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My Story: Magnetic Communication Project

Going wireless the old-fashioned way

By Philip Kane

Difficulty Level: Beginner-Intermediate
Required Time: 1-2 hours

Several years ago I adapted an idea from an old electronics project book and built a wireless system that allowed me to silently listen to my hi-fi receiver from just about anywhere in my apartment. Although the reception was not hi-fi quality it was more than adequate for my purpose (I used it mainly to listen to the news through headphones while doing other things). My wireless system was based on an old idea that is still in use today. However, unlike last Jameco's feature of make's FM transmitter project, it did not require transmitting the signal over an RF carrier. In fact, I didn't need to build a transmitter at all. My hi-fi receiver was the transmitter and the "antenna" was just a length of speaker wire that encircled my living room.

Parts List

Transmitter
4W - 10W Audio Amplifier (Hi-fi Receiver, etc)
Speaker cable P/N 100280
8Ω 10W (minimum) Resistor P/N 2144593

Receiver (Audio Amplifier Module - Fig. 3)
C1 - 0.22µF capacitor P/N 25540
C2 - 0.1µF capacitor P/N 25523
C3 - 10uF electrolytic capacitor P/N 94221
C4 - 0.047µF capacitor P/N 57621
C5 - 220µF electrolytic capacitor P/N 93772
IC1 - LM386 Audio Amplifier IC P/N 24125
L1 - Telephone Pickup Coil P/N 2144585
R1 - 10KΩ potentiometer P/N 29082
R2 - 10Ω resistor P/N 690380
SPKR1 - 8Ω speaker IC1 P/N 2099577

A Simple Wireless Audio System

The block diagram (fig. 1) shows the basic elements of a very simple unidirectional wireless communication system. Notice that this system contains no RF components. The transmitter is simply an audio amplifier, (or radio, CD player, etc) connected to a large loop of one or more turns of speaker wire that serves as the transmitter antenna. The receiver consists of another audio amplifier and a small induction pickup coil for the receiver antenna. The transmitter signal can be received from anywhere inside of the loop and for a short distance outside of the loop.

Block diagram of system
Figure 1: Block diagram of system

In this simple system the audio signal at the transmitter generates a magnetic field around the transmitter loop. This field varies directly with the intensity and frequency of the audio amplifier output. When the receiver coil is introduced into the field, a voltage is induced across its windings. The voltage across the receiver coil varies with the frequency and intensity of the changing field.

Orientation of Transmitter and Receiver Coils

The magnetic field strength decreases relatively quickly as the distance from the transmitter coil increases. Also, the relative orientation of the transmitter and receiver coils determines the strength of the signal at the receiver. For example, minimum coupling occurs when they are orthogonal (at right angles) to each other (fig. 2a).

Orthogonal
Figure 2a
Coplanar
Figure 2b

Maximum coupling, at a given distance, occurs when they are oriented in the same direction (fig 2b).

DIY Induction Loop System

The Transmitter

For the transmitter, I used an old 10W stereo hi-fi receiver with external speaker connections. The transmitter antenna was a length of speaker cable (if you are using two conductor speaker cables, twist the wires together at each end of the cable). I attached one end of the cable to one terminal of a speaker connection (fig. 3). I attached the other end of the cable in series with the impedance matching resistor to the other terminal of the same speaker connection. The impedance and power rating of the antenna loop should match that of the transmitter. For example, if the transmitter output is rated at 10W into 8Ω then use an 8 ohm resistor with a power rating of at least 10W or greater. One note of caution, the impedance matching resistor can get hot. Handle it carefully while the transmitter is operating. Keep it away from material that is flammable or can melt easily.

Stereo hifi receiverFigure 3: Stereo hifi receiver

The Receiver

The receiver is an audio amplifier built around an LM386 audio amplifier chip (fig. 4). The version used in this circuit (LM386N-1) has a supply voltage range of 4V to 12V. In this circuit, amplifier gain is fixed and set to maximum by connecting capacitor C3 between pins 1 and 8. Potentiometer R1 controls the output level. Input coupling capacitor C1 should be selected so that, at the lowest frequency of interest, its impedance is small compared to R1 (about 1/10 of R1). If you find that you are getting significant interference from a local radio station (especially when the input is unconnected) try placing C6 between the input and ground but may not be required.

Circuit schematicFigure 4: Circuit schematic

The receiver antenna is a telephone pickup coil (fig. 5).

Telephone pickup coilFigure 5: Telephone pickup coil

Construction and Testing

You can first assemble the receiver circuit on a solderless breadboard for initial testing (fig. 6).

Receiver amplifierFigure 6: Receiver amplifier

A permanent version of the receiver amplifier can be constructed using a general purpose prototyping board with through holes and solder pads. This will help to reduce component wiring. In order to reduce power line hum, RF noise etc., make sure that component leads and connecting wires are as short as possible. The LM386 should be mounted on sockets. To eliminate the possibility of damaging it while soldering. Remember to observe the proper polarity for all electrolytic capacitors.

Note: the National Semiconductor data sheet for the LM386 indicates that high gain applications might require a bypass capacitor between pin 7 and ground. I found that it was not required for this application.

To test the system, start by checking all connections. Make sure that the transmitter and receiver antennas are connected properly. Place the receiver inside the transmitter antenna loop. Connect the pickup coil to the input of the receiver amplifier. Connect an 8Ω speaker or low impedance earphones or headphones to the output of the receiver amplifier. Turn on the receiver and set the volume control to between one quarter and one half full volume. Power up the transmitter amplifier and set the output to a minimum level. Now, gradually increase transmitter output level until you can hear the signal at the receiver. If you notice power line interference try changing the orientation of the receiver coil.
Phil Kane has been a technical writer in the software industry for more than 10 years. He has also occasionally authored articles for electronics enthusiast magazines.

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