piclist 2002\09\20\220702a >
Thread: Inverting IR LED driver with 1 transistor
face BY : Russell McMahon email (remove spam text)

Now the men have had their input :-) - there is an interesting variation you
can use here.
Instead of putting both LED and resistor in the collector lead, place the
resistor in the emitter. This then becomes a VERY rough current source as
the current is limited to (Vdrive-0.6)/R

E --> resistor --> +5V
C --> LED --> GND
B --> resistor --> TTL output from uP

Odds are its not much different in result than the other arrangement in this
This would be of more affect if you drove the LED from a different voltage
source than the cpu - eg with an NPN drive & the LED supplied from the
unregulated battery supply while the processor runs on a regulated supply.

Have a look at the circuit inside typcal IR remotes and see what they do
(will vary no doubt)


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See also: www.piclist.com/techref/io/irs.htm?key=ir
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Subject (change) Inverting IR LED driver with 1 transistor

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