Mechanical engineer Florian Wilhelm Dirnberger has tried out a design for building a "very basic" radio frequency (RF) detector — using a Texas Instruments LM1458 dual op-amp and a speaker to turn RF energy into audible sound.
"The concept of this radio frequency detector (originally advertised as [a] 'Radar Detector') is anything but new and schematics are floating around in books and on the internet for decades," Dirnberger writes of the project, "but barely anyone ever seem to have actually built this particular device so I decided to test whether it is a viable design (it is, sort of)."
The detector is built with minimal components, the central of which is the TI LM1458 dual op-amp with a handful of resistors and capacitors. There's no microcontroller, and nothing that would be recognizable as a radio receiver — the detector instead working on the principle of rectification. As an RF transmitter — whether intentional or otherwise — nears the circuit, the energy enters the input of the op-amp along with the intended signal.
While such radio-frequency interference is usually notable only for how you can remove it from your design, in this case it's the whole purpose of the circuit: picking up RF signals from the aether and amplifying the interference they create to generate a new signal which can be fed through another amplifier and into a speaker.
"Capacitor C1 is the determining part here. Shortening its pins may or may not increase the performance drastically (tests ongoing). The value itself is not so very important (should be in the nF range though)," Dirnberger writes. "You can e.g. connect a LM386 amplifier and a speaker at the OUT Pin, but maybe you guys have a better suggestion what to do with the output signal."
The full project write-up is available on Dirnberger's Hackaday.io page, along with a schematic for the circuit.