Building a very simple AM voice transmitter
If a crystal radio is the distilled essence of a radio,
this transmitter is the matching distilled essence of
The transmitter goes together in about 10 minutes, and
is small enough to fit in the palm of your hand.
Click on photo for a larger view
Depending on the antenna, the transmitter can send voice and
music across the room, or across the street.
I put together my first version with simple clip leads
(no soldering, no printed circuit board, not even a battery
clip). This version is much sturdier and convenient.
An AM transmitter from simple parts
Our transmitter will need these parts:
Building the transmitter
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
. 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:
Click on photo for a larger view
The transformer has two leads on one side, (red and white
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.
Putting it together
The transformer has two metal tabs on the bottom. These can
be bent out flat, so the transformer can be glued to the
printed circuit board, or two holes can be drilled in the
board, and the tabs can fit into the holes and be folded over
to hold the transformer in place. If you choose to drill
the holes and fold over the tabs, the tabs can be soldered
to the copper pads on the back of the printed circuit board
for a more secure anchor.
The transformer should be placed on the left side of the
printed circuit board, leaving plenty of room on the right
for the oscillator.
Insert the leads of the oscillator into the printed circuit board,
placing it far to the right. The copper side of the board should
be down, with the oscillator on the side without copper.
Gently bend the leads of the oscillator over, so it is held firmly
onto the printed circuit board.
Solder the pins of the oscillator to the copper foil of the printed
circuit board. Be careful not to use too much solder, or it may form
bridges of solder between copper traces that are not supposed to be
Insert the stripped end of the red wire into a convenient unused hole
in the printed circuit board (such as the bottom left hole). Insert
the red wire from the battery clip into a nearby hole that is connected
by copper foil to the first hole, so the two red wires are electrically
connected. Solder the two wires to the copper foil.
Insert the white transformer wire into a hole whose copper foil is
connected to the upper left pin of the oscillator. Solder this wire
to its copper foil.
Cut one of the clip leads in half, so you have two pieces of wire
each with an aligator clip attached. In the photo, I used two
different colors for clarity (yellow and green). Strip the insulation
from the last half inch of each piece.
Insert the black wire of the battery clip into a hole whose copper
foil connects to the lower right pin of the oscillator. Insert
the stripped end of one of the aligator clip leads into a hole that is
also connected to the lower right pin of the oscillator. Solder the
two wires to the copper foil. The aligator clip will be the ground
connection, just like in the crystal radio.
Insert the stripped end of the other aligator clip into a hole that is
connected to the top right pin of the oscillator. Solder the wire to
the copper foil. This will be the antenna connector.
Open the phone plug, and insert the blue and green wires of the
transformer into the plastic handle. The metal part of the plug
has two pieces, each with a small hole. Put one of the transformer
wires into one hole and solder it, then put the other wire into
the other hole and solder it. When the metal has cooled, screw
the plastic handle back onto the metal phone plug.
Using the transmitter
We are now ready to test the transmitter.
Plug the phone plug into the earphone jack of a convenient
sound source, such as a transistor radio, tape player, or
Plug the batter into the batter clip.
Hold the transmitter near an AM radio, and tune the radio to
1000, so you can hear the your sound source in the AM radio.
Adjust the volume controls on the sound source and on the
AM radio to get the best sound.
Without any connection to an antenna or a good ground connection,
the transmitter will only transmit to a receiver a few inches away.
To get better range, clip the ground wire to a good ground, such
as a cold water pipe, and the antenna to a long wire, like the one
we used for the crystal radio. Many countries limit the length
of the antenna you are allowed to use without a license, so check
with your local laws before using a wire more than a yard or two
For a science fair project, the transmitter and receiver can be
placed within a few feet of one another, and a short wire antenna
should be just fine.
How does it do that?
The oscillator is connected to one end of a long wire antenna.
It alternately applies 9 volts of electricity to the end of
the wire, and then 0 volts, over and over again, a million
times each second.
The electric charge travels up and down the wire antenna, causing
radio waves to be emitted from the wire. These radio waves are
picked up by the AM radio, amplified, and used to make the speaker
cone move back and forth, creating sound.
The sound source (your CD player, or tape recorder) is normally
connected to drive a speaker or earphone. It drives the speaker
by emitting electricity that goes up and down in power to match
the up and down pressure of the sound waves that were recorded.
This moves the speaker in and out, recreating the sound waves
by pushing the air in and out of your ears.
In our transmitter, the sound source is connected to the transformer
instead of to a speaker.
The transformer is connected to the power supply of the oscillator.
The sound source causes the transformer to add and subtract power
from the oscillator, just as it would have pushed and pulled
on the speaker.
As the power to the oscillator goes up and down, the power of the
electricity in the antenna goes up and down also. The
voltage is no longer simply 9 volts. It is now varying between
0 volts and 10 volts, because the power from the transformer
adds and subtracts from the power of the battery.
Power into antenna
The varying power in the antenna causes radio waves to be emitted.
The radio waves follow the same curves as the waves in the antenna.
However, because the transmitter and the receiver are not connected,
the receiver does not know what the transmitter is using for the
value of zero. All the receiver sees is a radio wave whose amplitude
is varying. In the receiver, zero is the average power of the wave.
This makes the wave look like this:
Radio waves in free space
If we sent this wave to the earphone, we would hear nothing,
because the average power is zero. This is why our crystal
radio has a diode.
The diode does a neat little trick.
A diode only lets electricity flow in one direction.
This means that the part of the graph where the power is rising
from zero can get through the diode, but the part where the
power is going down
from zero is blocked.
Electrical signal after the diode
All those little peaks of power happening a million times
per second are too fast for human ears, and too fast for
the earphone to reproduce. But since they are all pushing
on the earphone diaphragm, all those little pushes add up,
and the earphone moves. Since some of the little pushes
are stronger than others (taller blue bars in the illustration)
they move the earphone more than the weaker ones. We hear
this variation as sound.
Sound waves reaching our ears
The sound is a faithful reproduction of the original sound
wave at the transmitter.
For more information on radio, see the
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Simon Quellen Field