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[EK-TM4C123GXL] Red LED not working

Francisco Suarez
Prodigy 30 points

First time posting in TI forums!

I've had a Tiva C Launchpad for quite some time now (about 1.5 years), and I just noticed that the red LED is not lighting up anymore. At first, I thought I was not configuring the pin properly, but then applied the same configuration to PF1, PF2 and PF3. Blue and Green LEDs are working fine.

I can write to the to the register just fine, and tried a red LED with a 470 ohm resistor on a breadboard attached to PF1 and it's working fine.

To me, it seems the red LED is busted, but since I'm a newbie in electronics, I would appreciate some validation. Is there any other reason to this problem?

Also, how hard would it be to replace D1? It looks like I would need a special iron tip, and my soldering skills are not very good.

Thanks for your help!

  • 0
    Intellectual 695 points
    Hi Francisco,

    Try checking the voltage at either side of R3 from +VBUS with the Red LED turned on. You should see a low here if things were OK and therefore see a voltage to +VBUS. If not the driver transistor Q1 may be faulty. Measure the voltage at R2 from ground to see if you have a signal here. It should be a high here so you should see a voltage to GND. I got these details from the schematic in the User's Guide.

    Cheers,
    Lee.
  • 0 in reply to Leethal
    Prodigy 30 points
    Thanks for your time Lee.

    I measured +VBUS to R3, and I get 5 V. Then, I measured R2 to GND, and I see 3.3 V.

    The conclusion I can draw from this is that the driver transistor is functioning, but the LED is not since I'm not seeing the voltage drop caused by it when measuring from +VBUS to R3 (which should be ~1.8 V for a red LED).

    Can you confirm this? Is the only solution replacing D1? Is it doable with a regular soldering iron?
  • 0 in reply to Francisco Suarez
    TI__Guru**** 244400 points
    Hello Francisco,

    If the RED LED is not working then you may check that quickly by putting a through hole LED across the existing LED to ensure that the circuit does respond before taking the soldering route.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Prodigy 30 points
    Hello Amit,

    Yes, I just tried that. Tied an LED from +VBUS to R3 and it is coming on. So it is the RGB LED, indeed.

    However, I would prefer not to have a through hole LED dangling on my Launchpad.

    So is replacing the RGB LED doable with a regular soldering iron?

    Do you think the probable cause for this is user action, or just the LED failing? What could I do to prevent this from happening or extend the service life of the remaining LEDs?

    Thanks!
  • 0 in reply to Francisco Suarez
    TI__Guru**** 244400 points
    Hello Francisco,

    Depends on the pitch of the solder tip. Note that it is a Integrated RGB LED, so you would be changing all 3 of them.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Guru 47900 points
    Before I'd "touch" the Led w/an iron - suggest that - via an insulated (plastic) probe - you apply gentle downward force upon the Led's (slight) leads/solder joints.

    If current limiting resistors are in place - and correct - hard to imagine anything which would damage that Led. Thus I lean toward a bad connection at the (unique) "red" solder joint - that R-G-B Led.