Make Your Own Low-Power AM Radio Transmitter


Objective

The goal of this project is to build a simple AM radio transmitter and to test its broadcast range with a radio receiver.

Introduction

Electromagnetic (EM) radiation is pretty much all around us. For example, light is electromagnetic radiation and so are x-rays. When you listen to an AM or FM radio station, the sound that you hear is transmitted to your radio by the station using EM radiation as a carrier—radio waves. Electromagnetic radiation is a propagating wave in space with electric and magnetic components. In a vacuum, electromagnetic waves travel at the speed of light.

Electromagnetic waves such as light, x-rays, and radio waves are classified by their frequency or wavelength. For example, EM radiation at frequencies between about 430 THz and 750 THz can be detected by the human eye and are perceived as light. EM radiation at frequencies ranging from 3 Hz to 300 GHz are classified as radio waves. Radio waves are divided into many sub-classifications based on frequency. AM radio signals are carried by medium frequency (MF) radio waves (530 to 1710 kHz in North America, 530 to 1610 elsewhere), and FM radio signals are carried by very high frequency (VHF) radio waves (88 to 108 MHz).

So how does a radio wave carry sounds such as voice or music to your radio receiver? The radio station broadcasts a carrier wave at the station's assigned frequency. The carrier wave is modulated (varied) in direct proportion to the signal (e.g., voice or music) that is to be transmitted. The modulation can change either the amplitude or the frequency of the carrier wave. The "AM" in AM radio stands for "amplitude modulation," and the "FM" in FM radio stands for "frequency modulation." A radio receiver removes the carrier wave and restores the original signal (the voice or music). Figure 1, below shows graphically how amplitude modulation works.

Illustration of amplitude modulation of a carrier wave by a signal.
Figure 1. Illustration of amplitude modulation of a carrier wave by a signal. The top diagram shows a carrier wave at a set frequency and amplitude (green) and a signal to be broadcast (red). The signal is used to modulate the amplitude of the carrier wave. The bottom diagram shows the resulting output signal (blue). Note how the peaks of the output trace (its envelope) follow the form of the input signal. (Wikipedia contributors, 2006a)

In this project, you will make a simple low-power broadcast circuit, using a crystal oscillator integrated circuit and an audio transformer. You can connect the circuit to the headphone jack of a portable music player (e.g. mp3, CD or cassette tape player). You'll see that you can receive the signal through the air with an AM radio receiver. Although the circuits used in radio stations for AM broadcasting are far more complicated, this nevertheless gives a basic idea of the concept behind a broadcast transmitter. Plus it is a lot of fun when you actually have it working!

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

  • electromagnetic radiation and waves,
  • electromagnetic spectrum,
  • wave model,
  • speed of light,
  • wavelength,
  • frequency,
  • amplitude,
  • crystal oscillator,
  • transformer,
  • amplitude modulation,
  • heterodyne.

Bibliography

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • 2 crystal oscillators, notes:
    • Each oscillator should be at a different frequency, within the AM broadcast band (0.53 to 1.71 MHz in North America, 0.53 to 1.61 MHz elsewhere).
    • For use with the solderless breadboard in this project, you want the 'full can' package.
    • Suitable oscillators are available from online suppliers:
  • solderless breadboard (e.g., Radio Shack 276-175),
  • 1000 ohm to 8 ohm audio transformer (e.g., Radio Shack # 273-1380),
  • 1/8 inch mono phone plug (Radio Shack # 274-286A),
  • a 6 V AA battery holder (holds four batteries),
  • four 1.5 V AA batteries,
  • a set of alligator jumpers,
  • jumper wires for breadboard.

Experimental Procedure

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 your music player from the rest of the circuit, and also amplifies the signal from your music player. The amplified 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.

Simple AM transmitter circuit diagram.
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.

Example of a solderless breadboard.
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. Use two alligator jumpers to connect to the terminals of the phone plug. (If you have a soldering iron, you can solder connecting wires instead.)
  2. Connect the other ends of the alligator clips to the 8 ohm side of the transformer (red and white wires).
  3. Insert the 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.
  4. Use the breadboard to connect the + (red) and − (black) terminals of the battery holder and the 1000 ohm side of the transformer (blue and green wires) as shown in the diagram and in the photo below. Note that the 1000 ohm side of the transformer has a center tap (black wire) which is not used in this project.
  5. Connect a long jumper wire to the output of the crystal oscillator (pin 8). This will serve as the antenna.
  6. Double-check to make sure that all of your connections correspond to the circuit diagram.
  7. Figure 4, below, shows photographs of the completed setup and a detail view of the circuit on the breadboard.

Photo of the completed circuit on the breadboard.

Detail of the completed circuit on the breadboard.
Figure 4. The top photo shows the completed setup, including the music input source (portable tape player) and an AM radio receiver. The bottom photo is a detail view of the completed circuit on the breadboard. On the Radio Shack audio transformer, the blue and green wires are the 1000 ohm side, and the red and white wires are the 8 ohm side. We used small pieces of masking tape on the transformer tabs to hold it in place on the breadboard. The 8 ohm side of the transformer is connected to the phone plug (yellow oval). The 1000 ohm side of the transformer is connected to the positive terminal of the battery pack ("+6 V") and the oscillator, pin 14 (blue oval). The black wire from the transformer is a center tap from the 1000 ohm side and is not used in this project (no connection needed; we taped it off to the side to keep it out of the way). Pin 7 of the oscillator is connected to the negative terminal of the battery pack ("ground"). The wire from pin 8 of the oscillator is the antenna.

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?

Variations

  • Try receiving the signal from your AM transmitter with a crystal radio that you build yourself. You can explore how the relative placement of the receiving and transmitting antennas affects signal strength at the receiver. To see how to build a crystal radio receiver, see the Science Buddies project Rock'n'Roll Radios.
  • Try using a 9 V transistor radio battery instead of 4 AA batteries. What differences do you notice in the signal?
  • Advanced. If you have access to oscilloscope in school, try to see the signals coming out from the antenna with your mp3 turned OFF and then ON. Also connect the +6V of the battery directly to the oscillator bypassing the transformer and look at the signal. What difference or similarities do you see between these three signals?
  • Advanced. This is an extremely rudimentary transmitter and therefore the sound quality is not going to be good. However, you could add more blocks to the present circuit and make improvements. What could you possibly add to the present circuit so that you are able move your antenna further away from your radio and still hear the music? For a slightly more complex circuit, try the following link: Bowden, B., 2006. "Micro Power AM Broadcast Transmitter," Bowden's Hobby Circuits [accessed April 12, 2006] http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#amtrans.gif. How does this circuit compare to the simpler one in the project? How does the broadcast range compare? Can you relate the difference in performance to the difference in the circuits?

Credits

Written by Niraj Subba, AMD logo

Edited by Andrew Olson, Ph.D., Science Buddies

Sources


Last edit date: 2007-03-06 15:00:00


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