Project files: LED tester

What is it?
The PCB files for my version of Håvard Skrodahls LED-tester as described here.

How big are the boards?
The board measures 2.0” x 1.6” (app. 51 x 41 mm.).

What is the status of the boards?
This is version 1.0 as everything (for once) worked the first time 🙂

Does it use any special/expensive/hard-to-find parts?
None, really. The 16mm pots can be bought from ebay and everything else you should be able to get from multiple different sources. If possible, I would suggest using a stereo 5k-10k pot and the fully-isolated version of the LM317. The former gives the best adjustment range and the latter helps protect against mishaps with flying test leads 😀

Anything else I need to know?

  • For information about how the circuit works, read the hackaday-post linked above.
  • Output current can be calculated as 1.25V/Rtot. For max. current Rtot = R1 and for min. current the value is Rtot = R1 +  the pot value (with the decks in parallel if you are using a stereo pot obviously)
  • There is a difference between Lin/Log pot as described in the build article, so you’ll have to decide up front which adjustment profile fits you best (or keep the pot offboard so you can change – or just build two boards 🙂 ).
  • If you want to use the “high-current” mode, populate R2 as well and short the jumper. Remember that power dissipation in both the resistor and the LM317 regulator increases with higher current. The calculations for min and max current above have to be adjusted to reflect the fact that R1 and R2 are in parallel.
  • The connection for the ammeter is required as it is in series with the LED being tested. If you don’t want the ammeter, bridge If+ and If- connections as shown in the picture. The connection for the voltmeter is optional.  Note that I have tried using a cheap LED meter from ebay for the ammeter and I had some problems with it, whereas if i connect my normal multimeters everything works fine – YMMV.

Download design files here

Related information:
Be sure to read the original post for the exact circuit description, information and tips.

Note: Always read the “intro post” for additional important information about my designs.

LED-tester deluxe…

A few months ago I stumbled upon a presentation thread for an “LED-tester” circuit by Muffsy-creator H. Skrodahl on a Norwegian audio forum. Two things immediately occured to me:

1) I want one!
2) I think I can improve this a bit 😀

So rather than simply downloading his posted Eagle files and ordering boards from there, I started doing my own board instead. With the final result arriving earlier this week it’s time to put it to the test.

The basic idea is to use an LM317 regulator as a variable Constant Current Source (CCS) to test unidentified LEDs and confirm what currents are required for acceptable brightness – something that isn’t always easy to guess based on the published specs. I’ve kept the basic circuit intact but my modifications basically consist of:

– “Real” connectors for all connections instead of just solderpads.
– Additional outputs for LED connections to allow direct plugging in, permanent wired connections and also temporary connections via test leads/crocodiles clips.
– Space for a stereo pot to give a bit more mechanical stability.
– Optional “high-current” mode for testing constant-current LED bulbs as a supplement to just normal LEDs.
– Four real mounting holes to allow the board to be fixed to a bit of scrap metal or similar for use in a lab environment.

I need to do a bit more validation on the prototype before I publish my board files, but at least I can confirm that it works and that it is a very useful way to identify the operating parameters of e.g. LEDs in pushbutton switches.