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This receiver is designed to pick up the signal from a hearing loop and will drive a pair of headphones. You can use it with a hearing loop you install in your own home or with commercial loops already installed in halls and churches. HEARING LOOP RECEIVER By JOHN CLARKE E Connect a hearing loop to your TV or stereo system, use lsewhere in this issue we introduce the concept of hearing loops for those with hearing loss. They’re this Hearing Loop Receiver and an earbud or two – and specifically intended for use with hearing aids fitted they will be able to hear everything in the program, with no need to have the volume cranked up! with T-coils (the other article explains T-coils). But there are many people in the community who have hearing loss and, for various reasons (cost, denial and van- Our Hearing Loop Receiver It’s housed in a small case which can attach to a belt or slip ity are the main ones!) don’t own or want a hearing aid, into a pocket, so it’s fully self-contained. The user can walk particularly one of the more advanced types. This project, in fact this whole series of related projects, is around without the sudden jolt of reaching the end of a headphone lead! It’s intended for them equipped with – and anyone else a power switch, who “suffers in siCurrent Consumption............10mA power on LED, lence” (or perhaps volume control suffers in muffles!). Frequency response..............-3dB at 100Hz with stereo 32Ω headphones and of course a You might have ..............................................connected. Upper response to beyond 5kHz standard 3.5mm experienced it in jack outlet for your own house- Signal to noise ratio.............-67dB A-weighted with respect to a 400mA/m headphones or hold: someone who (with stereo 32-ohm .............field strength and VR1 at mid setting. earphones. wants the TV or headphones connected)........Noise is dependent upon background Current constereo turned up ..............................................environmental noise from mains wiring sumption is beyond everyone ..............................................and equipment about 10mA, else’s comfort level which should so they can hear it. Battery voltage indication.....Down to 7V Specifications 62  Silicon Chip siliconchip.com.au give up to 40 hours of use before the 9V battery needs to be changed (a rechargeable battery could be used). The power LED also functions as a battery indicator where its initial brightness when power is applied is dependent upon battery voltage. By now, we hope you’ve read the article in this issue on the design and installation of a hearing loop. That will give you a much better understanding of how the Hearing Loop Receiver works, so we won’t go into a lot of detail here. But if you haven’t seen that article, a hearing loop at its most basic simply consists of a loop of wire around a room, driven by a standard audio amplifier. The magnetic field it produces induces the audio signal into a coil in a hearing aid equipped with a T-Coil or in this case, our Hearing Loop Receiver. Circuit description The circuit for the Hearing Loop Receiver is shown in Fig.1. It comprises two low-cost ICs plus a handful of other low-cost parts. The magnetic field from the hearing loop is detected using inductor L1. This is actually the secondary winding of a standard Xenon flash tube trigger transformer (eg, Jaycar MM2520). Because of the very large number of turns, it has a high inductance – around 8.2mH. Best of all, it is quite cheap and is suitable for the task of hearing loop monitoring. One side of L1 is biased at about +4.05V using two 10kΩ resistors connected in series across the 8.1V supply. A 100F capacitor bypasses this half-supply. The 4.05V rail biases the output of IC1b so that its output can swing symmetrically within the available power supply rail. Tying one side of the transformer secondary winding to the +4.05V supply means that it is effectively grounded while the other end of the winding provides the signal to op amp IC1b. The DC resistance