Roomba IR LED failures

[james_s]

I've been maintaining a fleet of Roombas for myself and friends and family for quite a few years now, previously the old 400/Discovery series and more recently the 500 series and later models. One of the most common failures I've encountered behind bad battery packs and mechanical problems is faulty IR LEDs. These things use a lot of them, proximity sensors, bumper switches, wheel tachometers, etc and across both series I've seen what I find to be a surprising amount of IR LED failures. They don't normally fail outright, they just get weak and that causes erratic operation before it gets so bad that it stops working completely. Just the other day I fixed a 500 series my friend gave me that I found had a weak LED in one of the bumper switches and a few days later I finally went over and grabbed my mom's 500 which had stopped working some time ago and found the same fault in hers. Does anyone else work on these things? I'm curious as to why these LEDs seem to fail so often, whether they're just cheap LEDs, or they're overdriven, or damaged by spikes from the motors or what. It's been a while since I've looked at the circuit but IIRC most of these LEDs are wired with several in a series string with a current limiting resistor fed from the switched battery voltage that is live whenever the robot is running.

Also in case anyone is looking, I found an exact drop-in match for the LED used in the bumper switches and wheel tach sensors, available for something like 37 cents each from DigiKey. There are sellers on ebay scalping replacement LEDs for like $50 a pair. iRobot of course will not sell you the LEDs or even the complete sensors, for the bumper sensor you are expected to replace the entire robot chassis.

[fzabkar]

Do the Digikey replacements also fail in this application?

[JXL]

I have not worked on them for a while, but I had the same issue with IR LED failures. My first line of attack was to increase the LED drive to 20mA by piggy-backing a SMD resistor on top of the existing resistors.  I don't recall the R values, but I think the original drive was about 10mA.  The increased LED current normally "fixed" the problem for a few years. When the IR LEDs finally fail, I then replaced them with whatever I had in my parts bin.

[james_s]

Do the Digikey replacements also fail in this application?

They haven't yet, but that doesn't mean they won't. Most of the Roombas I deal with are close to 10 years old and some older so the repairs probably have not reached the number of hours on the originals yet. I'm still surprised to see so many LED failures though, none of the visible LEDs they use have failed, and I have lots of other equipment with LEDs much older than that and very few of them have ever failed.

In case anyone needs them, I looked up the replacement I found, it's IR928-6C-F‎  made by Everlight. They're 28 cents each from Digikey. There is a second type of IR LED used in these things that is a conventional 3mm clear package, those do not seem to fail as often and it's a standard package that is easier to find than the flat side emitting type used in the bumper switches and tach sensors.

[fzabkar]

I notice there is a huge variation in the light current spec for the Everlight LEDs -- 306uA min / 1870uA max. Maybe you should set up a jig (see page 3) and do some select-on-test measurements?

[james_s]

I notice there is a huge variation in the light current spec for the Everlight LEDs -- 306uA min / 1870uA max. Maybe you should set up a jig (see page 3) and do some select-on-test measurements?

It might be interesting to see the results, but frankly I've got enough things on my plate that I'm not going to spend time testing a bunch of replacement LEDs, the ones I found drop right in and work perfectly. I'm still curious why so many of the originals fail, I do not recall having any IR LEDs fail in anything else, although most things that use them pulse them at a very low duty cycle and these run continuously for up to an hour or more at a time.

[longboard1210]

Having repaired many of these and having a fleet myself of over 6 i can probably point you in the right direction

Its not the ir leds  yes they do go weak but it is usually caused by the red plastic cover in front of them it gets filled with dust and light scratches on the outer surface  with everyday use

I used a buffing wheel and cutting compound followed by a polish  to buff the trims up and 9 times out of 10 they started working fine again one to  try

[james_s]

This is definitely the LEDs. I'm not talking about the soft touch proximity sensors, I'm talking about the bumper switches and wheel tachometers, these have a side emitting IR LED directly facing a phototransistor and a little plastic flag or toothed wheel interrupting the beam. There is no lens in between them and cleaning does not help, the LED itself goes weak, easily confirmed using a camera that can see IR and comparing side by side with other IR LEDs in the machine.