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Induction loop tester

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Phenoxyethanol
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Username: Phenoxyethanol

Post Number: 46
Registered: 05-2005

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Posted on Tuesday, 30 May, 2006 - 08:48 pm:   Edit Post Delete Post Print Post

Dear all

I hope that someone may be able to help me.

At our church, I am partly responsible for our PA system, which also includes an induction loop. Unfortunately, at present, I have no way of testing the output as I do not own a hearing aid.

Usually the only way I have of knowing if the output is great enough is when someone approaches me at the end of the service and says "was the loop on, I couldn't hear it"

Any ideas/diagrams appreciated. I have tried google and all I seem to get are ready made (expensive) units.

Regards

Bob
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Externet
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Post Number: 63
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Posted on Tuesday, 30 May, 2006 - 10:15 pm:   Edit Post Delete Post Print Post

Hi.
The simplest way would be to have visual contact with the led VU meter on the front panel of the transmitter (if it has it)

Otherwise, you could use a receiver (hearing aid or not) to drive a simple LM3915 circuit located wherever can be seen to confirm the signal is on.

Building a receiver would be a more involved project, but still, my suggestion is centered to provide a visual indicator.
The schematics are available on the net as application notes for such IC.
Miguel
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741
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Post Number: 29
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Posted on Tuesday, 30 May, 2006 - 10:16 pm:   Edit Post Delete Post Print Post

Try
http://www.marcspages.co.uk/tech/deafspec.htm
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Obiwan
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Post Number: 510
Registered: 12-2005

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Posted on Tuesday, 30 May, 2006 - 11:15 pm:   Edit Post Delete Post Print Post

If it's what I'm thinking of, you should be able to build one very easy.

Remember that the induction loop is just one side of a transformer.

Just a few turns of wire and hook it up to a o-scope or maybe the mic input of a tape recorder or something similar, and you have a tester.

Depends on the wire you use too. I'd use small gauge, and many loops. You really can't have too many. It will only increase it's output.

I'd start with at least 20 or so. Make your loops and then "listen" to them.
Do Not Hit The Fly That Lands On The Tigers Head.
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Ant
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Username: Ant

Post Number: 12
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Posted on Wednesday, 31 May, 2006 - 10:09 pm:   Edit Post Delete Post Print Post

Hello all,

This is a subject I've been gently investigating...

I made Mom a little loop driven off a microphone amplifier; the loop (a few turns over a bit of ferrite) fits on the end of her specs arm, the mic is a camcorder "gun" mic - actually a cancelling arrangement with two elements. The idea is that she could hold it over the edge of the table in noisy environments and point it to whoever of her friends was speaking. I arranged the strength of the loop by comparison with a telephone handset - not very scientific as her aid has avc and I'm afraid I didn't note the details anyway!

A lunch club I go to has a member with loop moitor, it's a baby alarm fitted with an input coil; I've never asked but probably 20 turns or so about 1" diameter, air-cored and wound pretty casually!

Hoping that I won't get accused of hijacking the thread - it would be handy also to consider the wider details of a loop. I'd been considering providing the lunch room with a permanent loop (we borrow a portable kit at present) and perhaps a permanent amplifier etc. My suck-it-and-see feeling is to make a loop with a series resistor of about 4-5 ohms for safety and a largish amplifier, say 40W max. How might that figure sound, would any equalisation be necessary to counter loop impedance? I'd probably just use the tone controls...

To sum up, and to drag myself back to the original question; get a small microphone-level amplifier and at least an earphone, preferably a meter on the output. Feed it with a coil, perhaps have several coils to try on site. Go to a place with a loop - bank, station or whatever and find a loop that gives you a reasonable volume/meter reading. Just to check your church, that would do. In my case I could then set out an experimental loop/amplifier system and when I was happy with it, get a genuine user to try it.

But not tonight, eh!
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Obiwan
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Post Number: 513
Registered: 12-2005

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Posted on Wednesday, 31 May, 2006 - 10:41 pm:   Edit Post Delete Post Print Post

I'm curious. Do these induction loops work with all hearing aids?

My mom wears them and has a really hard time talking on the phone.

The biggest issue is she can't place it next to her ear because of the feed back.

So tghat causes volum issues.

Could I make a simple induction loop and pickup for the phone to fix her problem?

