Simple LED Checker

Never too many LEDs! You can't have too many. You can't use too many in your projects. But sometimes sorting out what you have to find what you need can be a problem. I got tired of finding a battery and a resistor to check an LED to see what color it was, if it worked at all, how bright it was, or which pins were which. So I finally built the LED checker to keep handy on my bench. It had to be small and simple, cheap, and dedicated to only one purpose. Maybe you will find it handy, too.

This is a simple project. It is good for beginners who don't have much experience building things. For that reason I will go into a lot of detail. The circuit is simple and very easy to get working. It is a good project to "get your feet wet" without much trouble.

As I mentioned, the circuit is very simple. Rather than load up KiCad and draw it that way, I just hand drew it on a 3x5 card. The battery provides approximately 9 volts and the switch chooses either the path with the 680 Ohm resistor or the path with the 6800 Ohm resistor. The 680 Ohm will allow about 10 milliamps to flow and the 6800 Ohm will allow about 1 milliamp to flow. No current can flow until something is connected (usually the LED) to the test jacks, so no on-off switch is needed. The LED completes the circuit when connected in the right direction. The current is calculated using Ohm's Law ( I = E / R). A 9 Volt battery minus about 2 volts drop across the LED gives us about 7 Volts. So we get (9-2) / 6800 = .00103 Amps or 1.03 milliamps and (9-2) / 680 = .0103 Amps or 10.3 milliamps.

Parts you will need. This is what I used. Use what you have or can easily get. I do recommend using the 9V battery since you need a battery voltage higher than the forward voltage of any LED you test. Some can be over 5V.

Case -- any small case that will hold the rest of the components. I used a small plastic one I bought at Radio Shack years ago. Plastic is easiest to work and you don't have to worry about wires shorting to it.

680 Ohm Resistor -- 1/4 watt through-hole resistor. The value isn't critical. Anything from about 500 to 1500 Ohms will do. We want it to provide about 10 mA through the LED.

6800 Ohm Resistor -- 1/8 or 1/4 watt through-hole resistor. Again, the value isn't critical. Anything from about 5000 to 10000 Ohms should do. It will provide about 1 mA through the LED.

Switch -- Single pole, double throw switch. Anything small that will fit your design. Toggle, slide, rocker, whatever style floats your boat. It will be easiest if it is designed for panel mounting with screws instead of PC board mounting, but that isn't critical, as long as you can mount it in your case.

Battery Snap -- Standard 9V (transistor radio / smoke alarm) battery snap with wire leads. And a battery.

Female Header -- A female header such as used on Arduino boards is a good way to plug in "standard" through-hole LEDs for testing. You want a section with 2 or more holes/pins. More is good in case you have LEDs with leads more than 0.1 inch apart. I used a six pin section.

Grabber Clip-On Connectors -- These are convenient for grabbing leads that you can't plug in. Alligator clips are good, too. You can also use them to clip to the probe leads of a voltmeter to measure the forward voltage of the LED.

Wire -- A foot or so of red and black, or whatever colors you want. It's good to have them color coded for positive (anode / red) and negative (cathode / black.)

Glue (not shown) -- Some of the parts don't have a good way to mount them. Since they won't usually be subject to a lot of stress, we can make them fit fairly tight and glue them with some strong glue.

You will need some tools. If you don't have exactly what I have, improvise. This is what I used.

Drill and Bits -- These are handy. If you don't have them, a strong knife can be used on a plastic case -- CAREFULLY!

Wire Stripper -- Or the knife again. Using the knife isn't the best idea. If you knick the copper wire it makes it much easier to break. But the knife works if you are careful.

Soldering Equipment -- We need to actually connect the stuff together!

Strong Pocket Knife -- For final shaping of holes in the plastic case.

Before you start cutting and soldering, decide where everything is going to go. Make sure all the parts will go where you want before you cut holes in your case. After about fifty years of trying, I still haven't figured out how to uncut a hole.

In the picture you can see I put the parts in roughly the places I want them to go. The female header is hard to see, but it is inside the case at the top of the picture. I will cut a rectangular hole for it and glue it half in, half out. I will drill a small hole below it for the wires to the clip leads.

Notice the switch on the left side of the case. That is where I wanted to put it, but you can see it may interfere with the battery. Good thing I checked before cutting! I will move it a bit until I find a place it fits well.

At first, I thought the battery wasn't going to fit unless I modified the case. Turns out, a bit of force and it snaps into place. That also holds it tight without adding any kind of extra holder.

