Optical LED test

[david_liechti]

In our test system we are testing PCBs which are adapted with needle adapters. We use the NI USB-6229 and some custom electronics for data aquisition.

We would like to test the (blinking or just On) green LED on the frontpanel of the PCB automatically. Until now the test person had to confirm the correct light of the LED. Any ideas how to detect the LED light automatically? The sensor should work in different ambient light conditions...

Thanks for tips!

[F._Schubert]

You need a photodiode or phototransistor which is sensitive in the same wavelength as your LED. Then play around with the detection threshold (so LED on vs. LED off under various light conditions). You also can enclose the photodiode in a way that it will only see the LED.

Felix 

[johnsold]

Use one of your green LEDs as a photodiode.  LEDs used as photodiodes are only sensitive to light near the wavelengths which they emit.  Sometimes they are less sensitive than regular Si photodiodes, but the wavelength specificity can be very handy.   You can use the same amplifier circuits you would use with other sensors.  LEDs cannot withstand much reverse voltage (typically 5 V), so do not use a reverse biased circuit.

As Felix suggested setup the fixture so that stray light is blocked as much as possible.  

Lynn 

[F._Schubert]

Lynn, thanks for the idea of using LEDs as sensors, never thought about it.

One more info: You could be able to use the Voc (so just measure the voltage and nothing else required), it is very sensitiv: any light will give you high (so the ambient light might prevent this mode). Second next simple is to shorten the diode via a small resistor and measure the Isc. As long as the resistor is small (little voltage drop to keep in the Isc) it is linear with the irradiance.

Felix 

[david_liechti]

Thanks for the inputs. I was also thinking about using a photo diode, but I just wanted to ask if there would be a complete and already tested solution without any additional handycraft work.