LED Tester (20ma Constant Current Source)




About: Electronics and Software Hobbyist. Let's see what I come up with... into #DoctorWho, #ESP8266, #Arduino, #WebDevelopment

Are you tired of having to change the resistor every time you change the voltage to power your LED??

Here is your solution! a constant current source. Built with only two transistors, fits in a 3 by 5 perfboard.

Step 1: Materials

You are going to need:

  • NPN Transistors (2)
  • 1kΩ Resistor (1)
  • 220Ω Resistor (1)
  • 9v Battery Snap / Power (1)
  • LED / Header (1)
  • Breadboard / Perboard (1)

Step 2: Schematic / Plan

This is the circuit diagram, very simple only two transistors.

Step 3: Get Ya Breadboard/perfboard

just get it.....

Step 4: Place Your First Transistor

Just look at the picture, by the way my transistor have a CBE pinout.

Step 5: Place Your Second Transistor

one row down from the first.

Step 6: Place Your 220Ω Resistor

just like in the picture....

Step 7: Place Your 1kΩ Resistor

pictures are worth 1000 words

Step 8: Place Your LED

Place your led or your header.

Step 9: Apply Power

Plug in your power wires/battery snap and you get 10-20ma.

Step 10: Done

Now you have a 20ma led tester. Run it off USB, car battery, 9v it will light your leds!



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    9 Discussions

    Eric Brouwer

    2 years ago

    Thanks for sharing. It is always nice to see an Instructable using just basic components.

    Although your circuit does work as a current limiter, variance in the transistor gains will greatly affect the current flowing through the LED.

    You can experiment with the attached circuit, which will reduce your component count to only 2 components, using a LM317 voltage regulator and single resistor.

    Have a look here for more details:


    5 replies
    Luigi PizzolitoEric Brouwer

    Reply 2 years ago

    I did know about the LM317 as a constant current source but I did not happen to have any laying around and wanted an instant solution, no waiting for shipping.....

    I do agree that that would make a good led tester but if you only have basic parts then I think this is the closest you can get.

    Eric BrouwerLuigi Pizzolito

    Reply 2 years ago

    I agree - use whatever components you have available.

    In fact, I only use my LED Tester to verify the LED colors. The actual mA is not that important to me.

    Eric BrouwerLuigi Pizzolito

    Reply 2 years ago


    But a constant current source is still useful to determine which LED will work best in your application. Some applications require a bright LED, others dim. Nice to see the difference at a constant current.

    OmniventEric Brouwer

    Reply 2 years ago

    Did something very close to this some months back (same exorbitant component selection and all), but decided that it wouldn't really be "meaty" enough for an instructable, but perhaps I should reconsider :)

    Just have to edit a truckload of pics from back then first (oh and fix the X-mas lights so they can be put on a timer without going berserk each time they're switched on - note to Chinese manufacturers: Just because it's got a microcontroller doesn't mean that the customers want a lot of flashing programs, that takes 8 button presses to settle to a stable light :-/)

    Have a nice day :)


    2 years ago

    So looking at your circuit and setup, It looks more like a .010 ~ ma circuit then a .020 ma. via your pictures from the power supply readings. Did you actually measure the current to see how much it varied and if so what were the readings. But it is a nice circuit seeing the changing voltages and LEDs it worked with. Just imagine if you used SMD for this how small it could be. Thumbs Up!

    2 replies
    Luigi Pizzolitogm280

    Reply 2 years ago

    I tried measuring the current but accidentally blew my 200ma multimeter fuse.....

    I did not have proper resistors and so I played around until I got something decent. The current changes a bit with voltage it goes from 10-20mA.

    If you want a more accurate current then you can calculate a resistor value and use a slightly modified circuit...


    You cant see it in the photos but my power supply amp meter keeps switching to 0.001A to 0.002A in the video