Re: Simple RF/IR distributi on

[Nigel Orr <nigel.orr@xxxxxxx>]

At 12:52 01/03/00 -0000, you wrote:
>I think I was being naive in thinking that you could just amplify and
>re-transmit all the noise including any IR signals present and let the

Yup.  If you reamplify it, your signal is already noisy before it gets
retransmitted and re-received... with all the static and fluctuating IR in
the air (sunlight and fluorescent flicker for example), it's a wonder that
it works at all :-)

>I have just had a look at the Maplin circuit on the recommended web
>site.  As far as I can see it does not have a 38-40kHz oscillator in
the
>transmitter.

I haven't analysed the circuit in great detail, but there is mention of a
pulse generator in the transmitter- it says the pulse length is 40us, so
it's less than 25kHz, but I'm not sure exactly what is going on there!

>used (TBA2800) does not demodulate the signal?

I'm not certain, it's data sheet doesn't give too much away... it may be
that the Maplin project transmits the modulated cleaned IR signal.
Personally, I'm using the all-in-one receivers TSOP1338 or something like
that... seems much more straightforward, with a 555-based transmitter.
Transmitting modulated signals probably gives them a fighting chance of
passing along a UHF cable unscathed... 40kHz is at the bottom of the LW
band...

>Does anyone know how to alter this circuit to work with multiple
>receivers?

Again, I've not looked hard, but I think you could just connect up multiple
IR receivers or IR transmitters.  You will get errors when 2 people fire
commands at one time, but that's unlikely to happen often.

>  I guess it would be OK as long as the power supply can
>handle all the receivers

All the receivers (I'm not sure whether you are talking about IR receivers
or at-the-other-end-of-the-coax receivers (ie IR transmitters!) could have
their own power supplies.

>to bypass the distribution amp!  I suppose you need to decouple the DC
>at the input and output connections with 100p ceramic capacitors and
use
>an inductor to link the source with the destination (dc only).  Am I on
>the right track?

Something like that... make sure your distribution amp doesn't also provide
power for external masthead amplifiers, or you will then pass that on to
the TVs, which might not be good.

I tried to design a suitable filter off the top of my head at this point,
but it went embarassingly pear-shaped, so I'll leave that as an exercise
for the reader... or you could have a Tx/Rx pair at the distribution amp
and pass the signal across as infra-red :-)

Nigel


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