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Hearing-aid wearers often have difficulty understanding conversations in noisy environments, especially if they have switched their personal hearing aid from microphone mode to T-coil mode. This projects solves that problem. By JOHN CLARKE Microphone To Neck Loop Coupler For Hearing Aids I N ORDER TO LISTEN to a hearing loop via a hearing aid fitted with a T-coil, the wearer needs to switch off the inbuilt microphone receiver. Instead, the hearing aid is switched to T-coil mode so that it can receive and process signals from the hearing loop. Such loops are often installed in public buildings, churches and halls. However, while this allows signals from the hearing loop to be heard, it prevents the user from hearing ambient sounds. It also prevents them from hearing other people around them, making conversation difficult. This Microphone to Neck Loop Coupler is the answer to this problem. It comes in two parts: (1) a small battery86  Silicon Chip powered unit that can be slipped into a shirt pocket; and (2) a wire neck loop coil that the user wears around . . . well, yes . . . their neck. This neck loop plugs into the battery-powered unit via a 3.5mm mono jack socket. The battery-powered unit has an inbuilt electret microphone, a microphone preamplifier and an amplifier to drive the neck loop. In use, the microphone picks up local sounds (or conversations) and sends them to a neck loop. The neck loop then couples the signal into the hearing aid via its T-coil. A volume control allows the level to be adjusted to suit the listener’s requirements, or it can be turned right down (or the unit switched off) to eliminate ambient sound. In summary, this unit can be thought of as a personal version of the much larger inductive loop systems installed in public places. It can operate in parallel with such systems or on its own. Presentation As shown in the photos, the preamp/coil-driver unit is housed in a small hand-held case. A power switch, power indication LED, volume control and 3.5mm jack socket are located on an end panel, at the top of the unit. Power comes from a 9V battery and the current consumption is around 10mA. This should give up to 40 siliconchip.com.au +8.7V 10k 470 µF 100 µF 10k K K IC1: TL072 470 µF A 100k 5 6 10k IC1b 7 10Ω 4 15nF IC1a 2 1 470 µF 10 µF VR2 10k LOG 1k 2.2k VR1 10k LEVEL 100nF 3 VOLUME 2 6 1 IC2 LM386N 5 7 10 Ω 1W 100 µF 10Ω 10 µF TO NECK LOOP 3.5mm JACK SOCKET 10 µF 47nF ELECTRET MIC (WITH PINS) 1N4148 1N5819 SC 8 4 + – 2011 27k 220pF 2.2nF 10 µF 9V BATTERY 8 3 10k ELECTRET MICROPHONE D2 1N4148 A ZD1 4.7V 1k 100k + S1 K λ LED1 A K A POWER +4.05V D1 IN5819 POWER 150Ω +8.1V NECK LOOP DRIVER AMPLIFIER A A K A LED K ZD1 K K A Fig.1: the circuit uses a microphone preamplifier stage (op amps IC1b & IC1a) to drive an LM386N audio amplifier (IC2). IC2 in turn drives a neck loop via a 3.5mm jack socket, with VR2 acting as the volume control. hours of use before the battery needs changing. The power LED also functions as a rough battery-level indicator. Its initial brightness when power is applied is dependent on battery voltage. Once power has been applied, the LED brightness is automatically reduced to conserve the battery (more on this later). Circuit details Take a look now at Fig.1 for the circuit details. It uses a TL072 dual op amp, an LM386 audio amplifier IC, an electret microphone and a few sundry bits and pieces. Signals picked up by the microphone are fed to the non-inverting input of IC1b via a 15nF capacitor. IC1b is the microphone preamplifier and is wired as a non-inverting stage with a a gain of about 5.5 as set by the 10kΩ and 2.2kΩ feedback resistors. The 15nF capacitor and 100kΩ load resistor at the pin 5 input set the low frequency roll-off to about 100Hz. As shown, the 100kΩ resistor connects to a supply that’s nominally at 4.05V, as set by two 10kΩ divider resistors across the 8.1V supply rail. This is siliconchip.com.au