cell phone detector circuit
As Andy Aka has pointed out this is an alternating electric field detector. I'm no expert on radio but this circuit is easy enough to understand.


U1-A is wired as an inverting amplifier. R1 and R2 bias the + input to half-supply voltage (BAT+ / 2).
R3 is the amplifier feedback resistor but the usual inverting mode input resistor is missing. Therefore the input antenna will provide alternating current to the virtual earth of the inverting input. For every nA of current flowing towards the inverting input the output will decrease by \$ 1~nA \cdot 6.8~M\Omega = 6.8~mV\$ (to keep the inverting input at the same potential as the non-inverting input).
U1-B is configured as a comparitor. The inverting input is held at a reference voltage by sensitivity adjust pot R5. In setup this would be adjusted downwards until LED D1 turns off. (For LED off, pin 7 must be high so inverting input must be lower than non-inverting input.)
When the antenna signal is strong enough U1-A output will oscillate about the half-supply point and when it goes below the threshold set by R5 the LED will begin to light.