Introduction: LED Tester With Adjustable Current

This is an easy to build LED tester with several useful features.

- Adjustable current of 1 mA to 20 mA - Evaluate brightness and efficiency

- Vf (forward voltage drop) measurement - Needed to calculate resistor value

- LED will not be damaged if connected backwards - Voltage is limited to 5 volts

- Power on / open circuit / reverse polarity indicator - Indicates bad or improper connection

Step 1: Parts and Schematic

Parts required:

47 ohm resistor
100 ohm resistor
1 K ohm resistor
1.5 K ohm resistor
2N4401 or similar (2N3904, 2N2222) NPN transistor
Three 1N4148 diodes
Three red LEDs (must be red)
5 K ohm log taper pot
Transistor socket or similar
Two banana jacks
9 Volt battery clip
Knob (for pot)

Step 2: Voltage Reference

Solder three 1N4148 diodes an a 1K resistor together as shown.

Do not trim the leads at the junction of the 1K resistor.

Step 3: Current Adjust

Solder a 47 ohm and 1.5 K ohm resistor to the diode as shown. Do not trim any of the leads.

Solder the 1.5 K ohm resistor to the left terminal of the 5 K pot.
Solder the junction of the 1 K ohm resistor and diode to the right terminal of the 5 K pot.

Step 4: Current Source

Solder the base lead of the transistor to the center terminal of the 5 K pot.

Solder the emitter lead of the transistor to the 47 ohm resistor.

Step 5: Open Circuit Indicator

Solder together 3 red LEDs as shown. The anode of one connects to the cathode of the other.

Solder the negative (black) wire of the battery clip to the junction of the 47 ohm resistor, 1 K ohm resistor, and diode.

Make a temporary solder connection from the collector of the transistor to the cathode of the LEDs.

Make a temporary solder connection from the 1 K ohm resistor to the anode of the LEDs and the positive (red) battery clip wire.

Connect a 9 volt battery and rotate the 5 K pot from CCW to CW. The LEDs should go from off to maximum brightness.

Measure the voltage a the left and right terminals of the 5 K pot. Reading should be approximately 450 mV left, and 2.1 V right.

Step 6: Enclosure

Drill holes for the 5 K pot, socket, and LED.

Install the components and wire up the LEDs and socket.

Solder some wire to the banana jack ring terminals and from one of the ring terminal to the switch. Solder the positive (red) wire from the battery clip to the switch.

Drill holes for the switch and banana jacks.

Connect the wire from the black banana jack to the collector of the transistor. Connect the wire from the red banana jack to the junction of the 1 K resistor and LEDs.

Step 7: Calibration

Put a 100 ohm resistor in the LED socket and connect a volt meter to the banana jacks. The voltage reading will indicate the current flow through the resistor.

Mark a 1/2/5 sequence...

0.1 V = 1 mA
0.2 V = 2 mA
0.5 V = 5 mA
1.0 V = 10 mA
2.0 V = 20 mA

Step 8: Usage

Turn it on and place a LED in the socket. If the red LED is on, reverse the LED in the socket.

A meter can be connected to the banana jacks to measure the forward voltage drop of the LED (Vf).

Vf is different for the various LED colors and materials used to make it.

With 20 mA of current...
Red is usually 1.9 to 2.0 V
Yellow / orange is usually 2.0 to 2.1 V
Green may be 2.2 V or 3.0 V (true-green or ultra-green)
Blue and White are usually 3.0 to 3.5 V - there are many ways the make these.
IR is usually 1.0 to 1.2 V - If the LED doesn't light up and the meter reads 1.0 to 1.5 V, it is probably IR.

Surface mount LED can be tested with leads connected to the banana jacks - they are in parallel with the test socket.


holecutter (author)2016-07-22

Hi APossum,

I've only just come across this instructable, and it's very good. I do wonder what the 3 diodes in series is for? Ditto the three LED's?



Gadien81 made it! (author)2015-11-10

I made it, Was the first project I ever took on-board. Thank you for making this instructable easy to follow :)

Kjata1013 (author)2012-12-10

This rules and I can't wait to build it. I'm a newbie at this, so please forgive me if this is a dumb question: you have 3 red LED's wired but it seems that you're only using one. What are the other 2 for?

fcat1 (author)2011-09-06

Very nice ! Thumbs up !

Built one following your steps, and it works like a charm, I further enhanced a little bit by tapping two wires onto a small board consists of two rows of turn pins configuration, which screw on with brass pole (insulated the copper board side), so that it could use to test normal led & superflux led as well, thank you again : )

todell-1 (author)2011-06-04

Can you clarify "rotate the 5 K pot from CCW to CW"? If the POT is turn Counter Clockwise until is stops the what should the reading be? Likewise, if the POT is turned Clockwise until it stops what should the reading be?


todell-1 (author)2011-06-03

Sure would nice to see this on a breadboard. Compairing your photo's to schematic is quite a challenge for us newbies.

lyndon94 (author)2010-01-06

nice im going to make this

i was wondering if you could test laser diodes

NarNar (author)2009-08-01

Congrats, you got the thumbs up from Goodhart!
I'm going to try this once I manage to get 3x 1N4148 diodes and a transistor socket, whatever that is.

Goodhart (author)2009-06-15

Very nice. I really like this.

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