So the layout is planned and workable. Time to build.

Mark an outline of each hole that you need to make in your case. My case is pretty soft plastic so it is easy to scratch the outlines.

My switch needs a rectangular hole. I couldn't find any rectangular drill bits on Amazon, so I drilled some smaller round holes inside the outline. Now I can trim to the lines with the pocket knife. Pay no attention to how crooked the lines are or the line of holes. No one has ever accused me of being a machinist.

If you use this method, I recommend starting with small holes and making them as big as you can by re-drilling until they are just inside the lines you marked. The rest of your work will be much easier. You can trim with a sharp knife or use files. Aim for a snug fit of the part. If the hole is a little too big it isn't a great concern. That's why we have strong glues!

If you do use a knife, be very careful. I'm not going to try to teach you how to use a knife safely; that's between you and your mother. She probably told you. I hope you listened. If you have tools available that are more appropriate to the job, please do use them. Files work very well.

The header is rather long and very thin. It would be hard to cut a long slot that fits it well. So I took a slightly different approach. I cut into the top edge of the case with the knife. When the top is attached it will make a closed slot, like we would have gotten by cutting a hole. But it is easier to cut. The top will stay on most of the time except when the battery needs to be changed. If you do it this way, make the slot slightly less deep than the header is tall so that the top will push down on it a little bit to help hold it sturdily in place. Just a bit. You don't want to have to stand on it to close it up. About 1/64 inch or 1/2 mm is good. If you accidentally make it too deep, a bit of glue or tape on the top will make it stick up and allow the top to press down on it. The header will get the most use, so it needs to be pretty sturdy. It doesn't take much force to put an LED in it, but over time it adds up. On my case the top has a lip that sticks down inside the rest of the case. I had to trim that off where the header goes.

One last hole to make. We need a hole for the test leads to come out. You want this hole to be a pretty tight fit for the wires. It should take a bit of effort to get both wires through it. I used 22 gauge wire and drilled a 1/8 inch hole. That was perfect (a good guess on my part!) Start small. It's pretty easy to remove more material. A bit more difficult to put it back. The tight hole provides some strain relief for the wires. Wiggling wires inside your device is almost always bad.

With the mechanical parts all prepped, we are ready to wire up the electronics. The first thing to do is tie an underwriter's knot in the wires for more strain relief. This knot isn't hard, but it can be confusing. There are lots of diagrams and videos online to show how to tie it. Put the wires through the hole for them and pull them about half way through. Tie the knot on the INSIDE of the case. Make sure to leave about an inch of wire to connect the the other parts inside. Once the knot is tied, strip about 1/2 inch (12 mm) of insulation off the inside ends and pull the wires back out until the knot stops them.

Now solder the resistors to the switch. A Single Pole Double Throw switch will have three terminals (pins.) One is "common" and will connect to one or the other of the other two, depending on the switch position. Most often, the three will be in a single line and the center terminal will be the common. Use an ohmmeter to check. Solder one resistor to each of the terminals that is NOT the common one. Electrically it doesn't matter which resistor goes to which terminal. But their placement will determine which switch position is 1 mA and which is 10 mA. Try to make the leads for the resistors kind of short, but not so short you have trouble soldering them. Leave the non-soldered ends full length for now. Wrap one lead of the resistor around the terminal and solder it. Do the same with the other resistor on the other terminal.

Now take your battery connector and strip the ends of the wires about a half inch. You might want to trim the leads a bit shorter. They are usually fairly long. I didn't trim mine and now I have all this excess wire. If you trim it, make sure to leave it long enough that you can get the battery out to replace it. Run the red wire from the inside of the box out the switch hole and solder to the common terminal of the switch. Make SURE you feed the wire out the hole. If not, you will probably end up unsoldering it and re-doing it a second time. Don't ask me how I know. You can see in the picture what you should end up with. Now twist the two non-soldered ends of the resistor leads together. Then slide the switch into position in its hole. Don't glue it yet; you may need to remove it again before you are finished.

Now take the two twisted resistor leads and the red wire going to the probe clip and solder them to to half the pins of the female header. If the resistor leads are long enough to reach where the header will go without too much stress then great. If not, use a bit of wire to connect them. You will end up with both resistors and the red clip lead wire soldered to one side of your header strip. This will be the LED anode connection. I used a six pin strip of 0.1 inch female header. The most common through-hole LEDs have leads spaced 0.1 inch apart. But there are some larger ones that have larger spacing, so three pins on either side allows larger LEDs to be plugged in easily. Half (3) of the pins will be soldered for the positive side, and half are soldered together for the negative side. Adapt as you see fit.

