Q: Check if an LED in my circuit is still "good"
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) Monitoring forward voltage across LED. (b) Monitoring LED current by opto-isolator.

Figure 1a may provide the best solution. The analog input should read around 2 V for the LED when on, 0 V if short-circuited and +5 if open-circuited. In addition the ADC can be read when the LED should be off to confirm that the micro output is OK.



Figure 2. Use the chart to estimate the expected forward voltage drop across the LED based on its colour. The current is the y-axis value of the point where the loadline crosses the chosen coloured LED curve. The expected forward voltage drop is the x-axis value of the same point. Source: LEDnique.com (mine).

Figure 1b: If analog inputs are not available monitoring by opto-isolator is an option but not as precise as the analog monitoring. It will not be able to differentiate between a working and short-circuited LED.

In either case the LED can be toggled periodically to test the LED whether on or off.