This circuit uses a 1458 dual op-amp to form a radar detector. C1 is the detector of the radar signal. The first op-amp forms a current-to-voltage converter and the second op-amp buffers the output to drive the piezo transducer. R5 sets the switching threshold of the second op-amp; normally it is adjusted so that the circuit barely triggers on background noise, then it's backed off a bit. The response of the circuit may be tuned by adjusting the length of the leads on C1. For typical road-radar systems, the input capacitor's leads should be about 0.5 to 0.6 inches long.

Email received form a visitor: The circuit on the following link seems to be fake. Detection/reception on GHz range requires the employment of specialized point contact microwave diodes (usually cartridge type) placed inside the resonant cavities with feed horn connected to the opening of cavity. I am surprised what this "Economy radar detector" detects with the open long leaded capacitor which can never ever be a replacement of any diode. Detection of radar signals, specially the police radars for vehicle speed detection requires highly sensitive input stages as the incoming signal is usually in the range of micro watts or below depending on the distance of radar and vehicle due to the inverse square law.

Answer to visitors email: Built the circuit out of curiosity today (cost $10). First test tonight it triggers when I use a mobile phone across the room, can't wait to test it properly tommorrow on the highway. Will keep you posted but the fact that a noisy mobile triggered across the room is a positive for a $10 circuit that I expected to be a total dud. Further refinement of this cct could be an interesting project. A better way of tuning the input than the length of cap 1 lead which in this case is soldered into some additional length of vero bd. Playing in the Ghz range is a new experience, will still be surprised if this ever has practical use.
Scott Nth QLD