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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, 2013 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, 2013 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, 2014 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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This is how old RC often worked, contrary to what many critics and the average scientific person thought these were not controlled by little people but used radio waves. if you want basic information about this you can look into Nikola Tesla's designs since that are the most barebone versions of RC machinery.

That said: It is quite easy to make a circuit that you can control using radio waves these days You only need to have a charge activated switch such as a mosfet, which do not need a as high wattage but only a static voltage. while flow-based like a normal transistor can work, those need a strong signal to work to full extend.

So what you need:

  1. a tuned receiver coil: you can look to old radio coils, or in the old patents and designs from N. tesla
  2. A (power) MOSFET to switch your circuit.
  3. A power source to power your circuit.
  4. A low voltage drop switching diode is helpful, and might be necessary at higher frequency(like those very cheap and small ones for low power)

Building the device

while there are many ways to do this,

connect one of the outputs of your tuned coil(connected to the antenna) through the diode to your MOSFET gate(trigger), connect a high resistance resistor from the same output of the coil to the ground and connect the gate of the MOSFET also to the power source ground with a high resistor. now connect your circuit to the mosfet and it works.

Extra info:

If you do not have too much radio pollution in your area than the tuner circuit is probably not necessary. so in that case:

You can just connect the antenna to the gate of the MOSFET(with the diode and resistors can be helpful still)

however in most areas, you'll find extreme amounts of radio pollution, so adding something to tune it either like a radio or using some of the other methods is probably recommended. (or you can test it in a metal container).

Explanation a radio signal generates a small amount of power in a coil or antenna, just like your finger can trigger an average mosfet or light a led bulb connected to the ground when you are near mains electric cables in your house(lamp cables). your antenna would send of some static voltage which is enough to trigger a mosfet, but there are other ways, especially when using surface wave radio transmitting, while at a really low frequency, it can transmit large amounts of power over high distances. so you could just make a coil or ring and use the lightning sparks between the terminals to for example trigger a plasma bridge in a vacuum or to directly drive your cirquit after stepping down the voltage with 2 simple coils(since it is ac it is simple)

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