Turn any Hearing Aid into an Inductive Hearing Aid.

This handy project turns almost any Hearing Aid into an Inductive Hearing Aid.
An Inductive Hearing Aid has a coil connected to the front-end of the amplifier in place of a microphone.
This coil picks up electromagnetic radiation.
Electromagnetic radiation is similar to a radio transmission but at a much-lower frequency.
It is generated by connecting a coil across the output of an amplifier. The amplifier can be a radio, TV or Public Address system, and if the wearer of a hearing aid has an inductive coil connected to the aid, he will be able to hear the output of the radio or TV etc.
This will be an enormous advantage to a deaf person as the level of volume in a church or meeting place is quite low compared to someone talking directly to them.
To those of us with normal hearing, we can adapt to the different volume levels.
But an electronic circuit such as a hearing has to be adjusted.
The wearer has to turn up the gain of the “aid” and this sometimes results in feedback. But the main problem is the aid is operating at its maximum gain and the result is a distorted sound.
Our project overcomes this problem.
An inductive loop is placed around the perimeter of a room, or placed nearby, so the setting of the aid can be reduced.
This prevents the possibility of an annoying feedback whistle and the clarity is increased.
Some churches, meeting places and theatres have an inductive loop installed around the room to allow those with inductive hearing aids to take advantage of the “pick-up.”
This project can be used when this loop is available. The need to turn up a hearing aid and listen to distorted audio is very annoying.
Any one in your family who wears a hearing aid will like this project.

There are other uses for this project:
A loop can be placed around the neck of the wearer to create a “wireless link.” The loop is connected to the output of a radio or CB radio and the wearer can listen without the need for wires. An “in the canal” heading aid can be purchased (very expensive) to create a link like this, for a security guard in a light-club, for example.

There are a number of very cheap hearing aids on the market through mail-order companies or they can be purchased as spy ear from toy shops. These can be converted to an Inductive Hearing Aid by adding the receiving coil described in this article.
Some hearing aids have an inductive pick-up already inside the case but they are very expensive. The cheap versions do not have this feature.
This is surprising as an inductive pick-up is simply a coil consisting of many turns of fine wire. It’s one way of “value adding” to a product and profiting from those who need a particular feature. That’s why hearing aids were notoriously expensive, until Chinese copies came on the market.Although it is possible to build the amplifier for the hearing aid, some of the parts are difficult to obtain (such as the earpiece) and the case is not available from any source.
That’s why we have suggested buying a ready-made unit and adapting it to detect the signal from an “audio-loop.”

MAKING THE RECEIVING COIL

To turn an ordinary hearing aid into an inductive hearing aid, we add a coil of wire in place of the microphone.
This coil of wire can be very small or as large as book.
A large coil will provide a longer range. It has no effect on the clarity or the loudness.
We have suggested three different coils and you can choose the type you want to use.

1. The receiving coil can be a 10mH choke as shown in the photo:
   In fact it can be any value of inductance but a 10mH choke will have the largest number of turns and this will produce the best results.

2. The receiving coil can be 100 turns of 0.2mm wire wrapped around the hearing aid or around a matchbox. This will give a better range than the 10mH choke.
3. But the longest range will be produced with a coil of 50 turns or more wound on a book or tin with the largest diameter. Place a ruler or pencil against the tin so that it can be removed and the coil taken off when all of 50 metres of wire has been used.
   Use sticky-tape to keep the turns in place. The coil can be any shape but the best is a smooth round circle. The turns can be jumble-wound as the winding makes no difference.
   Scrape the enamel off the ends and the coil is ready.

Open up the hearing aid and locate the microphone.
Solder one end of the coil (or the 10mH inductor) to either wire that comes from the microphone. It is best to do this where the wire joins the PC board so the microphone is not heated up.
Now get a 100n monoblock capacitor (see photo):

Solder one lead of the capacitor to the other lead of the coil and solder the second capacitor lead to the other microphone wire.
Here is the circuit:

Connecting the coil to a hearing aid

Connecting the coil across the microphone will reduce the sensitivity of some hearing aids and the volume will have to be turned up more but the advantage of placing the coil across the microphone means the listener can hear the audio from the room at the same time as hearing the audio from the loop.
The microphone can be removed or a switch used to select between microphone and inductor.

MAKING THE TRANSMITTING COIL

The other part of this project is the transmitter or transmitting coil.
We need to have a 50 turn transmitting coil at least 30cm diameter to create the electromagnetic radiation that will be picked up by the inductive hearing aid.
This transmitting coil is connected to the output of a radio by simply soldering the ends of the coil to a plug that fits into the “ext spkr” of a radio. The coil will now take the place of a speaker and instead of the energy of the radio being delivered to an external speaker, it get delivered to our coil.
The transmitting coil can be any diameter, any number of turns and any gauge of wire.
We have experimented with a number of designs and the best performance was 50 turns of 0.5mm enameled wire, 30cm diameter.
The coil can also be 10-20 turns of wire around the ceiling of a room. This will give everyone in the room an equal opportunity to receive the signal.

MODIFYING A RADIO OR TV

Most radios and TV’s have a “headphone” socket. When a set of headphones is plugged into this socket, the main speaker is disconnected. To stop the speaker turning off, the side of the plug is filed so that when the plug is inserted, it does not activate the switch.

TRANSMITTING RANGE

The transmitting range for an audio loop is very short, when compared to RF transmission.
Even though the energy is ELECTROMAGNETIC RADIATION, it is a very low frequency and the signal contains very little energy.
In most cases the receiver (the hearing aid) must be situated within the loop or a very short distance away.
The transmitting and receiving coils must be in the same plane to achieve the greatest range.
To explain this more clearly we place the transmitting coil on the work-bench and point the single-ended choke at the centre of the coil. The flux rises from the centre of the coil and passes through the ferrite of the inductor. This produces a waveform (a voltage) in the winding of the inductor and the amplifier increases the voltage level to drive the earpiece.
We achieved a listenable level up to a range of 80cm.
The range for a larger coil is much more. A 10cm to 30cm receiving coil will operate from 1 metre to 5 metres, depending on the size of the transmitting coil.

PARTS LIST

• 1 - Hearing Aid
• 1 - 50m of 0.5mm enameled winding wire
• 1 - 50m of 0.2mm enameled winding wire
• 1 - 3.5mm plug