of inductor L1 is 27Ω, presenting a low source impedance at low frequencies to the noninverting input of IC1b and thereby minimising low-frequency noise. A 2.2kΩ resistor is connected in parallel with L1 to lower the inductor’s Q and prevent the possibility of oscillation. The 220pF capacitor that shunts high frequency signals to ground also assists in this. Furthermore, the input siliconchip.com.au It’s all housed inside a “remote control” case which is small enough to fit into a pocket, or clip to a belt via an optional clip. So if Grandpa forgets he’s wearing it and gets up to walk around, he won’t leave his head back in his easy chair! of each amp stage has a 10Ω “stopper” resistor to help prevent oscillation. Any signal induced in L1 will rise in level with frequency, at about 6dB per octave, because the induced voltage is proportional to the rate of change of the magnetic field. To compensate for this and to provide a flat frequency response, a 33nF capacitor across the 100kΩ feedback resistor, between pins 6 & 7 of IC1b, rolls off signal above about 50Hz at 6dB/octave. This counteracts the rise in response from the inductor. At the same time, the frequency response is rolled off below 16Hz using the se- ries connected 1kΩ resistor and 10F capacitor between pin 6 and ground. Op amp IC1a provides the second stage of gain, adjustable via the 10kΩ trimpot, VR1. In the trimpot’s minimum position, the maximum gain is 101, as set by the 100kΩ and 1kΩ resistors. Minimum gain of about 10 is available when VR1 is set at maximum. Because of VR1, the low frequency rolloff at maximum gain is 16Hz (the same as for IC1b) and 1.45Hz at minimum gain. Further frequency rolloff is provided by the 330pF capacitor across the 100kΩ feedback resistor. This rolls September 2010  63 D1 1N5819 150 +8.1V +8.7V POWER LED1 470 F 10k  K 2.2k L1 8.2mH ZD1 4.7V 10 6 IC1b 7 10 4 3 2 IC1a 220pF 100 F 100k 10k 33nF 100k 330pF 1k 1k 1 VR1 10k 10 F S1 POWER K D2 1N4148 K 9V BATTERY A 1k A IC1: TL072 8 5 A K A 470 F 470 F 27k 10 F VR2 10k LOG LEVEL 100nF 6 3 VOLUME 2 1 IC2 LM386N 8 10 5 100 F T 7 10 4 LK1 R 10 F 10 F TO 32  HEADPHONES S 47nF 3.5mm STEREO SOCKET LED 1N4148 SC 2010 HEARING LOOP RECEIVER A ZD1 A K K 1N5819 A K K A Fig.1: the electromagnetic signal radiated by the hearing loop in the building is “intercepted” by inductor L1, which is then amplified and processed by IC1 before being fed to audio amplifier IC2, which drives the headphones. off signals above about 4.8kHz. Next, the signal passes through a 10F coupling capacitor to the 10kΩ volume control, VR2. This sets the level the of signal applied to the LM386 audio power amplifier, IC2. This can provide up to about 500mW into 8Ω with a 9V supply, with distortion typically 0.2%. When using stereo 32Ω headphones, the power is about 250mW; more than adequate for headphone listening. Note that both left and right headphones are connected in parallel, via link LK1. IC2 has a gain of 20 and its power supply is bypassed with a 470F capacitor. The separate 10F bypass at pin 7 removes supply ripple from the amplifier’s input stages. A Zobel network comprising a series 10Ω resistor and 47nF capacitor prevents amplifier instability. The LM386 drives the headphones via a 10Ω resistor Controls are simple: just a power switch and volume. Most 3.5mm phones/ear buds will be fine. The large “block” at the back of the case is an optional belt clip so the unit can easily be worn around. 64  Silicon Chip and 100F capacitor. The 100F capacitor provides low frequency rolloff below 61Hz, assuming that 32Ω stereo headphones are used. The circuit is powered by a 9V battery, while diode D1 provides protection against reverse polarity connection (which is quite easy to do with a 9V battery). LED battery condition indicator LED1 functions as a battery condition indicator, as well as showing when the Receiver is on. When power is first applied, current for the LED flows through the 4.7V zener diode ZD1, the 1kΩ resistor and the discharged 470F capacitor. If the battery is fresh, the 9V battery provides 8.7V at the