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Hearing Loop Level Meter Setting the correct signal level and minimising noise are critical factors when setting up a hearing loop. This easy-to-build tester can display field strength levels over a 27dB range. Here’s how it works, how to build it and how to use it. Pt.1: By JOHN CLARKE W HEN INSTALLING a hearing aid loop, it is important to set the magnetic field strength to the correct level. This ensures that a hearing aid with a Telecoil (or T-coil) will deliver the best signal-to-noise ratio without signal overload. The same applies if you are using a hearing loop receiver such as the one described in the September 2010 issue of SILICON CHIP (or a commercial equivalent). Additionally, when setting up a hearing aid loop, it is important to verify that any background magnetic noise is at an acceptable level. Both background noise and signal strength from the hearing aid loop can be measured with this Hearing Loop Tester. Of course, if you are setting up a small hearing loop in your home, you can usually get away without using a level meter. In that case, it’s usually just a matter of setting the level to give good results from the hearing aid without any overload occurring. However, for a system that will be used by more than one person or the general public, it is important for the level to be correct. That way, the loop will be suitable for all who use it. Specifications As shown in the photos, the SILICON CHIP Hearing Loop Tester is housed in a small hand-held plastic case that includes a battery compartment. A power switch and an indicator LED are located on the top panel, while the front panel carries 10 bargraph LEDs arranged in a vertical column on the lefthand side. In operation, this bargraph displays signal levels ranging from -21dB to Power supply: 9V at 18-26mA Display: –21dB to +6dB in 3dB steps Meter response: “S” (slow) response of 1s Weighting: A-weighting or wide (see Fig.4) 20  Silicon Chip Main features +6dB, with each LED representing a 3dB step. However, to conserve battery life, the display is normally set to dot mode which means that only one display LED is lit at any time. The current consumption is 18mA when no bargraph LEDs are lit and 26mA when one bargraph LED is lit. This is quite satisfactory for an instrument that is normally only used for short durations. Alternatively, you can install a link under the PC board to convert to a conventional bargraph display. This is not recommended though, due to the increased current drain. An important feature is that the unit can be accurately calibrated to indicate 0dB at a field strength of 100mA/m. This specification is based on the Australian Standard AS60118.4-2007 – “Hearing Aids: Magnetic Field Strength In Audio-Frequency Induction Loops For Hearing Aid Purposes”. Once calibrated, the meter can then be used to set the field strength level in a hearing loop to the correct level. It can also be used to measure the siliconchip.com.au environmental background noise, to determine whether this is low enough for a hearing loop to be successful. In operation, the unit is simply held at right-angles to the plane of the hearing loop for both signal level and noise measurements. Circuit details Refer now to Fig.3 for the circuit details. It’s based on four low-cost ICs, an inductor (L1), 11 LEDs and a handful of minor parts. Inductor L1 is used to detect the magnetic field from the hearing loop. This inductor is actually an Xenon flash-tube trigger transformer (Jaycar MM-2520) which has a high inductance, suitable for loop monitoring. In this circuit, we use only the secondary winding of L1, which is wound as an autotransformer. This winding has an inductance of about 8.4mH and is biased at about 4.15V using two 10kΩ resistors connected in series across the 8.3V supply. A 100µF capacitor bypasses the divider output. The 4.15V half-supply rail is also used to bias pin 5 of op amp stage IC1b (via L1). This allows IC1b’s pin 7 output to swing symmetrically about this half supply rail. L1’s coil resistance is 27Ω and, in conjunction with the 100µF bypass capacitor, it presents a low source impedance to IC1b’s pin 5 input at low frequencies. This minimises any low-frequency noise. The inductor’s impedance increases with increasing frequency but this is restricted by a parallel 2.2kΩ resistor. This 2.2kΩ resistor lowers the Q of the inductor, thereby preventing