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I would like to use a radio receiver to turn a device on and off.

The idea is that when the signal is received, the device is turned on, and when the signal stops, the device turns off.

I want to use an antenna and a tuner to receive the radio signal.

I am assuming these components are all that is needed to receive the signal, and that amplification of the signal is not necessary - is this correct? If the device is turned on by detecting the presence of the signal, and turned off by detecting its absence, is it necessary to process the signal (i.e. amplify, demodulate) once detected?

As long as the signal is received, will it matter how strong/weak it is?

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    \$\begingroup\$ In general, yes - it might be worth discussing which band, and thus licensing, power and range issues. \$\endgroup\$ – pjc50 Feb 7 '13 at 14:53
  • \$\begingroup\$ Note that a dumb system will also be turned on by any sufficiently powerful transmission of the right frequency. If you use 2.4GHz in an urban area it will probably be on most of the time. \$\endgroup\$ – pjc50 Feb 7 '13 at 14:55
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It is possible to use the power of the transmitted signal to run something, as long as you keep in mind that this power will be very limited. A "crystal radio" works on this principle. The power driving the headphones actually comes totally from the received radio signal.

With a sufficiently strong signal and good antenna, you should be able to rig up something like a crystal radio that turns on a transistor when the demodulated amplitude is high enough. There are various options, but ultimately power from elsewhere will have to make use of the transistor that is turned on/off directly by the received radio signal.

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  • \$\begingroup\$ If one could manage a high enough "Q" in the receiver and there weren't too many stray signals, it could be helpful to have a circuit which could power down until it saw something resembling a valid radio signal; a crystal-radio-equivalent should have no trouble producing enough power to switch a mosfet. The big problem would be that even if one could "probably" cut system power from e.g. 1mA to 1uA in the absence of an input signal, it would be hard to guarantee that stray noise wouldn't cause one to draw 1mA a significant fraction of the time. \$\endgroup\$ – supercat Jan 31 '14 at 18:28
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From experience I'd say that you can, but this will be very unreliable and you'd end up getting something else anyway. I'd really suggest a simple TX/RX system, for example using these parts:

TX Module RX Module

They are very simple to use and with a very small micro (arduino, launchpad) you can do anything you want. This is a very cheap solution as well.

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