Or do these induction loops work only with certian devices??
Do Not Hit The Fly That Lands On The Tigers Head.
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Ant
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Post Number: 13
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Posted on Wednesday, 31 May, 2006 - 11:05 pm:   Edit Post Delete Post Print Post

Hello Obiwan,

I'm just a suck-it-and-see expert, as you'll have gathered! These things are just audio transformers, effectively. Very inefficient because of the separation between "input" primary and the "output" secondary.

The "input" is just a coil of wire, possibly with a bit of a core to concentrate the field, driven by an audio amplifier.

The "output" is the corresponding coil in the hearing aid. It may have a core but it seems to me that that would make it too fussy about orientation to the exciting field. It feeds into an audio amp with enough gain to drive the earpiece.

I think one of the problems with modern digital aids is that the volume and thus gain is automatic, certainly my mother isn't wildly happy with it. She does find, though, that different 'phones have different characteristics and audibilities, might your Mom need to experiment with different 'phones?

Rereading your post again: the 'phone might howl unless the handset is pressed to her ear. If that's uncomfortable, could she hold it sideways so that her face is between earpiece and microphone to provide some acoustic isolation?

If the problem can't be sorted out, go back to the technicians - they will swear black's white that there's no problem, that the digits solve all problems, but be persistent - we speak from experience as well as hearsay!

Lastly: I've not played with the old-fashioned telephone induction coil for a long time, there may not be anything in a modern 'phone to give an external field except in the handset - older 'phones used a fairly simple transformer thing, a straight core with windings as appropriate.

I've probably said all I know or can guess on this subject, there's obviously scope for someone more knowledgeable...

Regards Ant
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Vk3kbr
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Post Number: 89
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Posted on Friday, 02 June, 2006 - 11:40 am:   Edit Post Delete Post Print Post

G'day Pheno....,
pardon the shortening of your username. :-)

I have installed several of these loops in my career, these have been in both loungerooms for listening to the telly, to churches and theatres.

The loop I used in a theatre was wired with about 6-8 turns of switch wire (240V ac type). This was done for two reasons,
1 To get the impedance up to the minimum that the amplifier could handle (2 Ohms in this case)
2 To get the maximum current flowing in the loop.

The test device I used then was a 1 Ohm headphone, complete with a small gain amplifier.

The loops worked quite well. The amplifier that I used to drive the loop was usually a 250Watt (RMS) type, the sound INSIDE the loop was fantastic, outside the loop the sound dropped off extremely quickly.

These days, most hearing aids are both accoustic and magnetic (switchable), I have never had to return to any of these jobs for updating.

Hope this has been of some help.
Regards, Rod vk3kbr
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Mike_b
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Posted on Friday, 02 June, 2006 - 05:52 pm:   Edit Post Delete Post Print Post

Just to say that I have had the same results as VK3KBR. It has worked for years at our church and I have always been amazed at the simplicity of the concept.

The output from the main amp connects to a 8ohm load resistor then to a wire running around the roof space and back to the amp.

Any old amp, with any old coil acts as a receiver.

rgds

mb
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Phenoxyethanol
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Username: Phenoxyethanol

Post Number: 47
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Posted on Friday, 02 June, 2006 - 08:41 pm:   Edit Post Delete Post Print Post

Hi Guys

Many thanks for all the suggestions. It has certainly given me an idea of where to proceed.

Regards

Bob
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Piers
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Registered: 04-2006

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Posted on Wednesday, 07 June, 2006 - 07:29 pm:   Edit Post Delete Post Print Post

Hi all.

Obiwan, These loops should work with all aids with a telecoil, activated by switching the the aid to the T position. You may even find that the aid in the T position will pick up sufficient magnetism from the phone speaker to work. In my experience as a aid user some phones are better than others, look for phones that are hearing aid compatible. As for the feedback issue there's something wrong somewhere, most likely the earpiece needs replacing as it's become mis-shapen with time. Is it an NHS aid? I can completely cover my aid with my hand with very litle or no feedback.

Sorry if I've gotten a bit off topic here.

Regards Piers
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Obiwan
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Posted on Saturday, 10 June, 2006 - 03:32 am:   Edit Post Delete Post Print Post

Thanks, that gives me a bit more information. You have to have a special hearing aid for it to work. Just any old aid won't do it.

My mom's issue is the degree of loss. So she's going to get feed back no matter what just becaue of the volume.

So if she could eliminate the acutal sound portion at times, that would do her a lot of good.

I'm going down there for a visit soon, I'll talk to her about it.

Thanks.
Do Not Hit The Fly That Lands On The Tigers Head.

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