oscillation. A 220pF capacitor at the output of L1 also shunts any high-frequency signals to ground. IC1b is configured as a non-inverting amplifier stage with nominal gain of 1001, as set by the 100kΩ and 100Ω feedback resistors. However, one aspect of using an inductor to receive the hearing loop signal is that the signal induced in L1 rises in level with frequency. This is because the induced voltage is proportional to the rate of change of the magnetic field. As a result, IC1b’s gain is reduced with frequency in order to achieve a flat overall frequency response. This is achieved by using a 33nF feedback capacitor and 100kΩ feedback resistor to roll off signal frequencies above about 50Hz by 20dB per decade. This siliconchip.com.au Fig.1: the basic arrangement for a hearing loop. The loop creates a varying magnetic field in response to the driving signal and this is picked up by suitablyequipped hearing aids and receivers. T-COIL OUTPUT VOLTAGE MAGNETIC FIELD Fig.2: this diagram illustrates the magnetic field generated by the hearing loop and shows how it couples into a hearing-aid T-coil. counteracts the 20dB per decade increase from the inductor. In addition, IC1b’s low-frequency gain is rolled off below 723Hz using a 100Ω resistor and 2.2µF capacitor connected in series between pin 6 (the inverting input) and ground. If link LK1 is installed, an extra 22µF capacitor is placed across the 2.2µF capacitor and this lowers the low-frequency roll-off point to around 66Hz. Op amp IC1a provides a further stage of gain. If trimpot VR1 is set to its minimum, IC1a’s gain is 1+ (100kΩ/150Ω) or about 667. However, if VR1 is set to its maximum value of 5kΩ, the gain is reduced to about 20. This range of gain adjustment allows the meter to be calibrated. IC1a’s high-frequency roll-off starts at about 10.6kHz due to the 100kΩ resistor and 150pF capacitor in the feedback path. In addition, both IC1b & IC1a have inherent reduced gain at high frequencies. IC1a’s low frequency roll-off depends on the setting of VR1 and occurs somewhere between 10.6Hz and 0.32Hz. ing. A-weighting is a tailored response that’s designed to match the way our ears perceive loudness with respect to frequency at a particular low-level sound pressure. The weighting rolls off the signal below and above 1kHz as shown in the graph of Fig.4. Inserting link LK1 extends the frequency response of the unit down to at least 200Hz, before rolling it off at the lower frequencies. As explained later, this wider response is better for checking background noise levels than the A-weighted curve. As a result, we recommend that LK1 be installed for all measurements (including loop level measurements), to provide a nominal frequency response of 200Hz to 10kHz (-3dB points). In fact, the relatively flat response of the meter between 200Hz and 5kHz with LK1 in is ideal for checking hearing loop response levels. If necessary, treble boost can be applied to the loop amplifier to counter the effect of drooping high-frequency response due to the loop inductance. A-weighting IC1a’s output is fed via a 100nF capacitor to a full-wave precision rectifier stage based on IC2b, IC2a and diodes D4 & D5. The capacitor rolls off the response below about 106Hz. This The high and low roll-off frequencies set for IC1b with LK1 out of circuit produce a nominal A-weighted overall frequency response for the level meter- Precision rectifier November 2010  21 Parts List For Hearing Loop Tester 1 remote control case, 135 x 70 x 24mm (Jaycar HB5610 or equivalent) 1 PC board, code 01111101, 65 x 86mm 1 panel label, 55 x 14mm 1 panel label, 113 x 46mm 1 miniature PC mount SPDT toggle switch (S1) 3 DIP8 IC sockets (optional) 1 DIP18 IC socket (optional) 1 Xenon flash tube trigger transformer (Jaycar MM2520 or equivalent) (L1) 1 2-way pin header (2.54mm spacing) 1 jumper shunt for pin header 4 M3 x 5mm screws 1 9V (216) alkaline battery 1 9V battery clip 1 40mm length of 0.7mm tinned copper wire 2 PC stakes 1 5kΩ horizontal trimpot (code 502) (VR1) Semiconductors 2 TL072 dual op amps (IC1,IC2) 1 LM3915 log bargraph driver (IC3) 1 7555 CMOS timer (IC4) 1 1N5819 1A Schottky diode (D1) 4 1N4148 diodes (D2-D5) 1 3mm red LED (LED1) 2 3mm orange LEDs (LED2,LED3) 8 3mm green LEDs (LED4-LED11) stage works as follows. When the signal from the 100nF capacitor swings positive, pin 7 of IC2b goes low and forward biases diode D4. As a result, IC2b operates as an