anode of LED1. This voltage is reduced by about 1.8V by LED1 and by 4.7V with ZD1, leaving 2.2V across the 1kΩ resistor. So LED1 lights with a current of 2.2mA. At lower battery voltages, there is less voltage across the 1kΩ resistor so the LED is dimmer. At a battery voltage of 7V, there is about 0.2V across the 1kΩ resistor and the LED barely lights. With LED current flow, the 470F capacitor charges up so that the LED current is reduced. A 27kΩ resistor across the 470F capacitor ensures that the LED stays lit but at a low current that allows it to be still visible. This indicates that the power is on and means that battery voltage testing happens only at power up. When the receiver is switched off, diode D2 discharges the 470F capacitor. The 8.7V supply is used directly by IC2 but it is fed to IC1 via a 150Ω resistor. A 470F capacitor decouples this supply and prevents any supply modulation from affecting siliconchip.com.au IC1, which could cause instability. Construction 10k 10 150 IC2 LM386 ZD1 4V7 10k 100k IC1 TL072 100k 9V BATTERY 220pF 10 F 1k 1k 330pF 4148 0 1 6 0A H CJ 27k RELPU O C/REVIE CER P O OL 5819 D1 D2 The Hearing Loop Receiver is LED1 constructed on a PC board coded S VR2 T 01209101 and measuring 65 x R S1 86mm. The PC board and comCON1 ponents are housed in a “remote 100nF control” case measuring 135 x 70 LK1 x 24mm. Panel labels attach to the 100 F front edge of the box and on the 470 F 10 F 47nF front face. The PC board is designed to fit 10 F onto the mounting bushes within 10 the box. Make sure the front edge of the PC board is shaped to the cor100 F 470 F rect outline so it fits into the box. It can be filed to shape if necessary L1 using the PC board outline shape as a guide. 33nF This PC board can also be used to 10 470 F build the Hearing Loop Neck Loop 1k Coupler (which we will describe + – 2.2k in a future issue) since most of the VR1 + 10k parts are the same. However, there 10 10 F will be a few unused component holes in the PC board for the Hearing Loop Receiver. Begin construction by checking the PC board for breaks in tracks or shorts between tracks and pads. Repair if necessary. Check the hole size for the PC board mounting and for the 9V battery leads. These are 3mm in diameter. Assembly can begin by soldering in the two PC stakes, followed by the resistors. Use the resistor colour code table and/or a digital multimeter to help in confirming the resistor values. The diodes can now be installed, mounted with the orientation as shown. IC1 & IC2 can be now be installed, either directly on the PC board or mounted on DIP8 sockets (which makes removal easier if necessary). When installing sockets or ICs, orient them using the notch positioned as shown. Install the 2-way header (LK1), followed by the capacitors. Make sure the capacitors are placed on the PC board so their height above the board is no more than 12.5mm, otherwise the lid of the case will not fit correctly. Electrolytic types must be oriented with the shown polarity. Trimpot VR1 and inductor L1 are next. Note the third wire of L1 is soldered to a spare pad on the PC board. Switch S1, potentiometer VR2 and the 3.5mm stereo socket can be soldered in next. LED1 mounts horizontally but at a height of 6mm above the PC board. Bend its leads down 90°, 12mm from the base of the LED, making sure the anode lead is to the left. To install the 9V battery clip, firstly pass the wires through from the battery compartment side of the case and loop the wires through the holes in the PC board. This secures the wires ready to be soldered to the PC stakes. Four 6mm M3 screws secure the PC board to the integral mounting bushes in the box. However, before fitting, you will need to drill out the small holes for the switch, LED, siliconchip.com.au Fig.2 (above left) shows the component layout on the PC board, with a matching photo alongside. Ignore the unused holes in the board – they’re for another project in the series! The photo below shows how it all fits together inside the case. September 2010  65 Parts List – Hearing Loop Receiver Fig.3 (right) the