Wednesday, March 31, 2010

Building a computer controlled radio transmitter

How would you like to send text messages to your friends without wires, and without an Internet connection, and without paying monthly fees?

In this project we will build a very simple radio transmitter that you attach to a serial port on your computer. The computer then runs a free program that converts words you type into radio signals that are decoded by another computer, using a cheap radio receiver, and a sound card.

With a little study, you don't even need the second computer, since the radio signals are in Morse code, which anyone can learn to decode in their head with a little practice. It also comes in handy as a secret language, or as a way to send long distance messages with a pocket mirror.

Click on photo for a larger picture

The computer controlled transmitter needs these parts: (We carry most of the necessary parts in our catalog.)

* A one megahertz oscillator
You can use other frequencies if you have a radio that can receive them. We carry this item in our

* A serial port connector
We use a 9 pin RS232 connector. You can take apart an old serial cable, or buy a new connector from an electronics or computer store. We carry this item in our catalog.

* Some insulated wire for an antenna
Just about any kind of wire will do, the longer the better.

* An alligator test lead
This is a piece of wire with alligator clips at each end. We carry this item in our catalog.

For our first transmitter, we will connect the parts with alligator clips. This lets us quickly change frequencies by replacing the 1 megahertz oscillator with an oscillator with a different frequency. Later we will show a version made with a socket for the oscillator, a printed circuit board, and a light emitting diode that flashes morse code along with the oscillator.

Click on photo for a larger picture

The first step is to cut the test lead in half. In these photos I have cut two test leads, one red and one black, to make it easier to see where the connections go. But unless you are making two transmitters (your friend wants to send messages back, doesn't she?) you can just use one test lead (cut into two pieces).

Remove a little insulation from the cut ends of the wire, and solder one of the cut ends to pin 5 and the other to pin 4.

Pin 5 of the serial port connector (the black wire in the photo) connects to the ground pin of the oscillator. Pin 4 of the serial port connector goes to the power pin of the oscillator. The drawing shows the transmitter from the top (pins pointing down). The photo below shows the oscillator upside down, with the pins facing up.

Click on photo for a larger picture

The green alligator clip attaches to the antenna, which can be any long wire. It is attached to the output pin of the oscillator. The remaining pin of the oscillator (the one nearest the sharp corner) is not used.

Your Computer Controlled Transmitter is now complete!

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