short as possible. On my board, all connections except one were made by bending component leads toward the lead it needed to be soldered to and only one small jumper was needed.

Using those plastic breadboards, the kind in which you push component leads and wires into the plastic socket-like block might work, but we are talking about UHF and Microwave frequencies -the parasitic inductance and capacitance might cause either poor sensitivity or other problems. If you can't tell, I don't like those breadboards much.