Sunday, 13 February 2011

Building a very simple AM voice transmitter

The oscillator is the heart of the transmitter. It has four leads, but we only use three of them. When the power is connected to two of the leads, the voltage on third lead starts jumping between 0 volts and 5 volts, one million times each second.

The oscillator is built into a metal can. The corners of the can are rounded, except for the lower left corner, which is sharp. This indicates the where the unused lead is. The lead is there to help hold the can down firmly on the printed circuit board, but it is not connected to anything inside the can.

The other main part is the audio transformer . In this circuit it is used as a modulator. The modulator changes the strength of the radio waves to match the loudness of the music or voice we want to transmit.

A pictorial diagram of the transmitter looks like this:

A photograph of the completed transmitter is shown below:

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Click on photo for a larger view

The transformer has two leads on one side, (red and white in the photo ) and three leads on the other side (blue, black and green in the photo). The two leads are the low impedance side of the transformer, (the 8 ohm side). The three leads are the high impedance side (the 1000 ohm side). The middle of the three leads is called the center tap, and we won't be using it in this circuit.

To get the best range, we put the low impedance side of the transformer in series with the oscillator. This means that the signal source must be capable of driving heavy loads, like an 8 ohm speaker.

If you are trying to use a weaker signal source, such as an iPod or some other MP3 player that can only drive 32 ohm earphones, you will want to reverse the transformer, so that the 1,000 ohm side is in series with the oscillator, and the 8 ohm side is connected to your signal source. You will get slightle less range, but your odds of getting some modulation of the signal will be much better.

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