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

[Nigel Orr <nigel@xxxxxxx>]

At 11:12 31/10/00 +0000, you wrote:
> > >Second, the LED's, I know that the shortest leg is the
cathode, does that
> > >mean the other leg is the one to connect to the +12V.
> >
> > Probably.  Some LEDs are the other way round, there's no
particularly
> > common standard, if you can see inside, it looks a bit like a
wire going
> > into a 'cup'.  The 'cup' is usually the cathode (I think, I
usually just
> > test them with a battery!).
> >
>I was worried about blowing it if I did this and got it wrong, but I
will
>now try tonight

If you have a 9V battery with a 1k resistor in series, it's _very_ unlikely
to blow up the diode if you connect it backwards, especially when it's only
for a second or two to find which is right.  If you leave out the resistor,
all bets are off, I've tried it...  you should be even safer with a 3V
supply (2 AA batteries) as most diodes have a maximum reverse voltage
around 5V.

> > The only way to test before the receiver arrives is using an
oscilloscope,
> > a multimeter with a frequency meter, or a camcorder/camera.  If
you have
> > any of those, let me know and I'll tell you what you can use it
for.
> >
>I do have a camcorder (Hi8) and a digital camera, please do send
>instructions for testing with these.

The camcorder is probably (almost certainly) sensitive to the IR from the
diode, so if you point it at the diode and it's connected right, it should
be visible on the video output.  The digital camera might work too, but
I've never tried one for IR.

Once you build the circuit, put a 10k resistor between the input + pin and
+12V, the IR led shouldn't be lit.  If you now short out the input (the
junction between the 10k resistor and the 2.7k input resistor) to 0V, the
IR LED should come on.  It should also be possible to check that it's
oscillating, not just stuck on.  Take the end of the 2k7 resistor that is
plugged into pin 3 of the chip, and plug it into +12V, the IR LED should
now appear brighter to the camera, as it's on constantly, not flashing 50%
on 50% off at 38kHz.

Don't forget to reconnect everything as it was before you try it for
real!  When you point a remote at the receiver and press a button, you
should see your remote's IR LED flash, and see your transmitter flash at
the same time (the flickering is usually visible when a signal is being
passed)

> other question, I am planning to link the receiver(s) to the
transmitter
>with CAT5 going through normal IDC connections. Should I avoid using
both
>wires in a twisted pair or is this OK ?

No, you should actually aim to use both wires in the same twisted pair, as
that should reduce radiated and received interference.  So you might have
one pair supplying +12V and 0V, and the other pair with 0V and IR
data.  There are some situations where that could cause problems, but
they're not very likely.

After prototyping on plugboard, I've built my receiver up on soldered
breadboard, with Krone PCB mount IDC terminals, but I've not tested that
one yet!

Nigel


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