inverting amplifier stage with a gain of -1, as set by the 15kΩ input and 15kΩ feedback resistors on its pin 6. This inverted signal at D4’s anode is applied to IC2a’s inverting input (pin 2) via a 150kΩ resistor. This stage operates with a gain of -6.66, as set by the ratio of the 1MΩ feedback resistor and the 150kΩ input resistor. As a result, the total gain for the signal path from pin 1 of IC1a to pin 1 of IC2a via IC2b is -1 x -6.66 = +6.66. In addition, the positive-going signal from IC1a is applied to IC2a via a second signal path, ie, via a 300kΩ 22  Silicon Chip Capacitors 1 470µF 16V PC electrolytic 4 100µF 16V PC electrolytic 1 22µF 16V PC electrolytic 3 10µF 16V PC electrolytic 1 2.2µF 16V PC electrolytic 1 1µF 16V PC electrolytic 1 100nF MKT polyester 1 33nF MKT polyester 1 1nF MKT polyester 1 220pF ceramic 1 150pF ceramic 1 10pF ceramic Resistors (0.25W, 1%) 1 1MΩ 4 10kΩ 1 300kΩ 3 2.2kΩ 1 150kΩ 2 150Ω 2 100kΩ 1 100Ω 2 15kΩ 1 10Ω Helmholtz coil 2 836mm lengths of 2.4mm diameter steel fencing wire (or similar stiff wire) 1 piece of timber, approximately 65 x 19 x 200mm 1 33Ω 0.25W resistor 1 wire clamp made from two solder lugs or metal scrap 4 small rubber feet (optional) 1 400mm length of medium-duty hook-up wire 1 1m length of shielded cable 1 3.5mm stereo jack line plug 4 solder lugs 3 small wood screws resistor. For this path, IC2a operates with a gain of -3.33 and so the overall signal gain from the output of IC1a to the output of IC2a is +6.66 - 3.33 = +3.33. Now consider what happens for negative-going signals from IC1a. In this case, diode D5 is forward biased and so IC2b’s output is clamped at about 0.6V above its pin 6 input. As a result, no signal flows via D4 and IC2b ceases operating as an inverting amplifier. This means that negative-going signals from IC1a are fed to IC2a via the 300kΩ resistor only (ie, via only one signal path). Because IC2a operates with a gain of -3.33 for this path, the signal is inverted. Therefore, the precision rectifier provides a positive output for both positive-going and negative-going signals from IC1a and both have a gain of 3.33. IC2a also provides low-pass filtering of the signal so that its response is slow to incoming signal level changes. The time constant is around one second (1s) as set by the 1MΩ feedback resistor and its parallel 1µF capacitor. This matches the slow (S) response requirement for measuring background noise for a hearing loop system. Bargraph circuit IC2a’s output is fed to the pin 5 input of IC3, an LM3915 10-LED bargraph driver with a logarithmic response. The bargraph displays a 27dB range with each LED covering 3dB. We have labelled the display so that is covers field strength levels from +6dB down to -21dB As explained previously, the unit is calibrated to read 0dB at a field strength of 100mA/m. The voltage range for the meter display is from 1.25V at full scale (+6dB) down to about 56mV for the -21dB LED. This range is set by connecting the RHI input to the 1.25V reference (pin 7) and the RLO input to ground (0V). The 2.2kΩ resistor between REF (pin 7) and ground sets the bargraph LED current to about 6mA. Link LK2 sets the bargraph mode. Power supply Power for the circuit is derived from a 9V battery, with diode D1 providing reverse polarity protection. S1 functions as a power switch, while LED11 is used as a power-on indicator. The 2.2kΩ resistor in series with LED11 limits the current through it to about 3.5mA. The resulting 8.7V rail is filtered using a 10µF capacitor and directly supplies IC2, IC3 & IC4. IC1’s supply is also derived from this rail but is decoupled using a 150Ω resistor and a 470µF filter capacitor. This is done so that supply variations due to changes in the LED bargraph display are not introduced into IC1, which contains two sensitive amplifier stages. A negative supply for IC2 is generated using 7555 timer IC4, diodes D2 & D3 and two 100µF capacitors. IC4 is wired as an astable oscillator and operates at about 72kHz due to the timing components on pins 6 & 2, ie, a 1nF capacitor to ground and a 10kΩ resistor which is connected back to pin 3. siliconchip.com.au siliconchip.com.au November 2010  23 8 WIDE IN VR1 5k 150 10 A K D2–D5: 1N4148 7 IC1: TL072 HEARING LOOP TESTER 'A' WEIGHTING OUT 2.2 F 33nF 100k IC1b FUNCTION LK1 22 F 100 220pF 6 5 LK1 100 F 2.2k L1 8.2mH CALIBRATE 150pF 100k 4 IC1a 100 F 2 3 1 A K 15k 300k K D1: 1N5819 100nF 470 F 150 5 6 D5 8 K A 4 