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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 ?

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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.

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    $\begingroup$ Welcome to hamSE and thanks for joining us. Looks like the item you cited will do the job. What need does it not fulfill for you? $\endgroup$ – Brian K1LI Apr 12 at 9:48
  • $\begingroup$ @BrianK1LI Thanks. I am not sure whether it accepts just anything in ISM band 433 MHz or specific codes from some well known transmitter chips $\endgroup$ – kellogs Apr 12 at 9:50
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It is not generally feasible to detect "anything". The problem is there's always something, even if it is just noise. To know if something is signal or just noise you need to know something about the signal, but when you want to detect "anything" this means you must enumerate every kind of signal that could exist, which is of course not feasible.

Your best bet is likely a cheap SDR, like many of those based on the RTL2832U. With such a device you can at least receive whatever signal and/or noise may exist in the ISM band and display it on a waterfall, and from there make some guesses about the presence or absence of a signal.

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  • $\begingroup$ I don't want to go back to the SDR... oh well $\endgroup$ – kellogs Apr 12 at 16:10
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    $\begingroup$ @kellogs then you must specify more about what kind of signal you are looking for. It is not enough to specify just "OOK or FSK", because even there, you can run noise though such a demodulation algorithm and you always get something, just random bits. Usually there's a checksum or a fixed pattern or something devices use to differentiate between signal and noise, but you haven't specified any of that. $\endgroup$ – Phil Frost - W8II Apr 12 at 20:59
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Here's an example of an inexpensive unit that might meet your requirements:

enter image description here

The receiver module, on the right, has part number XD-RF-5V. According to several internet searches for "XD-RF-5V datasheet," the system uses Amplitude Shift Keying - ASK. Inasmuch as the modules are intended for connection to an Arduino or other digital means for decoding, this implies that on/off from the transmitter produces on/off at the receiver's DATA pins. A YouTube video demonstrates this simple mode of operation using a breadboard.

Whether you can visually detect the presence of a particular input signal at the receiver by observing an LED may depend on the rate, duration and occurrence frequency of the transmitted signal. But, it would be a simple matter to connect the receiver to an Arduino and write a simple sketch to latch an LED "on" when a DATA pin goes high, or latch it with a simple IC.

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  • $\begingroup$ These circuits could surely help with it, but they are not an off the shelf solution. $\endgroup$ – kellogs Apr 12 at 12:25
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    $\begingroup$ these circuits are relatively insensitive, and they very likely won't be able to detect a spread-spectrum or frequency hopping signal, both of which exist within the 433 MHz ISM band. Also, say, a key fob transmits for milli- if not just for microseconds. Not quite sure how quickly you can look at an LED? If you have some latching logic, how do you tell a noise impulse from a transmit signal? $\endgroup$ – Marcus Müller Apr 12 at 14:18
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    $\begingroup$ yes, but that doesn't solve the problem: this thing simply isn't sensitive to most ISM-band transmission types more than to noise – so no matter what you do, you're just getting a noise floor or signal presence detector, which you cannot calibrate without the hardware that OP is missing – so you never know if you're not sensitive enough (even for the couple of modulations this might actually detect) or just constantly misdetecting noise as signal emission. $\endgroup$ – Marcus Müller Apr 12 at 17:21
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    $\begingroup$ That's a bit of the core problem of communications technology – telling one's signal apart from noise, and detecting it correctly with a high probability. A pure amplitude observation (it's actually a power observation, these things are not great as ASK modems, OOK does work – with great SNR) doesn't "do" for knowing whether your transmitter actually transmits something, or you're just picking up noise (and especially considering the frequency selectivity – or lack thereof – of these receivers, you can just as "guess" whether it was transmitting), I'm very afraid. $\endgroup$ – Marcus Müller Apr 12 at 17:22
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    $\begingroup$ you're still making assumptions on the signal structure – either you're too restrictive and miss relevant signals, or you're too open and stand a high chance of misdetecting noise as signal. $\endgroup$ – Marcus Müller Apr 12 at 18:52

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