Introduction: Build a Computer Controlled Radio Transmitter

Picture of 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

Picture of 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?

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


Joshuaa21 (author)2016-12-17

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!

redspl (author)2016-07-01


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?

Evan81r4ison (author)2016-02-01

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

JimZ7 (author)2016-01-11

More ideas for transmitters here using that crystal oscillator.

fortsackville (author)2015-02-06

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.

Gelfling6 (author)fortsackville2015-03-06

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.

fortsackville (author)Gelfling62015-03-06

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!

marcust13 (author)fortsackville2015-12-09

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

stoobers (author)fortsackville2015-03-17

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.

Gelfling6 (author)stoobers2015-07-03

USB is a totally different standard... Think of USB as a constant conversation between the device the computer is talking to, it is sending info on what the device is, what it is capable of, happening at approximately 1.2 to 2.0MBit per second.. RS-232 was an older serial data standard. BUT... it was a fixed device.. (Not flexible like USB.) RS-232 was simple digital data sent to the device, not requiring knowing what the device was... and it ran at a far slower rate, at max 115Kbit. It also had more connections besides just Data+ & Data-.. DTR (the pin used) was like a traffic cop, telling the device to hold-on, the computer is still processing, DSR would tell the computer if there was data to receive, CD (Carrier Detect), would tell the computer if a modem was connected to the phone line. Also all of the signals were that full +/- voltage swing.. (a data=0 was +15V, Data=1 was -15V. ) the following page described the whole RS-232 standard...

stoobers (author)Gelfling62015-07-04

The rs232 standard doesn't really matter, as it isn't being used. What is being done is to hijack the DTR pin and make is deliver a dc voltage to the oscillator. What i don't get is what happens when the dtr goes negative, 15v below the rs232 ground. Maybe there is a diode hidden in the oscillator, somewhere, else the amplitude of the oscillator would just increase as the voltage swings from +10 to -15.

fortsackville (author)stoobers2015-03-17

Thank you.

I am trying to teach myself radio electronics in my spare time and it's hard to keep all the theory relevant. I am starting to think I might try a few simpler circuits, without radio, to get my understanding of theory a little more concrete before I try too far into the radio world. Anyway, what you said seems to make sense, and that is appreciated!

Gelfling6 (author)fortsackville2015-07-03

No-no! USB is totally different!! The RS-232C connection is based on an old standard, where the output was +15 (0) to -15V (1)... (See ) It would require a RS-232 to USB converter, (converts the USB port to a serial I/O port, the adaptor is available at most electronics stores.)

MohammedW2 (author)2015-11-17

hey guyz, how can i make my laptop source of a radiowave i want?

acer5050 made it! (author)2015-07-17

I'm amazed at how well this works, I'll be making another one with some filtering and a proper antenna to try and make an amateur radio contact with one of these.

30wpm with a crappy rx was no problem for this thing.

AnirudhS (author)2015-06-11

sir can i connect my laptop with the transmiter on a 15 pin port (as my laptop does not have a 9 pin port).if yes , then what connections do i need to make.

Gelfling6 (author)AnirudhS2015-07-03

I don't think that will work... What you think is a 15-pin port, is your VGA-Video socket.. The above 9-pin port is an older-style communication medium.. RS-232C. The pins being used, 4 & 5, are actually DTR (Data Terminal Ready, which told the device the computer was talking to, was ready to receive data), and Ground) (Zero voltage, frame common).. (on the even older standard, it was a 25-pin connector, Pins 20 (DTR) & either 1 or 7 (Signal & Frame Ground) Most modern laptops are strictly USB ports, and would require a USB to RS-232C adaptor.

AnirudhS (author)2015-06-11

sir i could not understand the last part. how do you receive the code using your computer?

is it possible to not use a pre made-radio and make it too?

guddu09993 (author)2015-05-01

sir i want a FM radio transmitter through which i can send signal to a receiver connected to a computer application and the computer application will place a order on the pre defined if i press a key on transmitter it should open a new file and if i press B it should close file like that.

