In 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 battery-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.
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.
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 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).
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.