IC2b A K LEDS IC2: TL072 7 15k 10pF +8.7V A 150k –V D4 1nF 3 2 A IC2a 1M 1 F 100 F 2 6 7 D3 K 4 5 3 K A 3 V+ RLO IN V– 2 IC3 LM3915 REF RHI MODE 8 REF ADJ 4 5 7 6 9 10 F 2.2k D2 100 F LK2 OUT = DOT IN = BAR 1 10k 1 IC4 7555 8 K A 1 18 17 16 15 14 13 12 11 10 K           10k K K K K K K K K K K 10 F 2.2k  LED11 S1 A A A A A A A A A A 10 F LED10 LED9 LED8 LED7 LED6 LED5 LED4 LED3 LED2 LED1 9V BATTERY A POWER D1 1N5819 Fig.3: the circuit uses inductor L1 to detect the magnetic field generated by the hearing loop. The resulting signal is then amplified by IC1b & IC1a and fed to a precision rectifier based on IC2b, IC2a and diodes D4 & D5. The output from the rectifier then drives IC3 which in turn drives the 10 LEDs in the bargraph display. Power comes from a 9V battery, while IC4 and diodes D2 & D3 generate a -7V rail for op amp IC2. SC 2010 10k 10k +8.3V practice, will be close to -7V. +20dB +10dB 0dB 'WIDE' CURVE directions shown (note: IC3 faces the opposite way to the others). The ICs can then be fitted, taking care to ensure that IC4 is the 7555. Alternatively, you can solder the ICs straight in. The 2-way header for LK1 can now go in, followed by the capacitors. Be sure to install the electrolytics the right way around and keep their heights above the PC board to less than 12.5mm, otherwise the lid of the case will not fit correctly. If necessary, sit the electrolytics up off the board slightly and then bend their bodies over after soldering. Trimpot VR1, switch S1 and inductor L1 are next. Note that a third (thin) wire attached to L1 is soldered to a spare pad on the PC board. Construction –10dB All parts except for the battery are mounted –30dB on a single-sided PC –40dB 'A–WEIGHTING' CURVE board coded 01111101 –50dB (65 x 86mm) and this –60dB assembly is housed in –70dB a remote control case –80dB measuring 135 x 70 x 100k 10 100 1k 10k 24mm. Two labels are FREQUENCY (Hz) attached to the front Fig.4: this graph shows the frequency response of and top panels to give a the Loop Tester with LK1 installed (wide) and with professional finish. LK1 removed (A-weighting). The PC board is designed to mount onto It operates like this: when power is the integral bushes inside the box. first applied, pin 3 goes high and the Check that the top edge of the PC board 1nF capacitor charges via the 10kΩ has the corner cut-outs so that it fits resistor. When it reaches 2/3rds the correctly. If necessary, you can make supply voltage, the pin 3 output goes the cut-outs yourself using a small low and the capacitor discharges until hacksaw and then carefully filing them it reaches 1/3rd the supply voltage. to shape. Pin 3 then switches high again and so Fig.5 shows the parts layout on the the process repeats indefinitely while PC board. Begin by carefully checking power is applied. the board for any breaks in the tracks As well as charging/discharging the and for shorts between tracks and timing capacitor, pin 3 also drives the pads. The four mounting holes and the negative supply circuit. When pin 3 two holes that are used to anchor the goes high, it charges its associated battery clip leads should all be 3mm 100µF capacitor to the positive supply in diameter. rail (+8.7V) via diode D2. Then, when The assembly is best started by pin 3 of IC4 subsequently switches installing the two wire links and the low, the positive side of the 100µF resistors. Table 1 shows the resistor capacitor is pulled to 0V. As a result, colour codes but it’s also a good idea its negative side goes to -8.7V (or to check each one using a digital multhereabouts) in order to maintain the timeter (DMM). charge across the capacitor. Follow with the diodes, taking care This negative voltage now charges to orientate them as shown. Note that the second 100µF capacitor via diode D4 & D5 face in opposite directions. D3 to provide the negative rail for That done, install two PC stakes to IC2. The actual rail voltage obtained terminate the battery clip leads. depends on the load and the voltage Next, install DIP sockets for ICs1-4 drops across the two diodes but, in with their notched ends facing in the –20dB Installing the LEDs LEDs1-10 must be installed so that the top of each LED is exactly 15mm above the PC board. This can be done by cutting a 10mm-wide cardboard spacer which is slid between the leads during soldering. Take care with the orientation (the anode is the longer of the two leads) and be sure to push each