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.
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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.
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kill_them_now2 months ago

hi guys,i have an idea..can someone
program an android app that combine SDR Touch( an apps that can receive
am transmission offline with an RTL 2832 U driver,

) with this computer controlled radio transmitter and it software?so that we can have a pocket android am transceiver!!!..

please make it free or less than 1$ for the sake of am lover :)

kill_them_now2 months ago

hi guys,i have an idea..can someone program an android app that combine SDR Touch( an apps that can receive am transmission offline with an RTL 2832 U driver,

) with this computer controlled radio transmitter and it software?so that we can have a pocket android am transceiver!!!..

please make it free or less than 1$ for the sake of am lover :)


Can i use this as a Remote controller for a RC plane kinda things?

Could this be used to connect a Transmitter to a pc to use on a simulator?
i only have this one short two pin crystal from an original xbox mother board, would this work? and if not, would it be the same if i used a simple 555 oscillator circuit?
Using a xtal oscillator in the amateur radio bands could make a very nice QRP beacon
Joshua_ag1 year ago
can i use those small oscillators found on computer motherboards for making this?
Joshua_ag1 year ago
on the computer where should i connect that 9 pin serial jack?
ratheesh4u22 years ago
I want to transmit a particular text, just one word continously.
Maximum range is required.
Any possibility?
Please answer...
I badly needs this

This word later I will use it to display in a receiver.

bluescope934 years ago

Just a Question how do u send the Data to the Frequency Oscillator if all you are supplying to it is Power? , do you have a CW Key to turn the +9 Volts on and Off therefore creating a Morse Code Format?? , Or do u send the Audio for a Mono 3.5 or 6.5 Plug into a Radio or Computer and play a Tone or music??

Cheers and 73's
Turning on and off the oscillator via the computer's DB (De) 9 port IS keying the oscillator. It can be unmodulated - - CW mode -- or modulated. In this case the computer program handles the selection and transmission of those modes.

Turning on the Carrier Wave - and then off - to form Morse code characters, is the very definition of CW mode.

Joe, N2QOJ
An idea I had: Have the computer output go into the mic input of a walkie talkie. Then, have the "Brother" Walkie talkie have headphone input into my friend's computer. Would this work? Also, for a different project, how could I change this program's source code to make it output on usb port instead of serial? I don't want to use an adapter; just change the source code.
would you please release the source code? i have no idea how to use various i/o ports on a system, and i dont want to sord through pages of dissasembly to find my love it if you would release the source :D
bmlbytes (author)  abamailinator3 years ago
I didn't write that program.
Could you attach a microphone instead of the serial thing to send sound? I want to send sound around my town.
This reminds me of something. Oh yeah there was this other site that has the same pics and stuff. 
just checked this out, i wonder how hard it would be to implement it with a arduino?
Oh, and uh does anyone know the legal limit of broadcasting signals from point A to point B in the Philippines? Otherwise, I'm making a pocket morse code transmitter!
martzsam3 years ago
Will this program work with windows 7?
qazxsw210004 years ago
To those wanting to send audio (as in music/voice): You will need an audio transformer. The transformer has 5 leads: 3 on left, 2 on right. The 3 lead side: connect the audio jacks ground on bottom, input on top leaving the middle lead free. The right side: batteries + supply on bottom lead, the top lead connects to the oscillators input pin. What you used as the ground should be the same. Just plug it into the computers "audio out" plug. To transmit music, play it with whatever music player you like. Plug it into a mp3 player/ipod/cd player and take it with you where ever you go. To send voice, plug mic into pc "audio/mic/line in" plug.
HOMEPIE646 years ago
wait is it possible to use voice instead of morce code becuase you said something about a male to male audio jack and i would prefer using voice becuase morsecode is kind of hard to translate while typing for an ametuer
You would need an audio transformer. With the 3 lead side on the left, connect the jacks ground on bottom lead. Then connect the other jacks terminal on the top lead (the middle will not be used). On the right side (2 lead), connect the battery to the bottom lead then the top lead connects to the oscillators input pin. I believe that's how it went.
I have a 4 pin 1.8432 Mhz oscillator.
Question 1). Can I decrease the frequency to within the AM broadcasting range?
Question 2).When Transmitting Morse code using the application mentioned in the article, if I set it to AM Low tone will it broadcast at half the frequency of 1.8432 Mhz.

Radio Electronics
brakeman754 years ago
NOTICE!!! Anyone can listen to anything, but a F.C.C. LICENSE IS REQUIRED FOR TRANSMITTING ON AUTHORIZED FREQUENCIES. PLEASE consult the american Radio Relay League (ARRL) for help getting started in this fascinating hobby. I don't want anyone to start on the wrong foot. Have fun!!
Brakeman75 a.k.a. KC2SXT, Tom
 Gday Tom

Totally agree with you there, i already hold my license and i have a 4 pin Crystal on 7.030MHz and did it all in the same format except where the antenna connects i used a SO239 Connector and used my Diamond CP6 HF Vertical QRP With AM and made a contact 523 KM's away 

I also was thinking of this project and thinking about the reflected power and what the SWR would be like 

Cheers 73's
lazer1554 years ago
 How do I wire in the led?  I read on that it is some how connected to pin 7 on the serial port but it doesn't say which wire on the led goes to pin 7 and it doesnt say where the other wire goes.
Pin 7 is prolly a signal output and therefore a positive output. Connect the LEDs long leg to ground and the short leg to pin 7.
I'm doing this off the top of my head so I might be wrong.
owenaero4 years ago
very good way to practice the mores code good job
joinaqd4 years ago
LOL. another guy made the same thing on another web site.

check out this website(its my second favorite website after instructables)
Win7Maniac4 years ago
Great project. Is there a program like this one that will send music out the serial port like a radio broadcast? Could you use a 1.0000mHz oscillator and tune a normal radio into 1000kHz and listen to the music? Thanks Ryan
Buggy2855 years ago
Can you transmitt music and hear it on an am/fm radio?
bmlbytes (author)  Buggy2855 years ago
See my other instructable "Make a Simple AM Transmitter"
kroq-gar785 years ago
1. how would you receive a 24.3MHz signal with a radio? my pocket radio only goes up to ~1300KHz
2. my pocket radio is (# of MHz) * 100 , not (# of MHz) * 1000. I think that's how a lot of pocket radios are to save space
Athlon7 years ago
I can't find this oscillators with 4 pins, only with 2 :S Help pls!
buy them at click on oscillators, not crystals
bmlbytes (author)  Athlon7 years ago
Look in old computers and desolder them. They will be found on cards that control things like serial ports, ethernet jacks, modems, etc.
ya but those are usually around 40 mhz
They sell em in the catalog section of
kroq-gar785 years ago
is there any way to send music (or anything else) to the transmitter through the computer?
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