bypassed using a 100µF capacitor. The supply for the electret microphone is via another 10kΩ resistor from this 4.05V source. The low frequency roll-off for IC1b is 7.2Hz, as set by the 2.2kΩ resistor at pin 6 and the 10µF capacitor to ground, while the high-frequency roll-off starts at around 7.2kHz. IC1b’s output appears at pin 7 and is fed to the pin 3 (non-inverting) input of IC1a via a 10Ω stopper resistor. IC1a is also wired as a non-inverting amplifier and its gain is adjustable via 10kΩ trimpot VR1. In its minimum position, the gain is 101 as set by the 100kΩ and 1kΩ resistors, while its low-frequency roll-off is 16Hz due to the 1kΩ resistor and 10µF capacitor. At the other extreme, when VR1 is set to 10kΩ, the gain is about 10 and the low-frequency roll-off is at 1.45Hz. The high-frequency roll-off for this stage is set by the 220pF capacitor across the 100kΩ feedback resistor. This rolls off frequencies above about 7.2kHz. Following IC1a, the signal is AC-coupled via a 10µF capacitor to a 10kΩ volume control pot (VR1). This sets the signal level applied to audio power amplifier stage IC2 (LM386). IC2 can provide up to about 14mA RMS into a 43Ω load. This load consists of a 10Ω resistor at the output (pin 5) plus the neck loop itself, the latter consisting of a 150mm-diameter 4-turn wire loop in series with a 33Ω resistor. IC2 has a gain of 20 and is powered from an 8.7V supply rail which is applied to pin 6. This is bypassed with a 470µF capacitor. A separate 10µF supply bypass capacitor at pin 7 removes supply ripple from the amplifier’s input stages. At the output, a Zobel network comprising a 10Ω resistor and 47nF capacitor prevents amplifier instability. The output appears at pin 5 and drives the neck loop via the aforementioned 10Ω 1W resistor and a series 100µF capacitor. This capacitor provides lowfrequency roll-off for signals below 37Hz (assuming a 43Ω load), as well as removing DC from the signal. Power supply Power for the circuit is derived from a 9V battery, with diode D1 providing protection against a reverse polarity March 2011  87 TO NECK LOOP LED1 S1 CON1 150 100 F 10 F 470 F 10 ELECTRET MIC 15nF 2.2k VR1 10k 100k 9V BATTERY 10 F + 10k 10k IC1 TL072 470 F 100k 1k 1k 27k D1 220pF 10 2.2nF + 10k 100 F 10  1W 4.7V 470 F 10 F 47nF IC2 LM386 D2 ZD1 4148 RELPU O C/REVIE CER P O OL 10190210 100nF 10 F 5819 VR2 10k LOG K 10k A connection. D1 is a Schottky type, so the voltage drop across it is only about 0.3V. Switch S1 provides power on/ off switching. LED1 is used to indicate both power status and battery condition. It works like this: when power is first applied, current for the LED flows through 4.7V zener diode ZD1, the 1kΩ resistor and the 470µF capacitor (which is initially discharged). If the 9V battery is fresh, it provides 8.7V at the anode of LED1 (due to the drop across D1). Further voltage drops of 1.8V and 4.7V take place across LED1 and ZD1 respectively, leaving 2.2V across the 1kΩ resistor. As a result, 2.2mA flows through LED1 and the LED lights. At lower battery voltages, there is less voltage across the 1kΩ resistor, so less current flows and the LED is dimmer. For example, at a battery voltage of 7V, there is only about 0.2V across the 1kΩ resistor and so the current is reduced to just 0.2mA and the LED barely lights. Regardless of the battery voltage, 88  Silicon Chip Fig.2: follow this parts layout diagram and the above photo to build the unit. The assembly is straightforward but take care not to get the ICs mixed up and make sure that all polarised parts are correctly orientated. Power comes from a 9V battery. when the LED lights the 470µF capacitor quickly charges. As a result, the LED current is progressively reduced and the LED automatically dims to conserve battery power. The 27kΩ resistor across the 470µF capacitor ensures that the LED stays lit but at reduced brightness, to indicate that the power is on. Basically, the 470µF capacitor is included only