Make Your Own Low-Power AM Radio Transmitter

Building the Circuit

Before we get into the step-by-step instructions for building the circuit, we'll first go over the circuit design and show you how the solderless breadboard works.

Figure 2, below, shows the connections you need to make to build the circuit. The transformer isolates the music player from the rest of the circuit, couples the music player and the crystal oscillatory, and "steps up" the signal voltage from the music player in proportion to the ratio of 1 kohm to 8 ohms. The stepped up signal from the secondary coil of the transformer modulates the power to the oscillator chip (+ power at pin 14 and − power at pin 7). A wire connected to the oscillator output (pin 8) serves as the antenna for broadcasting the amplitude-modulated radio wave.

Figure 2. Simple AM transmitter circuit diagram. The square corner of the oscillator corresponds to pin 1. The pins are numbered according to standard positions for a 14-pin integrated circuit.

Figure 3, below shows a small breadboard. The breadboard has a series of holes, each containing an electrical contact. Holes in the same column (examples highlighted in yellow and green) are electrically connected. When you insert wires into the holes in the same column, the wires are electrically connected. The gap (highlighted in orange) marks a boundary between the electrical connections. A wire inserted in one of the green holes would not be connected to a wire inserted in one of the yellow holes. Integrated circuits, such as the oscillator used in this project, should be inserted so that they span the gap in the breadboard. That way, the top row of pins is connected to one set of holes, and the bottom row of pins is connected to another set of holes. If the integrated circuit was not spanning a gap in the breadboard, the pins from the two rows would be connected together (shorted), and the integrated circuit wouldn't work. Finally, the two single rows of holes at the top and bottom (highlighted in red and blue) are power buses. All of the red holes are electrically connected and all of the blue holes are electrically connected. These come in handy for more complicated circuits with multiple components that need to be connected to the power supply. If you have never used a breadboard before you may want to take a look at a beginning breadboard activity, Electronics Primer: Use a Breadboard to Build and Test a Simple Circuit, before you start this science project.

Figure 3. An example of a solderless breadboard. The highlighting shows how the sets of holes are electrically connected. The red and blue rows are power buses. The yellow and green columns are for making connections between components. Integrated circuits are inserted to span the gap (orange) so that the two rows of pins are not connected to each other.

Now let's build the circuit!

1. Connect the terminals of the phone plug to the 8 ohm side of the transformer. You can either use alligator clips or a soldering iron to do this. See Figure 4 below for an example. Note: in the kit, alligator clips are used rather than soldering.

Figure 4. The terminals of the phone plug should be connected to the 8 ohm side of the transformer either by soldering or using alligator clips. In this picture the phone plug has also been plugged in to an iPod. The iPod serves as a music source.

2. Insert the 1 MHz oscillator across the gap in the breadboard, so that pins 1 and 7 are on one side of the gap, and pins 8 and 14 are on the other. You can identify pin 1 of the oscillator because it is next to the square corner (the other three corners are rounded). Be careful not to bend the pins. See Figure 5 below.

Figure 5. The oscillator should be inserted across the gap in the breadboard.

3. Use the breadboard to connect the positive and negative terminals of the battery holder and the 1000 ohm side of the transformer as shown in the diagram and in Figure 6 below. Note that the 1000 ohm side of the transformer has a center tap which is not used in this project.

Figure 6. The positive and negative terminals of the battery holder are connected to the breadboard (top). Then the 1000 ohm side of the transformer is wired into the breadboard and the antenna jumper wire is added (bottom).

4. Connect a long jumper wire to the output of the crystal oscillator (pin 8). This will serve as the antenna. See Figure 6 above.

5. Double-check to make sure that all of your connections correspond to the circuit diagram.

6. Figure 7, below, shows a photograph of the completed setup including an iPod for generating the music and an AM radio receiving the signal.

Figure 7. The completed circuit looks like this. In order to test the circuit you will need to connect the phone plug to a music source, for example an iPod as shown here, and use an AM radio to receive the signal.

Experimenting with the Circuit

Now that you have built the circuit, here is the fun part—experimenting with it!

1. Connect the phone plug to the output (headphone) jack of your mp3 or CD player and tune your AM radio to 1 MHz. Bring the antenna within an inch of your radio antenna. Can you hear the music that you are playing on your mp3 or CD on the radio?

2. Now tune your AM radio to a different frequency say 700 kHz. Can you still hear your music?

3. Tune your radio back to 1 MHz where you can hear your music. But this time remove the 1 MHz crystal oscillator and in its place put the 1.2288 MHz oscillator. Can you still hear your music?

4. Without changing the oscillator back to 1 MHz, instead tune your radio now to 1.23 MHz. Can you hear your music?

5. Use 1 MHz crystal oscillator and tune your radio to 1 MHz. Adjust the volume control of your mp3 or CD player, is there any change in the quality of the sound you hear in your radio?

6. Until now you have kept your antenna within an inch of your radio antenna, now move your transmitter's antenna further away slowly and hear what happens. Does the quality of your sound improves or gets worse? Why?

7. Rotate the radio receiver antenna relative to your transmitter's antenna (or vice versa). Does this affect the quality of the sound? Why?

8. Try using a longer wire for the antenna. Does this affect the quality of the sound? Does this affect the broadcast range for your transmitter? Why?