LED Resistance Finder

Full credit to Fun Led Checker Tester by Geek07Boy for the inspiration for this tool, consider this an updated version of his design.

This tool only tests LED's with two leads, cathode and anode, at 5 volts. This was designed for the types of LEDs and the voltage I use.

The tool is designed to be helpful in quickly determining a resistor for use with an LED in a 5 volt circuit, with no other test equipment needed. This tool DOES NOT not replace LED spec sheets but the 7 resistance values given are quite common. If you use a lot of LEDs in projects, think of this tool as a quick way to verify an LED is operational before soldering on to a circuit board. Another functional use of the tool is determine a brightness level of an LED with a specific resistor.

Only 7 resistor values are provided; 220, 330, 470, 1K, 2.2K, 4.7K, and 10K. If you have additional test equipment, such as a Resistance Decade Box, you can easily connect that (will be shown in the steps) to fine tune the resistance you need. If you have a multimeter, you can measure the LED current with a selected resistor (will be shown in the steps).

This is an easy to assemble project that would make a useful tool for your workbench. You need just basic schematic reading and soldering skills for this, and would make a great project for a school to teach soldering, schematic reading, and how all the parts are working together.

The circuit will be explained, what all the parts are for, during the assembly steps.

Parts List, all resistors ¼ watt 1% or 5% is up to you, there is nothing critical for an LED (I used both in this assembly)

LM7805 5 volt voltage regulator, x1 (no heatsink needed for this)

1N5819 Schottky diode 1A 30V, x1

220 resistor, x1

330 resistor, x2

470 resistor, x1

1K resistor, x1

2.2K resistor, x1

4.7K resistor, x1

10K resistor, x1

100 nF disc capacitor, x2 (I use monolithic)

10 uF 25v electrolytic capacitor, x2

mini slide switch SPDT 3 pin, x2

green LED 5mm, x1

DC jack PCB mount, 5.5x2.1, x1

female header 2.54 1x2, x1

male header 2.54 1x2, x1 (I use a single red 1x1 and a single black 1x1)

male header 2.54 2x7, x1

jumper cap tall, x1

jumper cap short, x1

standoffs 3x6mm, x4, plus 3mm screws, x4, (4 longer standoffs are useful during assembly if you have them)

wall mounted power supply 5, 9, or 12 volts DC, + on center pin with 5.5x2.1 (or 2.5 with spring) barrel plug, x1

PCB, x1 The gerber file can be found on my proton drive here, or you can order here on PCBWay (affiliated).

Tools Required:

Schematic

Electrical tape

Soldering iron, solder, plus solder wick & liquid flux if mistakes are made

Flush cutters

Small breadboard, (or pin header soldering tool), used for installing the header pins

Optional, for further Operation:

Resistance Decade Box with hook probes

Multimeter with hook probes

PCBs are easiest when assembled from lowest profile parts to tallest, normally I start with resistors, but since we have male header pins on this board, I am going to install the headers first. To do this I am going to use my pin header soldering tool, or you can also use a small breadboard.

Start with the 1x2 header, meaning that is a single row, two pins long, or if you want to use colored pins, such as red and black, feel free to do that. Insert the pins upside down in the tool or breadboard, turn the PCB over and line up the holes with the correct header position, which is H2, closest to the standoff. If you are using red and black header pins, the black one goes closest to the standoff. Now a single row of header pins on its side is the correct height to raise the board, so place one or two under the board to support the board so it is level while resting on the header pins, then place something heavy on the board, say a pair of pliers or flush cutters to hold the board stationary. Solder the pins in place. Remove from the tool or breadboard, the pins should be vertical when looking straight on from two adjacent sides.

Repeat this with the 2x7 male header, J2, which means it is a double row, 7 pins long. For the soldering, just solder one pin in opposite corners, carefully remove from the breadboard and check to see that the piece is sitting flat on the PCB, if not, just reheat one corner and press with the breadboard (not your fingers!) to seat the header row flat, repeat with other corner if needed. Now solder the other opposite corners, then solder all the remaining pins.

To install the female header, P1, I use tape to hold in place then soldered the leads, removed the tape and checked that it was vertical and flat to the board. Reheat a pin one at a time to adjust as needed. Then install the short standoffs on the bottom of the board.

Functions:

H2, this header is where we can measure LED current from if so desired. When the header has a jumper cap, the power to the LED flows as normal. When the jumper cap is removed, it opens the circuit to allow us to measure current. That will be shown in another step.

J2, this header will use one tall jumper cap to select the resistance shown next to each pair of pins.

P1, this is the female header and is our test socket for the LED we are testing. It is marked with + and - so the LED is inserted correctly.

You will also need the schematic as I use component references and not values on boards. You need a 2 or 3 inch piece of electrical tape for holding components.

Form the resistor leads. Place a resistor in the correct location, secure in place with the electrical tape, turn the board over and solder the leads. Turn board over, remove the tape and check the solder connections on the top of the board, then cut the leads off with the cutters. Repeat for all the resistors.

