Q: Check if an LED in my circuit is still "good"

Question

I have an electrical circuit which uses an LED, connected to a µC, to show the status of an x-ray source in another room. Obviously this LED is important to have in a working condition.

Now, LEDs can fail in in multiple ways, one being a broken circuit, another being a short circuit.

I have thought about using a light sensitive resistor to check the status, yet the surrounding light might break this. I then thought about PWMing a "signal" on to my LED power, trying to read the same signal from my light sensitive resistor - assuming that no outside light emits the same strobe signal (I might even modulate the signal further in its frequency over time).

Is there a "best practice" way of detecting if my LED is still working, emitting light as it should, without being disturbed by outside light?

Answer 1

^{simulate this circuit – 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.