U1 LM386 audio amplifier IC L1 800 turns on a 9 × 70 mm ferrite rod (see text) S1 SPST on-off switch B1 9volt transistor radio battery PH 8 ohm headphones
Misc PC board, wire,
1/
8” phone jacks,
battery clip, case
Induction loop technology has been around for many years, for use with the hearing impaired. Induction loop communication systems were originally designed to provide assistance in the school classrooms for the deaf, but today induction loop communications technology is now used for many other applications, such as museum displays, theater communications, mine and cave communication systems, and signal tracing. Induction loop systems have been installed in churches, public halls and auditoriums, schools, lecture halls, cinemas, service counter windows, drive-thru order and pick up windows, information kiosks, offices, airports, train stations, parks, tour and guide buses, automobiles, boats, riding academies, and homes.
Induction loop amplification operates on the basic principle of electronics called magnetic induction. When an electrical current is amplified and passes through a loop of wire, a magnetic field is generated around the wire. The field varies in direct proportion to the strength and frequency of the signal being
transmitted. When another wire is placed close to the field of the first wire, an identical electrical current is induced within it. The second current can be amplified and converted into an exact duplicate of the original sound signal.
In technical language, a current is said to be “induced” in the second wire. Hence the term
“induction.” In an induction loop system, the induction loop coil in the transmitter sends an electromagnetic signal, which is “induced” to the receiver’s induction loop coil in the receiving unit. An Induction Loop System consists of an amplifier and a loop, the amplifier
is connected to a sound source such as a TV, a radio, a public address system or a dedicated microphone. It then amplifies this sound signal and sends it out, in the form of an alternating current, through the loop. The loop itself consists of insulated wire, one turn of which is placed around the perimeter of the room in a simple loop system. When the alternating current from the amplifier flows through the loop, a magnetic field is created within the room. The magnetic field “induces” the signal into the Induction loop coil in the induction receiver unit.
In the portable induction receiver, the induction coil picks up the induced signal from the transmitting loop coil. The fluctuations in the magnetic field are converted into alternating currents once more. These are in turn amplified and converted by the induction loop receiver into sound. The magnetic field within the loop area is strong enough to allow the person with the hearing aid or Induction Loop Receiver to move around freely in the room and still receive the sound at a comfortable listening level.
Many modern day hearing aids incorporate a telecoil into the hearing device. So if a hearing aid user switches to the T or MT position on their hearing aid, the telecoil in the hearing aid picks up the induced signal from an induction loop transmitter and amplifies the signal for the hearing aid user to hear. This is a convenient feature for hearing aid users, in classrooms, churches, and museums with induction coil
transmitters.
The listener can receive an audio signal without being physically or electrically connected to the loop, so in effect induction loop transmission is a form of wireless telephony. The listener receives the sound in one of two ways; either through the use of the T-coil or telephone switch on a hearing aid instrument or through an induction receiver that can work with earphone, which we will discuss in our project.
The basic induction loop system consists of a “transmitter” section and a “receiver” section, as shown in Figure 15-1. The “transmitter” section consists of a microphone or signal source, an audio amplifier to boost the signal, and a large loop or wire around the room or listening area. The “receiver” section consists of a smaller loop coil, an audio amplifier which boost the incoming audio and a pair of headphones.
“Transmitter” loop unit
You can easily build your own induction loop communication system. First we will begin by describing the “transmitter” section of the system, as shown in Figure 15-2. In its simplest form an induction loop “transmitter” can consist of a mono or stereo Hi-Fi receiver, a resistor and a large loop coil placed around the perimeter of a house, room, or your easy chair. If you have a stereo receiver with a speaker output A-B switch, you can select the (B) output for your induction loop system. You would place an 8 ohm, 10 watt resistor across the output of the left channel’s output and then place an 8 ohm, 10 watt resistor in series with a large loop coil as shown. When you wanted to use the induction loop system, you would switch to the (B) speaker output; it’s that simple.
If you wanted to construct your own amplifier specifically for an induction loop audio system, you could utilize the 6 to 10 watt audio amplifier shown in Figure 15-3. In order to drive an induction loop, you would need an amplifier with at least about 5 watts of power or more to have a workable system. The amplifier shown in this diagram would work just fine for driving a large loop coil. An audio input signal is fed into the audio input section at the point marked (A) on the schematic. Your audio source could be adjusted via the potentiometer at R1. Note, that you may need an additional pre-amp ahead of the amplifier if you use a low output microphone. You can feed a radio or TV signal directly into the potentiometer at R1 to drive the induction loop “transmitter.” The audio amplifier module shown is an 8 watt National Semiconductor LM2002 module, but you could also use a higher power module such as their LM2005. The output of the audio amplifier is wired in series with an 8 to 10 ohm, 10 watt
Chapter Fifteen: Induction Loop Receiving System
Transmitter Tx Rx Audio amplifier Microphone Large loop coil Small loop coil Receiver Audio amplifier
Figure 15-1 Induction loop communications system block diagram
Tx L1 R1 Loop coil 5–18 watt audio amplifier or stereo receiver 1-channel
resistor in order to drive the coil. This resistor also protects the output of the audio amplifier.
