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#263 LigEmDio

An LED tester kit featuring a classic BJT constant current source. From The Boldport Club (Project #8).


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Another beautiful board from the Boldport Club.

The embossed patterns are truly marvelous (made with copper trace designs I think).

So does the world need another LED tester? Probably not, but this is certainly better than the ad-hoc breadboard test circuits I tend to reach for whenever working on a project with a significant number of LEDs.

Whether the pack of LEDs says "QC tested" or not, I've learned to expect a non-zero failure rate. I gather LEDs can degrade in storage too (esp in high humidity). So rather than find a dud in the middle of a LED regrets if you test;-)

And if you are going to test, then gazing at the intricate designs of the ligemdio makes the time pass just that much faster!

How It Works

The dual-BJT circuit is a classic constant current source. Essentially:

  • Given Vbe is fairly "stiff" (at around 0.6V for the PN2222ATA)..
  • So there is a constant voltage across R1 of ~0.6V
  • Therefore current thru R1 of 18.2mA
  • Assuming negligible base current (Ib) current for T2,
  • And assuming R2 and supply voltage are sufficient to saturate T2,
  • Then the current through the load (LED DUT) is also ~18.2mA

This holds to a degree, however Vbe is subject to temperature fluctuations and can also vary significanty with collector current.

But what this circuit does buy is constant current independant of the supply voltage.

This circuit is actually the inverse (NPN) version of the (PNP) voltage-independant current source discussed in 2.06/p75 of the Art of Electronics (second edition).

Parts Checklist

Item Qty Ref
33Ω through-hole resistor, Multicomp MF25 33R x1 R1
1KΩ through-hole resistor, Multicomp MF25 1K x2 R2, R5
USB connector, Multicomp MC32603 x1 J2
Orange LED, Kingbright L-9294SECK x1 LED3
NPN BJT, Fairchild PN2222ATA x2 T1, T2
3.3µF electrolytic capacitor, Multicomp MCMR50V335M4X7 x1 C1




Once again, a beautiful PCB. The embossed designs are amazing..



Let's not forget the circuit..



Somewhat OK solder job. Top:




Completed build, testing an SMD LED...


More testing. 999 to go ;-)


Credits and References