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LED Tester

August 17th, 2008
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While I was working for a manufacturing company, we would occasionaly go through our scrap parts from the machines, and at times would find hundreds of LEDs.  They could be saved and used on the next run, but we had two issues:

1.  What color was the LED?

2.  Polarity on the surface mounts?

This tester was made to make my personal life easier.  Output is limited and pulsed at a 50% duty rate.  Power is delivered to a set of multimeter leads that were lying around the shop.  A PIC12F675 was programmed for the 50% duty.

There are two modes in the tester, with a bicolor LED to indicate which mode is active.  The pic reads the switch position and is configured to reset on change.  Green indicates red lead = positve and black lead = negative.  Red indicates not polarity sensitive.  In this mode, the pic swaps the positive and negative continuously so that the LED will illuminate regardless of polarity.  This let me sort through the hundreds of LEDs quickly, placing each color in its own container, and then determining polarity later, during the production runs.

Overall, a very simple design.  The PIC12F675 is socketed to allow program updates. (Never needed any)  The battery here is the same one I built it with, which is now over 6 years ago.  Although I don’t comb through hundreds of surface mount LEDs, the tester is still handy.  The pushbutton switch is wired directly to the 7805 vreg, and I didn’t require any bulky capacitors in the circuit, since we’re dealing with pure DC.

Most of the parts were scavenged from scrap units we had at the time.  The container is an old toothpick canister.  Hot glue and tie-wraps were my best friends on this build.  The blue wire we had at the shop for repairing damaged PCBs.

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