Having completed the assembly, the Hearing Loop Tester is ready to be calibrated. As previously mentioned, the unit must be adjusted so that the 0dB LED lights for a magnetic field strength of 100mA/m. This is done by placing the meter in a known magnetic field and adjusting trimpot VR1.
One possible method involves using a single 1-metre diameter turn of wire fed with 100mA at 1kHz. An amplifier set to deliver 0.82V RMS via an 8.2Ω 0.25W resistor could be used to drive the coil.
However, to achieve correct calibration using this method, inductor L1 would have to be accurately located in the centre of the coil. That’s because the field strength varies depending on L1’s position relative to the centre position of the loop.
A more practical calibration method involves using a Helmholtz coil (see http://en.wikipedia.org/wiki/Helmholtz_coil). A Helmholtz coil comprises two identical parallel on-axis coils that are driven by the same signal. These two coils are separated from each other by the coil radius (Fig.6).
A feature of a Helmholtz coil is that it gives a constant field along the axis between the two coils. This field remains constant to within 1% inside a central concentric area out to about half the diameter of the coil.
Fig.6: follow this diagram to build the Helmholtz calibration coil. The two loops are made from steel fencing wire and are connected in series and driven with a 600mV 1kHz sinewave signal via a 33Ω resistor – see text.
The current required in each coil to give a field strength of 100mA/m is 0.1398 x R/n, where “R” is the radius in metres and “n” is the number of turns in each coil. In our case, we decided to design the coils so that they have only one turn each (to make construction easy) and can be driven by the headphone output of a PC.
In practice, a 130mm-radius coil is suitable and this requires a coil current of 18.16mA to give 100mA/m. This is achieved by connecting the coils in series and driving them with a 600mV AC signal via a 33Ω resistor.
Fig.6 shows the assembly details for our Helmholtz calibration coil. It’s built using 2.4mm diameter steel (fencing) wire, a 200 x 65mm length of timber, some hook-up wire and a 33Ω resistor. You will also need two screw terminals, a cable clamp, some shielded cable and a 3.5mm stereo plug.
As shown, the 2.8mm steel wire is looped to form two 260mm-diameter coils. To do this, first cut two 836mm lengths and bend them down by 90° about 10mm from each end. That done, drill two sets of 3mm-diameter holes at each end of the timber to hold the wire ends. Each hole pair should be 8mm apart and the two pairs should be separated by 130mm (see Fig.6).
The hook-up wire and 33Ω resistor can now be soldered to the ends of the steel wire. It’s then just a matter of bending the steel wires into loops and feeding the hook-up wires and the resistor down through the baseboard holes. The ends of the wire loops can be pushed into these holes to hold them in place.