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ill titleHome-Brew AM Transmitter

The simplest low-power AM transmitter you can build yourself.

ill_1

Occasionally one wishes to hear something on the radio other than what is currently being offered.

A situation I am often faced with whenever the cricket comes on the ABC.

In looking around the "interweb" I found the essentially same circuit on two different sites and set out to build one, but each of the designs had a fundamental flaw that would impede the performance of the transmitter.

So I provide here for your edification a complete and error free circuit that will give you the best transmission strength from such a basic design.

circuit diagramA photo showing a completed transmitter built on a generic circuit board.

The circuit uses a crystal oscillator to generate a 1 Mhz carrier wave upon which we will transport our music or other programming.

Marrying the crystal with an impedance matching transformer and a battery or DC power supply gives you a very simple circuit that can be connected to a CD Player, iPod or what have you and with an antenna of about 3 to 4 meters will broadcast to an area about the size of your house and yard and not much further.

Tuning your radio to 1 Mhz somewhere in the middle of the dial will tune in to the signal sent from the little transmitter.

circuit diagram

The crystal was by far the hardest part to find. Other sites provided details of suppliers in the US but here in Australia I eventually tracked one down from R.S. Components.
The actual RS Components part number is 471-8856. It should cost you a little under $8.00 AU.

The Pin for connecting to your headphone socket on your CD Player, iPod etc is easily obtainable from Dick Smith Electronics or can be snipped off a cheap pair of headphones.

Likewise Dick Smith can provide you the 8 - 1000 Ohm transformer. Part no. M 0216(Ask for "the Red One") and the 9 volt battery clip, switch etc.

Follow the circuit above and you can't really go wrong. The two errors (or omissions) I found and which you should avoid are as follows:

A couple of points for clarification.

  1. Use a generic circuit board to build the transmitter on.

    Start with the Transformer and Crystal, put the transformer on the left side of the board and the crystal up close to its right. You will notice that 3 corners of the crystal are rounded and one has a right angle. The right angle denotes pin no. 1 and should be at the bottom left as you look at the top of the crystal. Pin one doesn't connect to anything in this circuit.

  2. There are only 4 pins on the crystal even though they are numbered 1, 7, 8, and 14. These numbers equate to modern 14 pin Integrated Circuit sockets but have no real bearing on how we use them here.
  3. If you are using a stereo pin you will want to connect the two separate channels to one transformer wire and the common to the other to "mono-it-out". This transmitter will only broadcast in mono, which is all your old radios would pick up anyway.

    Be sure to connect it to the wires on the 8 ohm side of the transformer.

  4. Other circuits on the internet don't include a switch but if you want to package it all up neatly in a project box a switch is very handy otherwise you'll need to pull the battery on and off all the time.
  5. This circuit is based on designs found on the internet. If you'd like to see how they do it and read more about how the transmitter works please visit Sci-Toys.Com and Science Buddies where I originally sourced the circuit.

circuit diagramHere it is all tucked away in it's box.

In future years, as the AM stations move to FM or even digital, you'll still be able to enjoy your old radios by re-transmitting broadcasts using this simple transmitter. But for now I get a kick using it to tune in to an MP3 recording of a Winston Churchill speech and trying to convince my friends that my old radio still picks up 70 year old broadcasts.