The inductive loop is a winding of wire through which an audio signal passes, which creates a field within which the audio signal can be detected by appropriate equipment, either a telecoil in a hearing aid or an induction loop receiver. The loop coil is basically a simple 4 or 5 turn loop placed around the room fed from an audio amplifier. The loop then converts the sound to a magnetic field that can be received by a second loop coil and amplified. Note that more turns in the transmission coil result in a stronger transmitted signal. The loop should have a DC impedance of at least 8 ohms, or you can add a resistor in series with the coil to reach 8 ohms.
Pick a path around the room on the ceiling, or under the carpet. A room measuring 15′ ×20′will have a perimeter of 70′, which will require a minimum of four complete turns of 22 gauge wire, six turns of 20 gauge wire, or nine turns of 18 gauge wire. Pick a path around the room near the ceiling, and wind (or hang) your “coil,” or loop, starting and ending at the amplifier. If the back of your amplifier says something like “8 ohms minimum,” double the required wire length (load). Buy 120% of the wire you’ll need to ensure that you can complete the loop. You should add a power resistor to ensure an appropriate load and reduce wire length, but the amplifier output will be divided between the loop and the resistor, proportionately, which means less radiated energy. This solution requires a lot of energy so try not to compromise. If required, get 4 ohms
at 10 watt minimum. There’s another and simpler approach: I have a coil built into my favorite chair. It’s only 18′′in diameter, but includes 120′of 28 gauge wire (0.071 ohms per foot). That’s about 25 turns of that relatively thin wire.
Another approach to constructing a large loop is to take a long length of 25 pair telephone cable ‘Inside Wire’ that’s long enough to go around the perimeter of a room. Then get a 66 block (or some other kind of punch down block) and punch down the wires so they connect in a multiple turn loop. In this way you can add or subtract turns as you wish if you need that later. You could tack this “telephone” cable to the ceiling of the basement underneath the room you choose.
Connect either end of the loop coil wire to the plus (+) or red terminal and the other end to the minus (−) black terminal, on the audio amplifier’s speaker terminals. Most amplifiers or stereo receivers have SPEAKER A and B switches; put your coil circuit on the unused output (B). Remember to place an 8 to 10 ohm, 10 watt resistor in series with the speaker output and the coil loop windings.
“Receiver” loop unit
The induction loop receiver, shown in the photo Figure 15-4 and the schematic diagram in Figure 15-5, is very sensitive and can be the basis for a monaural
Chapter Fifteen: Induction Loop Receiving System
Loop coil L1 Audio input (A) R1 C1 + 1 U1 + – 4 + 2 C4 C2 C3 + R2 R3 C5 R4 C6 R5 + 3 5 +6 –12 V DC
induction loop wireless headphone system. The induction coil receiver is used inside the area of transmission loop. The receiver coil at L1 consists of 800 turns of #26 ga. enamel wire wound on a 9×70 mm ferrite core.
Another approach to constructing a small loop coil for your inductive loop receiving coil is to obtain a “telephone pickup coil” if available or a suitable coil from some other device. The coil in the prototype was salvaged from a surplus 24 volt relay. Actually, two relays were needed since the first was destroyed in the attempt to remove the surrounding metal so that a single solenoid remained. Epoxy putty was used to secure the thin wires and the whole operation was a bit of a challenge. A reed relay coil will give reduced sensitivity but would be much easier to use. The experimentally inclined might try increasing the inductance of a reed relay by replacing the reed switch with soft iron. Avoid shielded inductors or inductors with iron pole pieces designed to concentrate the magnetic field in a small area or confine it completely (as in a relay or transformer) unless you can remove the iron. The resulting coil should be a simple solenoid like wire wrapped around a nail. Don’t try to wind your own—it takes too many turns. Evaluate several coils simply by listening. Coils with too little inductance will sound “tinny” with poor low frequency response, and other coils will sound muffled, especially larger iron core coils. This prototype was tested with a large 100 mH air
core coil with superb results but the 2′′diameter was just too big for this application. The induction receiver develops a 1–2 mV signal across its internal coil from the fluctuating magnetic field. Transistor Q1 provides pre-amplification to drive a headphone amplifier. The tone at R3 control varies the amount of gain in the high frequencies. Capacitors C1 and C2 block AC hum (C2 also blocks DC to the amplifier input). C3 functions as a low-pass filter to prevent oscillation. The output of the volume control at R5 is fed to the input of an LM 386 audio amplifier IC. In this configuration the amplifier has a gain of 200. Capacitor C7 and R6 form the output “boost” circuit which drives the output to the speaker through C6. The output of the LM385 at pin is used to drive an 8 ohm headphone. The induction loop receiver circuit can be powered from a 9 volt DC power supplies or batteries.