to provide battery voltage indication at power up. When the power is subsequently switched off, diode D2 discharges the 470µF capacitor. This ensures that the battery condition indicator circuit is ready the next time power is applied. Power amplifier IC2 is powered directly from the 8.7V rail, while IC1 is powered from this rail via a 150Ω resistor, giving a rail of about 8.1V. A 470µF capacitor on pin 8 of IC1 filters this 8.1V rail and prevents instability. Construction The Microphone To Neck Loop Coupler is constructed on a PC board coded 01209101 and measuring 65 x 86mm. This is housed in a remote control case measuring 135 x 70 x 24mm. Separate labels attach to the top end panel of the case and to the front. Note that the same PC board was used for the Hearing Loop Receiver described in the September 2010 issue (the two circuits are almost the same). As a result, there are a few unused component holes in the PC board for the Microphone To Neck Loop Coupler project (the unused component positions are for extra parts used in the Hearing Aid Loop Receiver). The PC board is designed to mount onto integral mounting bushes in the base. Before installing any of the parts, check that the top edge of the PC board is correctly shaped at the corners, so that it fits into the case. If not, it can be filed to shape using the PC board overlay outline as a guide. Begin construction by checking the PC board for any defects and repair these if necessary. Check also that the PC board mounting holes are correct; siliconchip.com.au Fig.3: the neck loop is made using a 750mm-length of 4-core cable. This is wired to form a 4-turn loop and connected to a 500mm-length of single-core microphone cable. The other end of this cable is then wired to a 3.5mm jack plug, with a 33Ω resistor in series between the inner wire and the plug tip terminal. 33  RESISTOR BETWEEN INNER WIRE & PLUG TIP TIP WIRE ENDS JOINED TO FORM 4-TURN LOOP, THEN EACH JOINT COVERED WITH INSULATING TAPE OR HEATSHRINK SLEEVING SECOND 3.5mm PLUG COVER PUSHED UP TO ENCLOSE THE CABLE JOINTS 500mm LENGTH OF SHIELDED CABLE 3.5mm JACK PLUG LOOP MADE FROM 750mm LENGTH OF 4-WAY TELEPHONE CABLE PLUG COVER LOOP ENDS JOINED TO INNER CONDUCTOR AND OUTER SHIELD OF CABLE, THEN COVERED WITH INSULATING TAPE SLEEVE they should be 3mm in diameter, as should the holes for the battery leads Fig.2 shows the parts layout on the PC board. Start the assembly by installing the resistors. Table 1 shows the resistor colour codes but you should also use a DMM to confirm each value as it is installed. Once these are in, install the diodes, taking care to orientate them as shown. Next, install the two PC stakes at the bottom lefthand corner (for the battery leads), then install the two ICs. Be careful not to get the ICs mixed up and be sure to install them the right way around. The ICs can either be soldered directly to the PC board or mounted via sockets. The capacitors are next on the list. Take care with the polarity of the electrolytics and make sure that no capacitors are higher than 12.5mm, otherwise the lid of the case will not fit correctly. Trimpot VR1, switch S1, potentiometer VR2 and the 3.5mm mono socket can now all be installed. That done, install LED1. This mounts horizontally, with its leads 6mm above the PC board and its anode lead to the left. To do this, first bend its leads down by 90° about 12mm from its body (make sure it is orientated correctly). That done, cut a 6mm-wide cardboard spacer, then push the LED’s leads down onto this before soldering them. The board assembly can now be completed by installing the electret microphone. Make sure it is correctly orientated – its positive side goes towards the top and its face must be no higher than 12mm above the PC board. the top end panel of the case and drill out the holes for the power switch, indicator LED, the pot and the 3.5mm socket. The PC board can then be secured to the base of the case using four M3 x 5mm