1. Description

The concept of this radio frequency detector is anything but new, and schematics with the LM1458 op amp have been shared in electronic books and on the internet since I think the 80's and 90's. 

Yet barely anyone ever seem to have actually built such a "Radar Detector", so I wanted to know whether it is a viable design (it is, sort of).

2. Schematic

Capacitor C1 is the determining part here. Shortening its pins may or may not increase the performance drastically. The value itself is not so very important apparently (should be in the nF range though).

One can connect a LM386 amplifier and a Speaker at the OUT Pin for an immediate feedback (below is a further schematic), but maybe you guys have a better suggestion what to do with the output signal (see example oscillogram photo).

The first op amp stage together with the two 100k resistors resemble a supply splitter (what makes sense for we supply the detector with a 9V battery, not with a symmetrical voltage), the second is an inverting amplifier.

C3 and C4 provide capacitive coupling.

3. Demo video

Tests with various radio frequency emitting devices, such as a Raspberry Pi Pico W, a Bluetooth Low Energy module, and a mobile phone.

4. "Radar detector"

Military and civil radar operates (or used to operate) also in the lower GHz regions (L-Band, S-Band, etc.). Example: Airport Surface Detection (X-Band).

So it would be possible to detect these high frequency sources with my RF detector, at least in principle (haven't tested that yet).

5. Sound generator

For the Speaker variant, the following set-up with one LM386 IC is applicable. The generated sound doesn't have to be pleasant, just loud enough to be heard, so we can ignore most of the design guidelines regarding the LM386.