Is it possible to have a circuit that does something (fires a pulse, turns on a voltage, whatever) when it detects a radio signal at a specific frequency? One crucial point is that it should do this without recourse to any sort of clock within the circuit (which rules out logic gates, standard digital ICs, etc., I believe). I would need the frequency it detects to be within unrestricted frequencies (i.e. no government restricted ranges).

I'm not too fussy about the frequency detection being super accurate, as long as it detects within a relatively small frequency band - with extra points for being able to change that frequency, though that last is not essential. I'm also not too fussy about any specifics of that radio signal (I'm fine with it being a pulse, an oscillating volume, a consistent signal, or whatever), though a reasonable degree of sensitivity to a low power signal would definitely be useful.

I'm also not too fussy about power usage, though low power usage would definitely be useful.

I mainly need to know if that is possible, though of course anything you can tell me about what that would look like would be much appreciated.


closed as too broad by Chris Stratton, RoyC, Finbarr, Michel Keijzers, Dmitry Grigoryev Apr 10 '18 at 13:58

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    \$\begingroup\$ It's possible but your question detail is severely under-constrained. Why not just buy a radio receiver and use simple circuits connected to the speaker to detect what you want. \$\endgroup\$ – Andy aka Apr 9 '18 at 10:28
  • \$\begingroup\$ Is it possible to... Yes. But is someone here going to design it for you? Nope. Also your requirements are extremely vague, "not too fussy about..." that means nothing. \$\endgroup\$ – Bimpelrekkie Apr 9 '18 at 10:29
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    \$\begingroup\$ I would suggest you look at components designed for radio controlled models. Many have on/off outputs for lights etc. as well as controlling servos. \$\endgroup\$ – Finbarr Apr 9 '18 at 10:36
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    \$\begingroup\$ Why is there a restriction on a internal oscillator? That's how this is usually done. Without justification, this just seems like a arbitrary, and therefore silly, requirement. Also, what carrier bandwidth is acceptable? What frequency range are you considering? \$\endgroup\$ – Olin Lathrop Apr 9 '18 at 11:22
  • \$\begingroup\$ @OlinLanthrop and others, 1) Apologies for the vagueness - I'm not an electronics or radio expert and it's a long time since school. 2) I asked primarily if it is possible because if it is then my larger concept should work and it is worth expanding on the overall concept with an expert. I'm not looking for free work, though I would never say no! :) 3) While I am reluctant to explain the why of it, suffice it to say having this system driven by an internal clock, so that it is a digital circuit as opposed to an analog circuit, DOES break the needs of the project for a very good reason. \$\endgroup\$ – James Carlyle-Clarke Apr 9 '18 at 12:11

Of course it is possible. Whether it is reasonably achievable with components you can source resulting in a sufficiently narrow reception band is not clear.

The first thing to look at would be simple LC tuned resonant circuits. Attach the antenna to one end of a tuned parallel LC, and ground to the other. Then amplify and detect the result.

Note that detecting a narrow frequency band is exactly what an AM radio does. It detects the amplitude so fast that the resulting detected signal is in the audio range, and it drives a speaker with that.

Selectivity is made much easier when an internal oscillator is used. Without one, the narrow selectivity and out-of-band rejection of a typical AM radio can't be easily achieved.

Look up crystal radio circuits. That's basically a tuned LC driven by the antenna as I described above. You then add an amplifier and detector to the output. These don't need any internal oscillator.

You really should examine why you don't want any internal oscillator. That would make the quality of the result better.

  • \$\begingroup\$ @OlinLanthrop, thank you so much for your reply. That's exactly what I needed to know, with some extra info thrown in. \$\endgroup\$ – James Carlyle-Clarke Apr 9 '18 at 12:37
  • \$\begingroup\$ I have to say that the key facepalm moment for me was realising that an analog radio's output is essentially just a fluctuating voltage that is transformed into fluctuating speakers and thence into sound waves. So it most definitely can be done, and with simple technology. [Facepalm!] That's what I get for spending too long in the digital world, my 13 year old self would be ashamed! But I'll leave this question here in case anyone ever wonders the same thing. \$\endgroup\$ – James Carlyle-Clarke Apr 9 '18 at 15:38

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