screws into the integral mounting bushes. The case lid also requires a small cut-out to clear the pot shaft plus a small hole for the microphone. This latter hole is drilled by first fitting the label to the lid, then drilling a 3mm hole in the position indicated, so that it is centred over the microphone. A Table 2: Capacitor Codes Fitting the battery connector The battery connector is installed by first passing its leads through the battery compartment of the case, and then looping them through the holes in the PC board – see Fig.2. This anchors the leads which can now be soldered to the PC stakes (watch the polarity). Once that’s done, attach the label to Value 100nF 47nF 15nF 2.2nF 220pF µF Value 0.1µF .047µF .015µF .0022µF   NA IEC Code EIA Code 100n 104   47n 473   15n 153   2n2 222 220p 221 Table 1: Resistor Colour Codes o No. Value o   2 100kΩ o   1 27kΩ o   4 10kΩ o   1 2.2kΩ o   2 1kΩ o   1 150Ω o   1 33Ω* o   2 10Ω * attached to jack plug – see Fig.3 siliconchip.com.au 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 orange orange black 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 orange orange black gold brown brown black black gold brown March 2011  89 Here’s another view of the fullyassembled unit. Note that you will have to make cut-outs in the end panel and in the case lid to clear the pot shaft. The completed unit is lightweight and can be easily clipped onto clothing or slipped into a pocket. The loop is worn around the neck and couples signals into the hearing-aid’s T-coil. 3mm LED bezel can then then fitted to this hole, to provide a neat appearance. Making the neck loop The top panel of the unit provides access to the 3.5mm jack socket, the volume control and the power switch. Also present is the power indicator LED. How To Make Your Own Labels If you are building this project from a kit, then the labels will probably be supplied. If not, the labels can be downloaded as PDF files from the SILICON CHIP website and printed out onto photo paper with a peel-away adhesive backing or onto clear plastic film. If you are using clear plastic film (eg, overhead projector film), you can print the label as a mirror image so that the ink is at the back of the film when it is placed onto the panel. Wait until the ink is dry before cutting the label to size. The film can then be affixed in place using an even smear of neutral-cure silicone. If you are affixing the label to a black panel (eg, if using the specified case), use coloured silicone such as grey or white so that the lettering will stand out. The holes in labels can be cut out using a sharp hobby knife after the silicone has cured. 90  Silicon Chip The neck loop is made using a 750mm-length of 4-core telephone cable. Alternatively, you can use RJ11 4P4C extension cable if you want black (telephone cable is usually white). Fig.3 shows how the cable is wired to form a 4-turn loop. This is then connected to a 500mm-length of white or black single-core microphone cable. A 3.5mm jack plug and a 33Ω resistor are then fitted to the other end of this cable, so that it can be plugged into the microphone pick-up/amplifier unit. Testing To test the unit, apply power and check that the power LED lights. If it does, check that there is about 8.1V between pins 4 & 8 of IC1 (assuming a 9V battery supply). Similarly, IC2 should have about 8.7V between pins 4 & 6. If this all checks out, plug the neck loop into the socket and check the performance of the unit. To do this, you will need either a hearing aid with a T-coil or a hearing loop receiver such as the one described in the September siliconchip.com.au Parts List 1 plastic case, 135 x 70 x 24mm (Jaycar HB5610 or equivalent – see text) 1 PC board, code 01209101, 65 x 86mm (or use 01209102 to suit Altronics H0342 case) 1 end panel label, 55 x 14mm 1 front panel label, 67 x 49mm 1 miniature PC-mount SPDT toggle switch (S1) 1 3.5mm PC-mount stereo socket 1 3.5mm stereo line plug 1 3.5mm line plug