Re: IR LED to repair remote control

In article <1190721429.562853.308820@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Andrew wrote:

Unfortunately my Logitech harmony 880 remote stopped working. When
pointing it into a digital camera, I can no longer see the IR LEDs
lighting up (there are two). Logitech tech support has confirmed that
the IR LEDs are probably dead, but they have been less than helpful.

In an attempt to fix the remote myself, I bought two sets of IR
emitter/detector packs from radioshack (quick and easy), part number
276-142 (http://www.radioshack.com/product/index.jsp?
parentPage=search&summary=summary&cp=&productId=2049723&accessories=accessories&kw=ir
+diode&techSpecs=techSpecs&currentTab=features&custRatings=custRatings&sr=1&features=features&origkw=ir
+diode&support=support&tab=summary).

I used just the emitters (tinted package) and replaced both of the
emitters that were on the remote. The old emitters were clear
package, FWIW.

The remote worked great again after this for two days, but now, has
apparently died again. I suspect that there is either some other
problem with the board that is blowing the LEDs, or the radioshack
replacements were not within spec. I'm really hoping it is the LEDs
not within spec, because I don't want to deal with logitech tech
support any more. Or, since I'm conveniently out of warranty, shell
out for them to repair it, when seemingly I can repair it myself for a
few dollars.

I've never really worked with IR LEDs before, so I was wondering if I
could get a few pointers in trying to order some IR LEDs from
digikey. I realize it may be difficult without knowing the details of
the design of the remote. My plan, though, is to get the highest
rated LEDs and hope for the best.

The wavelength of the emitter that worked from radioshack was 940nm.
On digikey I see that there are LEDs ranging from 860nm to 950nm.
This is one area where I'm not clear. Just because the 940nm LED
worked (and worked really well on all my devices I might add, it was
not flaky at all), does that mean that I should stick with that
wavelength? Or is it likely that the other wavelengths are close
enough that they will also work well?

And what ratings should I pay particularly close attention to when
trying to spec an LED for this purpose? I'm thinking a combination of
the highest pulse forward current and highest continuous current? For
that purpose I saw the LITE-ON LTE-5228A, digikey part no 160-1062-
ND. 250mW power dissipation, 3A peak forward current, 150mA cont
current, 7.2V reverse voltage, and 940nm wavelength. It seems to be
the highest ratings in all categories for the 940nm wavelength LEDs
digikey stocks, and is my top choice right now. It also uses a clear
package like the original emitters, if that even matters.

Any thoughts overall on the matter?

I expect wavelength anywhere from 860 to 950 nm to be OK.

I think better would be LEDs with higher current capability, both
continuous and peak.

Then again, I see enough inconsistincies between LED datasheets and
actual performance...
If the unit is indeed out of warranty, one thing I would do is add a
resistor in series with each LED. Go ahead, splice resistors in, do a bit
of surgery/hacking if the unit is out of warranty!
I think that a 10 ohm resistor in series with each LED should make a
difference - by dropping half a volt at 50 mA. 1/8 watt resistors should
be adequate and are very compact.

The above Lite-On LED does have a data*** making a claim for higher
current and power. I give some fair chance this is true and likely to
help.
But beware, it has a wider beam width (35 degrees), and that can detract
from the beam intensity and reduce your range - especially if you need
resistors to keep eve these from blowing.
Should you need resistors, go for higher mW/sr intensity LEDs (they tend
to have narrower beams) and be prepared to use 15 possibly 22 ohm
resistors to keep the LEDs from blowing.

==============================================

By any chance, are you using lithium batteries instead of alkalines?
The extra .1 or so volt per cell could make a difference if the product
design is not tolerant of that extra voltage.

==============================================

You may do well by using rechargeable batteries (NiMH or NiCd) - the
voltage is a little less than that of fresh alkalines - that may save your
LEDs.

=====================================

- Don Klipstein (don@xxxxxxxxx)
.