Now install the diode paying attention to polarity, then install the two disc capacitors, then install the two mini slide switches.

Functions:

R1 is the current limiting resistor for D2.

R2 through R8 are the limiting resistors for the LED under test in header P1.

D1 is a Schottky diode, prevents 5 volts from going back to the output of regulator U1 when using a 5 volt wall adapter. When using a 9 or 12 volt wall adapter, D1 simply passes the 5 volts from the regulator to the rest of the circuit.

C2 and C3 are part of the filtering for the 5 volt regulator. C2 is on the input side, and C3 is on the output, 5 volt side.

SW1 is a selection for which wall adapter voltage you are using, 5 volts to the left, and 9 or 12 volts to the right.

SW2 simply turns the tool on or off.

Now we install the remaining parts. If you have longer standoffs available, go ahead and install them on the top surface of the board.

Start with the green LED, insert D2, positioned correctly, tape, solder, and trim leads.

Now install the DC jack, remove the standoff next to it so you can tape that in place, solder opposite tabs first, make sure the jack is flat to the board, then solder the remaining tab.

I am not a fan of bending leads to hold things in place, but for the two electrolytic capacitors, I do do this. Insert C1, positioned correctly, bend the leads right at where they come out of the board, at about 45 degree angles, that should hold the cap well so you can solder it in place, then trim the leads, repeat with the second cap.

Last part to install is U1, the voltage regulator. Since we are just going to be power at most, two LEDs, no heatsink is needed. Install the regulator solder in place, trim the leads. Install the tall jumper cap on the 1K resistance pins (I used green), and install a short jumper cap on H2 (I used black).

Functions:

D2, lights green when power is applied to the tool with SW2.

J1, is the power jack to accept barrel plugs, positive is the center pin. The tool can accept 5, 9, or 12 volts DC.

C1 and C4, are electrolytic capacitors and are filtering for the voltage regulator. C1 is on input side (9 or 12 volts) and C4 is on 5 volt output side.

U1, is the 5 volt voltage regulator, when switch SW1 is in the 9-12V position, the regulator provides a clean 5 volts. When switch SW1 is in the 5V position, no power reaches the output side of the regulator due to D1.

You can use this tool without the need for any additional test equipment if you desire. First, see what voltage your wall adapter provides, 5, 9, or 12 volts DC. Set switch SW1 to the correct position, 5V on the left, 9-12V on the right. Plug in the barrel plug and turn the tool on with switch SW2.

Ensure there is a jumper cap on H2. Place the J2 jumper cap on the pins for 1K resistance, which I use as a good starting point.

Insert an LED you want to test into P1, positive (long lead) on the left and negative (flat side, short lead) to the right, if you have trimmed the leads or cannot see the flat side, look through the lens and the larger metal part is the anvil, the anvil is the negative side. If you cannot determine the negative side, I use a component tester.

Now you can use the jumper cap on J2, moving from rating to rating to see what you like best in brightness or dimness.

Photo 1, I am testing an orange LED with 1K resistance, actually a nice brightness to it.

Photo 2, I moved the jumper down to the 10K resistance, a lot dimmer.

Now if you want to use a Resistance Decade Box along with this tool, that is very easy to do, but you have to be careful. You have to make sure you do not set the decade box to 0 ohms or you will burn out the LED!

Turn the tool off! Remove the jumper cap on J2, connect the decade box to the pins for 220 resistance on J2. Now set your decade box to 200 ohms, doing this prevents you from providing zero resistance to the LED, which will burn out your LED.

Turn on the tool, keeping in mind, do not select lower than 200 ohms (to be on the safe side) or you risk burning out the LED.

Photo 1, shows my decade box connected and providing 200 ohms resistance, very bright LED.

Photo 2, shows the box providing 3,200 ohms resistance, the LED is dimmer now.

Now you can adjust the resistance on your box to find the perfect resistance you need for an LED.

To see what current your LED with a specific resistance is drawing, we are going to use H2 and a multimeter with hook probes.

Turn off the tool, remove the jumper cap from H2, set your multimeter to read DC current or A or mA, and make sure your red lead on your multimeter is plugged into the correct port for current, and your black lead is plugged into the COM or ground port. If you are unsure, refer to the user manual for you multimeter.

Now connect the red lead hook probe to the + side of H2, and the black lead to the - side of H2, or if you used colored header pins, connect red to red and black to black.

Photo 1, insert an LED into P1. Set the jumper cap on J2 to 1K, turn on the meter and the tool. Now you can read the current that the LED is drawing with that specific resistor, which in this case is for an orange LED is 2.97 mA.

Photo 2, move the jumper cap to the 220 resistance, now with this orange LED, the current is 12.53 mA.

Now if you are looking for good brightness yet low current, adjust the resistance to a higher value to determine what you want.

YES, you can use a Resistance Decade Box and a Multimeter at the same time with the tool.

I find this a very helpful tool for my workbench, I can test LEDs I have to just see if they are working, and I can determine current values and brightness levels before assembling in a project.