I was wondering whether quick testing of various commercial ISM band transmitters for operation can be accomplished with some sort of off the shelf universal receiver which only turns on a led upon getting anything in the ISM band. Think key fobs, wireless plugs and thermometers, etc. as transmitters.
Maybe something like this thinggie - https://www.ebay.com/itm/DC-12V-1-CH-433MHz-Relay-Switch-Receiver-Module-for-Garage-Door-LED-Light/274727253469?hash=item3ff70535dd:g:G8gAAOSwkp9gVcOq ?
Does the key fob emit a spread spectrum signal ? Oh, I had no idea. Ok, let's stick to OOK, FSK at most. @Brian K1LI's solution is actually viable. IT was about 5 years ago when I took one such receiver, plugged it in to my PC's sound board through a 100nF cap and surely enough, Audacity showed up my signal: different digital modulations interference. Does it happen?
I think that with a bigger cap / with this cap and a tiny led, what I am after can be done. But... I want it already built. User should have to only power on the device, maybe adjust a variable cap and watch the led.