Athlon (author)2007-04-07

I can't find this oscillators with 4 pins, only with 2 :S Help pls!

stoobers (author)Athlon2015-03-17

If it has 2 pins, it is a "crystal". if it has 4 pins, it is an "oscillator" or sometimes a "crystal oscillator". It it has 2 pins and is coated with brownish plastic, it is a "resonator".

kroq-gar78 (author)Athlon2009-06-04

buy them at click on oscillators, not crystals

bmlbytes (author)Athlon2007-04-10

Look in old computers and desolder them. They will be found on cards that control things like serial ports, ethernet jacks, modems, etc.

junits15 (author)bmlbytes2009-03-08

ya but those are usually around 40 mhz

locofocos (author)bmlbytes2007-12-29

They sell em in the catalog section of

conrad2468 (author)Athlon2009-02-07


Win7Maniac (author)2009-08-19

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

stoobers (author)Win7Maniac2015-03-17

You don't send music over a serial port, as it is a digital port. You send music over an analog port, as all sound is analog.

You could add a transistor to the oscilators output and then mix in an analog signal from an mp3 player.

radio.electronics (author)2010-01-30

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

No, for a prefab oscillator, that is too much a frequency shift. You need to buy an oscillator cut to the desired frequency.

ratheesh4u2 (author)2012-06-27

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.

stoobers (author)ratheesh4u22015-03-17

Build this:

It is a low power morse code transmitter.

You need an amateur radio license to use it.

kill_them_now (author)2014-05-29

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 :)

stoobers (author)kill_them_now2015-03-17

You don't need an ap for this.

You need an AM transmitter that plugs into your headphone port on your phone. Then you play an mp3 and it will broadcast over the transmitter.

If you want to transmit morse code, that might be a little different, as you need to turn the carrier on and off.

Also, there aren't and long, medium or shortwave frequencies that a fellow can use without an amateur radio license, so you would be limited to 100 or so feet of transmission range.

4lifenerdfighter (author)2011-07-20

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.

It would, except... Output from the oscillator would just be a dead frequency carrier, and a little out of range for the Mic input.. You could try using an audio level oscillator in the 28Hz to 3.951Khz (Well below the Crystal Oscillator range) for the transmitter side, and a simple LM567 Tone Decoder chip tuned to pick-up on the specific tone on the receiver side.. The drawback with USB, is it is not a standard On/Off signal.. USB is constantly sending/receiving data, to whatever device it is connected to. Hence the serial port.. One suggestion, would be springing for a microcontroller like the Arduino, which uses an on-board USB to TTL-Serial converter (TTL means Transistor-transistor Logic, plain 0 to +5V, Not RS-232C -15/+15 voltages),.. But, again, it would require programming the Adruino instead of the computer.

Joshua_ag (author)2013-05-13

on the computer where should i connect that 9 pin serial jack?

Gelfling6 (author)Joshua_ag2015-03-06

Covering both of your questions, You could, but I would stick to nothing higher than the 1.8Mhz Video Oscillator.

As for the 9-Pin serial jack, the original poster is using what used to be the original standard for communicating, the RS-232C serial jack, which most modern computers don't have.. But, as fortsackville had asked recently, you could use a USB to Serial adaptor dongle. (changes a USB port into the old standard.) but, I still question the direct connection to the voltages generated. The DTR (DB9 #4, Data Terminal Ready) , RTS (DB9 #7, Request To Send) pins are signals generated by the computer to tell external devices either the computer is ready for more data or to stop, while received is being processed, or that the computer is ready to start sending. They're mostly used in digital transmitting like PSK31 as the PTT (Push To Talk) switch to key-up the transmitter. DB9 #5, is a frame/signal ground, used for return back even with the RX & TX data pins

jwilkerson jr. (author)2013-06-09

Using a xtal oscillator in the amateur radio bands could make a very nice QRP beacon

Gelfling6 (author)jwilkerson jr.2015-03-06

It would make for a VERY Simple QRP transmitter! But, most oscillators are usually a fixed frequency and usually chosen outside any normal allocated radio (commercial or civilian) frequency band.. With exception of the ones that have trimmers. LOL! I wonder if anyone has done any QRP Contests with one? (DE KA1OBK)

kill_them_now (author)2014-05-29

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 :)

Rishikeshjha424 (author)2014-02-17

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

RocketPenguin (author)2013-06-27

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?

Joshua_ag (author)2013-05-15

can i use those small oscillators found on computer motherboards for making this?

bluescope93 (author)2010-01-26


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

Jasinqc (author)bluescope932012-02-01

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

abamailinator (author)2011-04-11

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)abamailinator2011-04-13

I didn't write that program.

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