front panel label, reproduced here same size, depicts the Hearing Loop symbol. Itself adapted from the international “hearing assistance” symbol (with the added “T”), it is displayed wherever a hearing loop is installed. In many cases, there will also be raised Braille dots giving the same message to blind people. Headphones Volume Power Fig.4: this label is glued to the top panel of the receiver volume pot and headphone socket in the top panel. A 1:1 photocopy of the label makes a good template for drill hole positions. Panel labels for this project can be downloaded from the Silicon Chip website (www.siliconchip.com.au). Go to the downloads section and select the month and year of publication. The file can be printed out using stationery suitable for your printer. Some choices would be adhesive-backed photo paper, ordinary bond paper which could be laminated and glued onto the panel, or perhaps plastic film such as overhead projector film. If you use the latter, print the label as a mirror-image so that the ink is behind the film when placed onto the panel (that protects the label). Once the ink is dry, cut the label to size. The plastic film can be glued to the panel using an even smear of neutralcure silicone. If you are using a black coloured panel, use coloured silicone such as grey or white so the label has contrast. For panels that are off white or aluminium the silicone sealant can be clear. Cut the holes out in the panel using a sharp hobby knife. Testing Apply power and check that the power LED lights. There should be about 8.1V between pins 4 and 8 of IC1 (assuming a 9V battery supply). IC2 should have about 8.7V between pins 4 and 6. If these tests are OK, plug in a pair of headphones and apply the “blurt” test: touch the inductor (L1) terminal on the lower section of the PC board. There should be a “blurt” noise in the headphones if the volume is turned up. Final testing can be made with the Hearing Loop Receiver and a Hearing Loop. Note that the receiver needs to be at right angles to the loop, ie, for a normal horizontally mounted loop, the receiver is held vertical. Trimpot VR1 is adjusted so that the volume range for VR2 is suitable without allowing the volume level to be adjusted to excessive levels. If you require a belt clip for the receiver, the Altronics H0349 belt clip is suitable (www.altronics.com.au). SC Resistor Colour Codes o o o o o o o No. Value 2 100kΩ 1 27kΩ 2 10kΩ 1 2.2kΩ 3 1kΩ 1 150Ω 4 10Ω 66  Silicon Chip 4-Band Code (1%) brown black yellow brown red violet orange brown brown black orange brown red red red brown brown black red brown brown green brown brown brown black black brown 5-Band Code (1%) brown black black orange brown red violet black red brown brown black black red brown red red black brown brown brown black black brown brown brown green black black brown brown black black gold brown 1 “remote control” case 135 x 70 x 24mm (Jaycar HB5610, Altronics H0343 or equivalent) 1 PC board coded 01209101, 65 x 86mm 1 top panel label 55 x 14mm 1 front panel label 75 x 49mm 1 miniature SPDT toggle switch, PC mount (S1) 1 3.5mm stereo socket, PC mount 1 knob to suit potentiometer 2 DIP8 IC sockets (optional) 1 trigger transformer for Xenon flashtube (L1) (Jaycar MM2520, Altronics M0104 or equivalent) 4 M3 x 6mm screws 1 2-way pin header with 2.54mm spacing 1 jumper shunt 1 9V (216) alkaline battery 1 9V battery clip 2 PC stakes Semiconductors 1 TL072 dual op amp (IC1) 1 LM386 1W amplifier (IC2) 1 4.7V 1W zener diode (ZD1) 1 3mm LED (LED1) 1 1N5819 1A Schottky diode (D1) 1 1N4148 switching diode (D2) Capacitors 3 470F 16V PC electrolytic 2 100F 16V PC electrolytic 4 10F 16V PC electrolytic 1 100nF MKT polyester 1 47nF MKT polyester 1 33nF MKT polyester 1 330pF ceramic 1 220pF ceramic Resistors (0.25W, 1%) 2 100kΩ 1 27kΩ 2 10kΩ 1 2.2kΩ 3 1kΩ 1 150Ω 4 10Ω 1 10kΩ horizontal trimpot (VR1) 1 10kΩ log potentiometer, 9mm square, PC mount (VR2) Capacitor Codes Value F value IEC Code EIA Code 100nF 0.1uF 104 100n 47nF 0.047uF 473 47n 33nF 0.033uF 333 33n 330pF 330 330p 220pF 220 220p siliconchip.com.au