LoPy4 RGB LED Failing due to Temperature

[d.alvrzx]

Hi all,

I have been using LoPy4 units in the field for several years now. Lately, some of these units have been exhibiting a behaviour where the LED no longer responds to pycom.rgbled() commands, they just stay in the last color they had.

I noticed that this seems to be related to the temperature of the LED, because if I slightly heat it with a blowdryer, the LED suddenly stops working. When temperature goes back down, it works again. Please note I am not overheating it: the temperature it reaches does not burn my skin, so definitely below 50 degrees C. And this does not happen on all LoPy4 units, only on some of the ones that have been deployed to the field. To be transparent, these units have been working constantly at temperatures between 30 and 50 degrees C.

I checked P2 with an oscilloscope on a faulty unit, and the control signal for the LED seems to be sent correctly by the LoPy4 (regardless of the LED being hot/unresponsive or cold/responsive). For example, for rgbled(0x0000ff), the control signal has the following characteristics:

1. Frequency: 833.33 kHz
2. Duty cycle for first two sections: 25% (300 ns high, 900 ns low)
3. Duty cycle for last section: 75% (900 ns high, 300 ns low)

I obtain the same signal when I heat up the LED and it becomes unresponsive, so it seems an issue of the LED itself. I also verified pulse widths and other timing aspects and they seem the same regardless of the LED state. Finally, this does not happen on new LoPy4 units, probably because the LEDs have not been degraded from use at hotter temperatures?

Can anyone confirm if these LEDs are sensitive to temperatures below 50 degC? Are they not rated for 85 degC as most other electronics?