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Old 15th Jul 2010, 4:29 pm   #1
camtechman
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Default Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

I'm knocking up a simple circuit to demonstrate to my young nephew a basic 'telephone', i.e: Type 16 Carbon Transmitter, wired with twin bell wire to a 4T Receiver, using a 1.5V battery in one line.

Does it matter which way the battery goes round, as it seems to work either way round ?
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Old 15th Jul 2010, 5:24 pm   #2
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

... that depends on whether you want the 'bringg' or the 'bringgggg' first!
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Old 15th Jul 2010, 5:47 pm   #3
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

As a carbon transmitter is merely a resistance which varies according to the variable air pressure caused by the speech, it will work either way round. What the receiver is responding to is the variations in the voltage thus caused and again will work either way round. Be aware, however, that this arrangement will result in a DC voltage across the receiver, but at 1.5V, I wouldn't have thought this likely to cause any damage. Those with more in-depth knowledge of telephone circuitry may be able to give definitive advice on this.
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Old 15th Jul 2010, 6:27 pm   #4
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

I'd suggest that the polarity is going to be slightly more important to the receiver than to the carbon microphone - whether the standing DC augments or diminishes the permanent magnet. Either will work, and I'm almost sure that telephone receivers are tolerant of this - but it used to be 'good practice' with headphones to connect so that the diaphragm is pulled slightly inwards, so that the standing DC doesn't have a demagnetising effect oin the magnet.
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Old 15th Jul 2010, 9:16 pm   #5
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

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Originally Posted by camtechman View Post

Does it matter which way the battery goes round, as it seems to work either way round ?
No. Not for what you're doing.

If you have an old relay or choke and a suitable capacitor, you could route your DC to the mic via the relay (choke) and connect your receiver, with the capacitor in one leg, across the wires that go to your carbon mic, thus forming a rudimentary 'transmission bridge', so that the AC speech goes via the capacitor to the receiver and the relay (or choke) prevents the speech from going around the battery.

The DC will be blocked from the receiver, but will energise the carbon mic.
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Old 16th Jul 2010, 11:28 am   #6
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

Many thanks everyone for the replies & info. This set-up is only for demo and not for long-term or continuous use.

I'm 'ejakating' him in sound transmission, started with 2 tins and string, then 2 GPO type 4T's & bell wire and now the carbon transmitter, battery & a 4T.

The next amazing trick will be transmitting sound on a light beam....he thinks I'm a magician!
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Old 16th Jul 2010, 11:32 am   #7
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

Just to add, I'm trying to catch him early so that he'll be become an obsessed collector like me.

I remember my Physics teacher at school who demostrated such 'magic' when I was about 11 and from then onwards I was hooked!
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Old 16th Jul 2010, 1:42 pm   #8
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

Quote:
Originally Posted by camtechman View Post
I'm 'ejakating' him in sound transmission, started with 2 tins and string, then 2 GPO type 4T's & bell wire and now the carbon transmitter, battery & a 4T.
Ah... I was going to mention the 'sound-powered' telephone, but you've done it!
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The next amazing trick will be transmitting sound on a light beam....he thinks I'm a magician!
Great Scott! It'll be fibre-optics next, then!

I remember an article in 'Pratical Electronics' from years ago, modulating a sealed-beam car-headlamp (I think it was) at one end and using a focussed LDR at the other. Something like that, but with LEDs instead?
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Old 16th Jul 2010, 1:55 pm   #9
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

When I worked at Chubb Alarms, they produced a valve operated 200ft beam system (walk between transmitter & receiver and the alarm was triggered). It wasn't IR but used a value detector and a focused light bulb.

One of the techie lads set up one on the outside of the factory wall and modulated it to transmit/receive sound. We used it to talk to the repairs dept at the other end of the factory.

One day we were playing with it and chatting with one of the guys in the repairs dept and we could hear a strange "plinking/plonking' interference on the sound....it turned out that it was raining outside and causing the noise.
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Old 16th Jul 2010, 2:03 pm   #10
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

T'other day I had my nephew puzzled by placing an 8" Base Speaker on the table (with another hidden one wired to it under the table)

Then, I placed a handfull of rice on the cone of the visible speaker and amazed him with bouncing rice (whilst tapping on the cone of the hidden speaker) !

That's when he told me that I was the best uncle in the world!..... (Bless)
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Last edited by camtechman; 16th Jul 2010 at 2:04 pm. Reason: spelling error
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Old 18th Jul 2010, 11:26 pm   #11
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

Quote:
Originally Posted by russell_w_b View Post

I remember an article in 'Pratical Electronics' from years ago, modulating a sealed-beam car-headlamp (I think it was) at one end and using a focussed LDR at the other.
I remember building a version of the sliding shutter modulator for this out of a deconed old loudspeaker, some aluminium strip and a cornflakes packet while I was at college. It beat the pants off just directly modulating a torch bulb by connecting a bulb and battery in series via the loudspeaker secondary of a valve amplifier!
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Old 19th Jul 2010, 1:23 am   #12
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

Quote:
Originally Posted by camtechman View Post
Just to add, I'm trying to catch him early so that he'll be become an obsessed collector like me.

I remember my Physics teacher at school who demostrated such 'magic' when I was about 11 and from then onwards I was hooked!
To paraphrase Jean Shepherd, you'll be leading him down a path that ends in a hamfest flea market....staring at ten year old transformers....with shorted primaries..."

http://www.qtl.co.il/img/copy.pnghttp://www.google.com/favicon.icohttp://www.qtl.co.il/img/trans.png
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Old 22nd Jul 2010, 7:30 pm   #13
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Default Re: Polarity Of Battery In Simple Carbon Transmitter To Type 4T Receiver?

LOL!
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