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Monitoring LEDs

Integrating LED monitoring on your custom test fixture

Does your DUT/UUT have an LED indicator that needs to monitoring during testing, or to notify the operator of some component? You may need an LED fixture. How can this be done when the LED is inside of a bed of nails custom test fixture being probed and plugged into? There are several ways to monitor an LED on a custom test fixture depending on your needs.

CTS custom test fixtures have come across this problem multiple times and we solve in multiple ways to create an LED fixture. To identify the best solution, we need to understand the test procedure and budget. If your engineering team identifies that the LED is critical to the test, not something that can be measured or identified another way it is important to analyze the test procedure and the location of the LEDs. A common LED for test is the indicator lights on an RJ45 for example. These can be added to board level test fixtures or final assembly test fixtures

The simplest solution that works for a lot of our customers is a transparent fixture plate. If the LED can be oriented where the operator can make visual confirmation then we leave a window for this to be accomplished. We often probe PCBs from all angles and if the LED falls onto the bottom of the unit under test it can make the visual inspection a little more challenging. Some customers find that a mirror can provide all the visual confirmation needed. If this isn’t the case, or the custom mechanical fixture will be inside one of our RF faraday boxes we need to look at other solutions.

If the LED reading needs to be only off/on and won’t be crowded by other lights LEDs while the test is running, a simple photo sensor may provide the necessary requirements. While we provide an isolated area for the LED to reject ambient light when possible, tightly grouped LEDs make confirmation in this method next to impossible.

The most popular option is to use what is known as an LED optical probe. This probe which is installed at a close distance to the LED collects the light and transmits through optical fiber to the processor that communicates to your PC. Some of these simple probes can be used for off/on signals. More than that they can be used for color sensing and intensity. We have tested several brands of optical probes and recommend the Feasa brand (

Past optical probes, when LED is not just an indicator but an integral part of the test comes to spectrometers or camera monitoring.  These solutions are more unique and must be identified by the test engineers.