Are you ready to begin constructing the induction receiver circuit? First you will need to locate a clean, well lit work table or work bench. You will want a large table so you can spread out all the components, the diagrams and tools that you will need to build the project. Next you will need to locate a small pencil tipped 27 to 33 watt soldering iron and some 60/40 tin/lead rosin core solder for the project. Try and locate a small container of “Tip Tinner,” a soldering iron tip cleaner/dresser which conditions the soldering iron tip; this is available from your local Radio Shack store. You should also locate a few small hand tools such as a pair of end-cutters, a pair of needle-nose pliers, a pair of tweezers, a small flat-blade and a Phillips screwdriver and a magnifying glass.
The induction loop receiver prototype was
constructed on a small glass-epoxy circuit board which measured 21/4′′ ×3′′. A circuit board makes for a more
reliable and professional circuit. Place all the diagrams in front of you on the work table along with all of the components for the induction loop receiver and we will begin building the project.
The induction loop receiver project contains about four resistors and two potentiometers. Locate the resistors using the color chart in Table 15-1, this will help you identify each of the resistors. Place the resistors into their respective locations on the circuit board, then solder them in place on the PC board. Locate your end-cutters and trim the excess component
Chapter Fifteen: Induction Loop Receiving System
leads, by cutting them flush to the edge of the circuit board. Potentiometer R3 is a CP board mounted type, you can go ahead and install it on the board and then solder it in place; you can remove any excess leads if necessary.
Next we are going to move to installing the capacitors for the project. There a two distinct types of capacitors— small ceramic, mylar or polyester capacitors and larger electrolytic types. The small capacitors will most likely have a three-digit code stamped on them and not their actual value, since often the capacitor body is very small. Refer to the chart shown in Table 15-2, which illustrates the capacitor codes. You can search through the parts pile and try and identify the capacitors against the code chart and place the correct value into its respective location on the circuit board. After all of the small capacitors have been placed on the PC board, you can solder them in place. Remember to cut the excess leads from the components. Now, locate the larger electrolytic capacitors and place them in front of you and determine where they will go on the circuit board. Once you are certain where the capacitors should be placed on the board, you should take another look to make sure that you have installed the capacitors correctly with respect to its polarity. If an electrolytic capacitor is not
installed correctly the circuit will not work properly and may be damaged upon power-up. So you will want to install them correctly the first time through.
Note, there are a number of capacitors in this project, and in looking over the schematic, you will notice that a few of the capacitors will have polarity marking on them. These capacitors are electrolytic types and you must observe the polarity when installing them on the circuit board in order for the circuit to work properly. Electrolytic capacitors will have either a white or black band at one side of the capacitor body or they may be marked with a plus (+) or minus (−) marking on the capacitor body and this denotes the polarity. Refer to the schematic and parts layout diagrams when installing electrolytic capacitors in order to install them correctly with respect to their polarity markings.
Before we go ahead and install the semiconductors, take a brief look at the diagram depicted in Figure 15-6. The semiconductor pin-out diagrams will help you orient the semiconductors when building the induction receiver. The induction loop receiver also incorporates one transistor at Q1 which is connected to R1/R2 and R4 and C2. Transistors generally have three leads protruding from the bottom of the body. A Base lead,
Chapter Fifteen: Induction Loop Receiving System
B1 – + +9 V R2 R1 S1 C1 L1 Q1 C3 C2 + R3 R4 C4 R5 C5 1 6 3 2 4 5 8 + U1 + – + C6 C7 R6 Phones
a Collector lead and and an Emitter lead. Referring to the schematic you will note that the vertical line on the transistor symbol is the Base lead, while the Collector lead is connected to the junction of R1 and R2. The Emitter lead has the small arrow pointing downwards which connects to the junction of C2 and R4. Refer to the actual transistor pin-out diagram to see where each lead exits the transistor case. Generally small plastic TO-92 transistor case bodies will have all three leads in-a-row with the Emitter at one end, the Base lead in the center, and the Collector lead at the opposite end of the device.
The induction loop receiver has one integrated circuit at U1, this is an LM386 audio amplifier chip, which has eight leads in a dual in-line package. Prior to installing the IC, you should install a good quality IC socket. An IC socket is good insurance in the event of a possible circuit failure at some point in time. Since most people cannot successfully un-solder an integrated circuit without damaging the PC board, an IC socket makes good sense. Place the IC socket on the circuit board and solder it in place on the PC board. Locate the LM386 and take a look at the chip. You will note that the IC package will have either a cut-out, a notch or small indented circle at one end of the IC package. Pin 1 of