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<H1 ALIGN="CENTER">Serial Port Transmitters</H1>
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The following text was taken from the
<A
HREF="http://www.metzlerbros.de/lirc/index.html">
lirc-0.3.6</A> package with kind permission of its
author, Ralph Metzler. I have just substituted the
ASCII arts by nicer GIFs (thanks to David Norris),
recommend another IR LED (the LED recommended
originally was transmitting at 880 nm which is not
suitable for remote control applications which use
950 nm) and added a small signal diode to protect
the IR LED from the voltage of the serial
port. Most IR LEDs have reverse voltage ratings of
only 5V. I received reports that the range of the
first circuit will be much smaller if you use it
together with an receiver. Sometimes it will even
stop working at all.
</P>
<P>
If you are looking for a circuit with better range
and don't want to use an external power supply you
should have a look at <A
HREF="improved_transmitter.html">Enrique Vidal's
circuit </A> that he <A
HREF="http://www.geocrawler.com/archives/3/3680/2001/1/0/5037984">
posted on the mailing list</A>. I haven't tried it
myself but it looks very promising.
</P>
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<!-- AD -->If you are looking for a ready-made
<!-- AD -->transmitter solution you should have a
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<!-- AD --><A HREF="http://www.irtrans.de/">www.irtrans.de</A>.
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<!-- AD -->and the driver software provides a LIRC
<!-- AD -->compatible socket interface.
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<P>
Simple transmitter circuit on serial port:
</P>
<CENTER><IMG SRC="images/simple_transmitter.gif"
ALT="Simple transmitter"></CENTER>
<P>
R should be greater than 1K so that the current is less than 10mA (my serial
port has about +/- 10V). I guess it is also save to leave out the resistor
since the serial port should also have a max. short circuit current.
Since the maximum current is so small almost any infra-red LED should be
usable
</P>
<P>
The range of this device is up to 1m depending on the
receiving device (e.g. with my Sony VCR) and if you aim right at the IR
detector. Not good enough to be practical but enough to e.g. invent your own
code to control your computer and program a programmable remote with it.
</P>
<P>
Usage of the this circuit is not recommended if you value your serial port.
If it is on the motherboard I would not use it!
I use a serial port on a cheap serial card for testing.
Actually I only say this, so that it's not my fault if your house burns
down ...
Does anybody know what can happen if you short circuit the serial port?
</P>
<P>
If you don't want strain your serial port, use a bigger resistor or a simple
amplifier circuit like this:
</P>
<CENTER><IMG SRC="images/better_transmitter.gif"
ALT="Better transmitter"></CENTER>
<P>
R in the amplifier circuit can be chosen as small as 5 Ohm (for 5V !).
Since the signal is pulsed and with a 40kHz carrier frequency, an
LED like the LD 271 (or others with max. 100 mA) can tolerate 1A for
a short period of time.
Some data sheets might mention how high the current can go for certain
pulse lenghts.
</P>
<P>
But take care that after the signal is sent the signal line is low or
your LED might become a little "brownish" like one of mine did :-)
(It even still works, but the range decreased to a few centimeters ...)
It should be safe if you use >50 (>90) Ohm for 5 (9) V, but the range
will be much smaller.
</P>
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<P>
Petteri Aimonen sent in the following suggestion to
better protect the curcuit from overloading without
significant loss of range. The trick is to add a
100nF - 220nF capacitor in parallel to the current
limiting resistor which he suggests can be then as
high as 50 Ohm:
</P>
<PRE>
+5V
|
_|_ -->
\ / --> IR LED
--- -->
BD237 |
|---'
--|
|->-o------------,
| |
| |50Ohm ----- 100nF-220nF
|_| -----
| |
o------------'
|
----- Earth
---
-
</PRE>
<P>
Another suggestion for a <a href="http://iguanaworks.net/ir/">
high-power LIRC transmitter/receiver</a> comes from
Brian Shucker. This design integrates both receiver
and transmitter in one circuit using one power supply
just ready to use.
</P>
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