How to dim your remote LED on your ALP system

[awj223]

Nice writeup!

I wish AL would have thought of this, the confidence LED is far too bright and dimming it by any other means than what you have designed makes the alert intensity insufficient. Dark mode just makes me wonder... did I lose power?

Great solution!

I don't know, the only thing a constant LED tells me is that the system is powered up. But it doesn't tell me that the CPU is still running. Assuming that the blinking LED is controlled by some process on the CPU, the blinking action to me indicates that the CPU is actually doing something (the watchdog timer is firing, the CPU is switching the LED on, and then off). But a constant LED? Well if the CPU were to hang, wouldn't the LED simply remain on? Knowing that the system has power is not the same as knowing it's running. That's why I'm probably just going to keep on using dark mode. If I want to confirm it's running, I'll hit a button and make sure it responds.

[radargeek]

I don't know, the only thing a constant LED tells me is that the system is powered up. But it doesn't tell me that the CPU is still running. Assuming that the blinking LED is controlled by some process on the CPU, the blinking action to me indicates that the CPU is actually doing something (the watchdog timer is firing, the CPU is switching the LED on, and then off). But a constant LED? Well if the CPU were to hang, wouldn't the LED simply remain on? Knowing that the system has power is not the same as knowing it's running. That's why I'm probably just going to keep on using dark mode. If I want to confirm it's running, I'll hit a button and make sure it responds.

If the LED is dimmed via PWM it's essentially blinking at a fast rate (faster than the eye can see), so if the CPU were to hang, and software PWM is used, the LED would either get stuck on full brightness or off.

That's not a 100% reliable indication of system stability, however. Depending on the nature of the failure the PWM loop could still be running but other functions stop working, or vice versa. Or PWM may be implemented in hardware or in a CPU peripheral that keeps working even if the main CPU hangs. A good design uses a watchdog timer to reset the CPU if it hangs (another feature built into most microcontrollers).

Pressing a button is a better way to know "for certain" that the system is functioning correctly.