for neck loop ‘Y’ covering 1 10kΩ horizontal trimpot (VR1) 1 10kΩ log potentiometer, PCmount, 9mm square (VR2) 1 knob to suit potentiometer 2 DIP8 IC sockets (optional) 1 10mm OD PC-mount electret microphone 1 3mm LED bezel 1 9V (216) alkaline battery 1 750mm length of 4-way white or black sheathed flat modular telephone wire 1 500mm length of white or black sheathed single-core shielded cable 1 9V battery clip 2010 issue of SILICON CHIP. Note that this receiver needs to be at right angles to the loop. So for a horizontally mounted loop, the receiver is held in the vertical plane. Trimpot VR1 is simply adjusted for best results, so that the volume control (VR2) works over its range without excessive levels at full volume. Another method of checking the unit is to plug headphones into the outlet socket and check that the microphone sound is amplified. Loop orientation In operation, the neck loop signal is magnetically coupled to the T-coil. However, because the loop is worn around the neck, it is not orientated in the optimum position for the hearing aid to receive the field. In addition, the T-coil is not exactly perpendicular to the neck loop and so the signal level is not as high as it would otherwise be. This has been taken care of in the design of the Microphone To Neck Loop Coupler circuit. Basically, it has siliconchip.com.au 2 PC stakes 4 M3 x 5mm screws Semiconductors 1 TL072 dual op amp (IC1) 1 LM386 1W amplifier (IC2) 1 4.7V 1W zener diode (ZD1) 1 1N5819 1A Schottky diode (D1) 1 1N4148 switching diode (D2) 1 3mm red LED (LED1) Capacitors 3 470µF 16V PC electrolytic 2 100µF 16V PC electrolytic 4 10µF 16V PC electrolytic 1 100nF MKT polyester 1 47nF MKT polyester 1 15nF MKT polyester 1 2.2nF MKT polyester 2 220pF ceramic Resistors (0.25W, 1%) 2 100kΩ 1 150Ω 1 27kΩ 1 33Ω 4 10kΩ 2 10Ω 1 2.2kΩ 1 10Ω 1W 2 1kΩ Miscellaneous Heatshrink tubing enough gain to drive the loop so that a satisfactory level is obtained in the hearing aid. However, if the neck loop is plugged into an MP3 player or similar, the signal may not be sufficient for satisfactory sound levels to be heard. In some cases, it may be simply a matter of adjusting the volume on the MP3 player but that’s by no means guaranteed. Note that if you do intend plugging the neck loop into a stereo outlet (eg, an MP3 player), you will need to install an additional 33Ω resistor in the jack plug. This extra resistor goes between the loop and the ring connection of the jack plug and is necessary to provide stereo-to-mono mixing of the signal. Give your lighting projects a SEOUL AS FEATURED IN ZZLER SILICON CHIP LED DA 11) (P24, FEBRUARY 20 Acriche A4 4W Pure White AC LED Mounted on PCB No Electronics Needed, Just add power AW3231-240V $16.00 +GST P7 Power LED 10W Pure White Emitter Approx. 900lm <at> 2.8A Ideal for torch applications PCB available to suit W724C0-D1 $16.00+GST P4 Star 4W LEDs Power LEDs mounted on 20mm Star PCB. Various Colours available. Pure White W42182 $3.90+GST Nat. White S42182 $3.90+GST Warm White N42182 $3.90+GST P3-II Star 2W LEDs Power LEDs mounted on 20mm Star PCB. Various Colours available. Pure White WS2182 $2.95+GST Warm White NS2182 $2.95+GST P5-II RGB Power LED High power RGB LED mounted On 20mm Star PCB Drive each colour <at> 350mA Ideal for wall wash applications F50360-STAR $14.95+GST SMD RGB LED General purpose RGB LED in PLCC-6 package Drive each colour <at> 20mA SFT722N-S $0.95ea+GST Top View SMD White LED High Brightness pure white LED in small PLCC package Great for strip lighting Typical luminous intensity 1600mcd KWT803-S $0.30ea+GST Belt clip Finally, if you require a belt clip for the unit, take a look at the Altronics H0349. It’s on their website at www.altronics.com.au We have also produced a slightly modified PC board pattern (01209102) to suit the Altronics H0342 SC hand-held case. AUSTRALIAN DISTRIBUTOR Ph. 07 3390 3302 Fx. 07 3390 3329 Email: sales<at>rmsparts.com.au www.rmsparts.com.au March 2011  91