LED Tester (20ma Constant Current Source)

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!