# Sending On/Off signal through RF

I want to have one circuit generate a radio frequency and have another circuit act on the presence of that radio wave, to turn a transistor ON and light up a LED for example. Just a simple as that, I don't need to send any data through the air.

I tried this by having the transmitter circuit generate AC from DC at I don't know exactly what frequency but fast enough not to see the oscillations through a LED, and making that current alternate through a long wire acting as an antenna. The receiver circuit was the one from this experiment: http://www.creative-science.org.uk/mobile_LED.html. It didn't work and I don't know if it is the transmitter or the receiver or both that don't work.

So, how do you send a simple On/Off signal through radio waves? Any schematic would be greatly appreciated.

• A cell phone operates at GHz frequencies which the loop antenna receiver in your reference was designed to receive at very close range. You don't describe your transmitter but it has to be at cell phone frequencies to work with that antenna which is probably illegal to begin with. It also has to have a power level on the order of 1 watt to be able to work with a passive antenna. You need to supply more information on what you want: what range for example. – Barry Feb 20 '15 at 1:58
• The ARRL Handbook is a good resource for ham radio circuits... not only circuits but RF techniques. Assuming you are in USA, Title 47 CFR Part 15 governs low-power, unlicensed radio transmitters. – MarkU Feb 20 '15 at 10:19

## 1 Answer

The easiest way to send an on/off signal is using On-Off Keying (OOK) which is a type of amplitude modulation. One example circuit might be

with the sine wave generated by an oscillator. If you make this transmitter you should stick to ISM bands because they do not require a license for low power transmitters. Or you could get a technical class HAM radio license which is very simple to get, at least in the USA.

To receive the signal you will need a resonant antenna at the oscillator frequency and a bandpass filter. Then amplify this signal and rectify with a diode. This output signal should match your input digital signal.

• I don't understand what a resonant antenna is or what a band pass filter is. How does the receiver circuit work? – nc404 Feb 21 '15 at 19:35