Re: Urgent - Time to kill, want to build IR Extender

[Nigel Orr <nigel@xxxxxxx>]

At 16:07 01/11/00 +0000, you wrote:
>I now have all components in place and I want to test it, but I have
one
>silly problem, how do I get 5V to the TSOP 1838. I am putting 12V to
the
>transmitter circuit no problem, but in the absence of ac 5V output from
the
>comfort system (not yet setup), can I convert 12V to 5V easily, or will
4.5V
>be sufficient (three AA batteries in series).

I tested mine initially with 4 in series, and a 1k resistor to drop the
voltage a bit.  3 in series might work, and certainly shouldn't do any
harm.

Doing it 'properly' for a 12V supply, either use a 78*05* (the '*'s can be
pretty much any combinations of letters with no real effect), or, for an
even smaller, cheaper option, use a zener diode.

For the zener, assuming a 15V supply, you'd need a 2k resistor in series
arranged like:

+12V ------ 2k resistor ------- 5V1 Zener diode ------ 0V

The cathode of the 5.1V zener (5V1 is a common way of marking devices,
similar to 2k2 for a 2.2k resistor, it means they don't have to worry about
someone missing a tiny '.' on the package!) is probably marked with a black
band, and connects to the 2k resistor.  The +5V connection of the TSOP1838
connects to the same place.  The zener diode will dissipate about 25mW
maximum, so just about any 5.1V zener will do the job.

The 2k isn't critical, if you happen to have anything between 1k and 5k to
hand... I'll call it 2k7, to make it the same as the ones used for the
transmitter!

I've not tried mine this way, as I happened to have some 7805 regulators
lying around when I was building the prototype, but I probably will for the
'proper' receivers, it's smaller and lower power.

I'll do a schematic and put it on the web sometime, as a few folk seem to
be interested in building it now- according to the geocities counter there
have been about 100 hits, but I presume most of those are from a few people
going back to check things :-)

Nigel


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