Build a Computer Controlled Radio Transmitter





Introduction: Build a Computer Controlled Radio Transmitter

This will allow you to send morse code through radio waves to another computer. This could be used as a sort of chat program.

Step 1: Gather Supplies

You will need very few supplies:

-A crystal oscillator (will talk about which kind in the next step)
-A 9 pin serial jack (D-SUB9)
-A little bit of wire
-Something to use as an antenna
-A radio
-A male to male audio jack (unless you know morse code)

Step 2: Which Oscillator?

To determine what kind of oscillator you want you will need to know what kind of radio you have.

Regular AM/FM radio - We will be using only the AM part. The numbers on the dial are frequencies in KHz. Most oscillators are marked in MHz. You will probally want something in the .3MHz(300KHz)-1.5MHz(1500KHz) range.

Shortwave radio - Go with whatever frequency your SW radio can play. My radio has 3 bands. One is a KC (KHz or AM) band and the others are MC (MHz). I use a 20MHz oscillator.

Step 3: Wire Some Stuff Up

You will need to solder a wire onto pin 4 of the serial jack. This one should be red to make it easier to work with later but not nessesary.

You will also need to solder a (black) wire to pin 5 of the serial jack.

Step 4: Wire Up the Oscillator

Take the red wire (from pin 4) and connect it to the pin directly above the black dot on the oscillator. Next, take the black wire (from pin 5) and connect it to the pin to the right of the black dot.

Step 5: Add Antenna

Now add an antenna to the pin opposite the black dot. You can use pretty much any insulated wire as an antenna, just as long as it isn't too thick. I used a 25 foot telephone wire.

Step 6: Send Some Morse Code

Download the morse code program (linked below).
Connect the serial jack into your computer.
Start MorseCode.exe
Type some stuff in.
Click the radio menu on the top.
Click transmit.

-You may have to choose a different COM port from the "Com Port" menu.

-If your using a SW radio you can select the CW setting by clicking the radio menu and selecting CW

Your radio is now transmitting.

Step 7: Receive Some Morse Code

There are to ways to do this.

-Learn morse code

-Use a computer: Find a radio.Tune to your freqency. Use a male to male headphone plug to connect the headphones part of the radio to the microphone jack in the computer. Download and install When running it SHOULD (doesn't work well for me) start to decifer the morse code. Read the manual to figure out how.

Step 8: Congrats

Your all done.

A few tips:
-Get a good antenna. I heard about someone who did this with a 24.3MHz oscillator and a 10 meter antenna and sent his signal from California to Texas. I haven't tried testing mine outside my house yet, but it works pretty well here.

-Get a higher frequency oscillator if possible. If you have a SW radio then you can to this, otherwise stick with AM. If you use AM though be warned that your messages won't be private. Anyone can pick up an AM signal.

-Check your frequency. If your frequency is in an amatuer radio range then you will need a licence to run it. If it is in an AM range, you can only send your signal a few blocks (I think it's a mile) without a licence.

Hope you can use this to send secret messages to far away friends.


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i dont have the db9 connectors on laptop,only have audio combo jack and usb,hdmi for connecting peripherals.can this software output to audio output port???? so that i can output it to a qrp pixie kit??

This is a really cool instructable, and when i find a 4 pin crystal, I want to try it. Only problem I have is that I do not have a db9 plug on my computer :/ I only have de15 (VGA plug) is there any other plug I could possibly use for this? I have done some research, and I do not think that a VGA plug has a DTR pin used in this instructable. Thanks!


How's legality of that? Is it below that 0,03W that everyone is allowed to transmit? If not, how can i possibly lower it down? Using a resistor?

so for the wire, i could use a headphone set?

More ideas for transmitters here using that crystal oscillator.

Hey there! Wondering how different this would be to use a usb cable instead of the serial jack.

Wondering if tablets with microusb cables might work as well? Would be cool to make your own wifi tech. I don't think it'd be much of a leap to code full html into morse code, so you could even have browser to browser kinda fun.

A USB connection would require conversion to a connection that can be digitally switched.. (On/Off).. the Serial port is a good idea, but I'm only questioning the choice of connection.. DTR usually swings from at worse -15 to +15V (As does ALL signals for the RS-232C Standard), though most now are within the -5V to +5V range.. Not sure how the oscillator would handle the -5V swing. (yes, it does go negative, turning the GND connection into the + voltage.) Doing so could harm the oscillator's internal circuitry.

Aye, a little over my head but thanks for the reply.

What do you mean by a convert to a connection that can be digitally switched? Are you saying that there needs to be an intermediary between the oscillator and the computer? Is this a single component or a whole circuit that's needed?

Could you use a diode to make sure the power stays in one direction, keeping the ground negative? Does this swing voltage only apply to the serial jack or would this also apply to the USB connection?


3 DATA + (is this similar to data 4 DATA OUT on the serial jack?)

Sorry for so many more questions! Thanks for replying tho!

What you need is a digital-to-analog converter (or DAC for short)

Here is what he means:

An rs232 serial port has a two voltages: -14v (low) and +10ish volts (high). That is where the -15v and +15v is from. If you use the serial port to activate the gizmo, you are feeding -14v for the ground, and +10ish for the power, which is 24ish volts, which is way to high for this oscillator (it is probably 5v).

The circuit needs (should? maybe should?) be built using "ttl" level signals (0v to 5v).

An example of the "converter" that could be used is the "maxim 232cpe", which converts the -14v to +5v and the +10ish volts to 0v. This is called "reverse ttl" but works out pretty well most of the time, only in reverse.

USB is 0v and +5v, and so would likely work "better" for this circuit, as it is sort of ttl-ish to begin with.

Another option is to fire the circuit of a "parallel" port (old school printer port) as those are already ttl level and have 12 output bits to play with, 3 of which are inverted.

Another option to limit the volts is to put a zener diode on each signal line, and use it to clip the voltage to a reasonable level. That way you won't overdrive the oscillator.