LED down onto the spacer before soldering it in place. Note also that LED1 is red, LEDs2 & 3 are orange and LEDs4-10 are green. The power LED (LED11) is installed so that it sits horizontally with the centre of its lens 6mm above the board. To do this cut a 6mm-wide cardboard spacer, then bend the LED’s leads down through 90° 12mm from its base, making sure that the anode lead is to the left. The leads can then be inserted into the PC board and pushed down onto the 6mm spacer before soldering. Now for the battery clip. This is installed by first passing its leads through the battery compartment and Table 1: Resistor Colour Codes o o o o o o o o o o o No.   1   1   1   2   2   4   3   2   1   1 24  Silicon Chip Value 1MΩ 300kΩ 150kΩ 100kΩ 15kΩ 10kΩ 2.2kΩ 150Ω 100Ω 10Ω 4-Band Code (1%) brown black green brown orange black yellow brown brown green yellow brown brown black yellow brown brown green orange brown brown black orange brown red red red brown brown green brown brown brown black brown brown brown black black brown 5-Band Code (1%) brown black black yellow brown orange black black orange brown brown green black orange brown brown black black orange brown brown green black red brown brown black black red brown red red black brown brown brown green black black brown brown black black black brown brown black black gold brown siliconchip.com.au S1 LED11 2.2k D1 D2 D3 4148 10k 100 F 15k 150 33nF 1 L1 2.2k K 150k LED10 15k 300k K 470 F 10k LED9 10 F 150pF 22 F 10 F + 100 F 10 100 100 F 10k K D4 4148 4148 D5 10pF LED8 1 100nF 100k K LK1 K LED7 5819 K RETE M P O OL LED5 LED6 1 IC2 TL072 K 1M IC1 TL072 K 2.2k LED3 LED4 100 F 1 F 100k K IC3 LM3915 LED2 LK2 RA B (UNDER) 10k K 150 LED1 IC4 7555 1 A 4148 10 F 1nF VR1 5k 2.2 F 220pF 10111110 Table 2: Capacitor Codes 9V BATTERY Fig.5: install the parts on the PC board as shown on this diagram and the photo at right. The bargraph LEDs must be installed using a 10mm cardboard spacer – see text. then looping them through the holes in the PC board as shown. This anchors the leads which can now be soldered to the PC stakes (watch the polarity). The PC board can now be secured to the base of the case using four M3 x 5mm screws into the integral mounting bushes. That done, attach the label to the top panel and drill the clearance holes for the power switch and indicator LED. If the label is not supplied as part of a kit, you can download the artwork in PDF format from the SILICON CHIP website. You will also need to drill 10 x 3mm-diameter holes for the bargraph LEDs in the lid. These holes must line up along the inside border of the inset section on the top lid. Note that the label does not extend fully to the left side of this inset and so it does not need to be drilled. If you are building this project from a kit, then the labels will probably siliconchip.com.au be supplied. If not, the downloaded PDF files can be printed out onto photo paper with a peel-away adhesive backing or onto clear plastic film. If 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 sealant. If you are affixing the label to a black coloured panel (eg, if using the specified case), use grey or whitecoloured silicone so that the lettering will stand out. The holes for the power switch and indicator LED in the top label can be cut out using a sharp hobby knife after the silicone has cured. Testing Before applying power, go back over Value 100nF 33nF 1nF 220pF 150pF 10pF µF Value 0.1µF .033µF .001µF   NA   NA   NA IEC Code EIA Code 100n 104   33n 333    1n 102 220p 221 150p 151   10p   10 your work and check for wiring errors. That done, connect a 9V battery, switch on and check that the power LED lights. If not, then either D1, LED 11 or the battery is the wrong way around (or a combination of these). Assuming the LED does light, check that pin 8 of IC1 is at about 8.3V (assuming that the battery itself measures 9V). Similarly, check that pin 8 of IC2 is at about 8.7V and that pin 4 is at about -7V. Pin 3 of IC3 should be at 8.7V, as should pin 8 of IC4. If these supply voltages check out, touch the bottom lead of inductor L1. This should cause some of the LEDs in the bargraph to light due to the noise introduced into op amp IC1b. Note: it can take several seconds for the unit to display a bargraph reading immediately after switch-on. That’s all for this month. Next month, we’ll give the calibration procedure and describe how the unit SC is used. November 2010  25