I built this simple 40 meter CW transmitter with a 7.200 MHz Crystalenter image description here

I could hear it on a short wave radio, but my goal is to use it for control purposes.

I want to control a digital HIGH/LOW from a receiver lighting up an LED or Buzzer.

How could this task be simply accomplished?

  • \$\begingroup\$ 7.2 MHz is in the 40 meter band, not the 80 meter. At these frequencies interference will be a fact of life (at certain times of day/11-year solar cycle), so you will need a robust detection scheme - probably looking with a correlator for some very specific sequence, but beware that regulations may limit what sorts of information you can transmit; generally VHF frequencies (or today ISM) are used for line-of-sight remote controls. \$\endgroup\$ Commented Sep 6, 2013 at 23:07
  • \$\begingroup\$ Ok. I edited it to 40 instead of 80. I got confused \$\endgroup\$
    – skyler
    Commented Sep 6, 2013 at 23:11
  • \$\begingroup\$ How far away is the receiver intended to be? Why don't you buy a matched pair of transmitter and receiver - these are not expensive things AND, bear in mind that you can't just attach a button to the transmitter and a LED to the receiver and expect any form of reliable operation; to send "reliably" you may need to use a coded transmission and have a decoder attached at the receiver to avoid spurious noise and other transmissions giving false demands to switch on or off the LED. \$\endgroup\$
    – Andy aka
    Commented Sep 6, 2013 at 23:16
  • \$\begingroup\$ Not very far away, just throughout a house \$\endgroup\$
    – skyler
    Commented Sep 7, 2013 at 0:55
  • \$\begingroup\$ And i don't really care about reliability, this is solely for experimenting purposes. \$\endgroup\$
    – skyler
    Commented Sep 7, 2013 at 0:56

2 Answers 2


By International regulation, I don't believe the 40M band is allowed to be used for control purposes. Among other things, you must be licensed and identify yourself periodically. You need to change to a frequency where control signals are allowed e.g., 318MHz or 433MHz. I think some portion around 27MHz is the lowest frequency where radio control signals are allowable.


First, keep in mind that 7.2 MHz is not a frequency where you are legally allowed to do this. However, if you're just experimenting, keep the power low, and keep the antenna small, you can probably get away with this in practise. The more are away from a city the better because there is a smaller chance of someone running accross your signal, getting annoyed at it, and complaining to the authoroties.

To have the transmitted carrier turn something on/off, you need a matching receiver. Giving you a whole receiver circuit here would be too much. Look around for crystal-based receiver circuits. Since you are transmitting at a known and well-controlled frequency, you can use a crystal as the narrow band receiving filter. That will cut down a lot of noise.

Once you have a receiver, you amplitude-demodulate the carrier, then run the demodulated signal into a comparator to decide whether it is receiving a real signal or just noise. The comparator output is then used to energize a relay, or whatever else you want to turn on.

Keep in mind that the receiver will likely get the occasional false positive. This is due to enough ambient noise in the receive band. This will happen more as you tweak the threshold in the receiver to catch a more distant signal from your transmitter. Eventually the signal from the transmitter will be so weak that to receive it at all will cause the receiver to randomly react to all kinds of stuff.

The usual solution for rejecting unwanted signals is to send some digital information that includes a checksum. The receiver then only reacts to packets with a valid checksum. As you get too far from the transmitter, the receiver will then just fail to react instead of reacting to whatever noise is on the band.

  • \$\begingroup\$ At the right time a few hundred milliwatts of "experiment" can be heard on the opposite side of the globe on 40 meters. \$\endgroup\$ Commented Sep 7, 2013 at 16:41
  • \$\begingroup\$ Is it still bad if I transmit my Call sign in Morse before transmitting the data, and what if I am using the LED as a Morse receiver for someone to see the message? \$\endgroup\$
    – skyler
    Commented Sep 7, 2013 at 20:08
  • \$\begingroup\$ That's more a question for the ARRL or a ham forum. However, if you want to do it, maybe you should tackle the difficult part of the problem first, and try to build a receiver which will indicate when one of the known low-power CW propagation beacons can be heard. You may want to look at mixing to audio, sampling with a sound card, and doing software filtering. \$\endgroup\$ Commented Sep 7, 2013 at 21:10
  • \$\begingroup\$ @Chris: A few 100 mW under just the right conditions with a good antenna, maybe. I was thinking more around a mW with a small piece of wire as the antenna. In reality, nobody is going to come after you for that because they mostly won't notice, care little if they do notice, and won't want to bother taking the time to localize the signal. That is a good reason for not leaving the transmitter on for very long at a time, though. \$\endgroup\$ Commented Sep 7, 2013 at 22:43

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