LED's dimly lit..

29 Jul 2010


I recently got a second mbed and I've noticed that the right two LED's are dimly lit when the device is on. It doesn't seem like its a software program or anything like that. Does anyone else have this issue with the newer mbeds?


30 Jul 2010

Hi Igor,

Thanks for reporting this, and yes, I think I can explain this.

We chose to use the pins to source current rather than sink current, so that writing '1' to GPIO will turn the LED on. It seems more common to use MCU pins to sink current (which works also for open collector pins), but this leads to '0' is on, and '1' is off, which might be confusing to someone new to electronics and software.

The pins the LED connect to have weak pull-up resistors by default, and because of the chosen configuration, a small current will flow through this pull-up resistor and through the LED.

When we made a batch of mbeds with different colour LEDs (a long time ago) all but the blue ones would be dimly lit. , Basically for some colours of LEDs have I-V characteristics that mean the current from the pull-up was enough to dimly light them up. Blue LEDs have always required higher Vf and If and hence not had this problem.

So, with all this in mind, my guess is that two of your LEDs have IV characterists that mean the LEDs light dimly with the current from the pull-up. This will be down to tolerance and production variance, and these two LEDs are probably close to the end of the tolerance window which means the light up.

I'd be interested to know if anyone else has seen this? We could increase the series resistor slightly to prevent this from happening, at the cost of the LED being slightly dimmer when on.

Feedback and Opinions?




30 Jul 2010

Thanks for your reply. I assumed it had to do with the particular LED's since the other two are fine.