Now take the black wire from the battery connector and the black test clip wire and solder them to the other pins of the header. I found this part, soldering to the header, the trickiest of the whole project. Take your time and get it right.

Once you have all your parts soldered together, check it over carefully. Make sure all the things that are supposed to be soldered together are, and the things that aren't supposed to be aren't.

Now the moment of truth! Try it out. Connect the battery. Find an LED and insert it into the header pins, anode to positive and cathode to negative. The LED should light. If it doesn't, turn it around and try again. If it still doesn't work, check all your connections again. If all else fails, get out a voltmeter and trace through the entire circuit starting from the battery, then to the switch, then to the resistors, then to the LED header, until you find where the voltage stops and why. Fix it. When it works, flip the switch to make sure both current modes work. Then strip the clip lead wire ends and hold the bare wires to the LED. Once it all checks out, attach the clips to the ends of the wires. Some screw on and some must be soldered.

There is one step you may need to take that I did not. Depending on your case and how you place the battery, you may need some sort of battery holder. Since the battery fits snug into my case, I don't need one. If you do, find one that works for you and install it. On the other hand, a piece of double sided foam tape will work really well. The battery should last a very long time, so you won't need to replace it very often.

Now that the wiring is complete and tested, it is time to finish assembling the complete unit. Decide what type of glue you are going to use. If you took my advice at the start and used a switch with screw mounting, you don't need to glue it. I didn't follow my own advice, so I started by turning the unit up on its side to let gravity help me out. I used a solvent-based "fix-all" adhesive that is similar to E-6000 glue, which would work quite well. RTV sealant would work well, too. Epoxy of some type would work well, but it would make things very hard to repair if needed later.

I glued the switch and put a dab of glue into the hole where the wires come out to offer a bit more strain relief. Let that cure for a bit. You can see from the pictures that my gluing skills aren't much better than my machining skills. But it works!

After the glue has cured a bit so that it doesn't run every where, turn it over to glue the header in place. If you made a cutout like me instead of a slot, the easiest thing to do is glue it just enough to hold it in place, then come back once that has set and put more glue on. I used a piece of tape to hold the header in place while I put the top of the enclosure on. Put the tape so that it covers the entire joint between the header and the top. That will keep the glue from gluing them together. Use a plastic type of tape instead of something porous so the glue doesn't soak through and glue it all together. Put just a dab of glue on the edge of the cutout and stick the header in.

It is a good idea to mark the header connections for Anode (+) and Cathode (-). Depending on your case, your desires, your abilities, and your resources, you can do this any way you want. A permanent marker or paint pen should work. My case is black, so permanent markers don't show up well. I took a piece of 3x5 card and made markings on it, then glued them next to the appropriate holes in the header. A bit of clear fingernail polish over the card after the glue dries will protect it from wear.

Give it a final check to make sure everything (still) works, then close it up.

Finished!

Using the LED checker is mostly pretty obvious; plug in or clip to an LED and see if lights. But there is more to it. You can see just how bright it lights with either ~1 mA or ~10 mA to get an idea what size resistor to use for the desired brightness. You can determine which lead is which if you aren't sure. For the modern "water clear" LEDs you can see what color it is. You can often check LEDs in-circuit to see if they are good by using the clip leads to touch the two terminals. Another neat thing you can do is connect an LED and use a voltmeter to some of the other LED connections (other pins on the header or the clip leads) to measure the forward voltage drop of that particular LED. The voltage drop is important in calculating resistor values as we did up near the top. It's also useful to know for other reasons. For instance, if you are running a microcontroller on a 3V battery and want to light an LED, one with a 5V drop simply isn't going to work. One with a drop of 2.5V will be trouble when the battery voltage starts dropping. And sometimes you might want to use an LED as a voltage reference for some reason. LEDs are pretty stable voltage references. You can find one with just the right drop or close enough.

Use a new 9V battery, but you don't need an expensive one. Buy the cheapest carbon zinc (not alkaline or anything fancy) one you can find. I guy mine at Dollar Tree for $1.25. Why? Even these really cheap ones will probably last up to their shelf life. If you figure the average current draw is 5 mA when in use (sometimes 1 mA, sometimes 10) and the cheapest, crappiest carbon zinc battery has a capacity of about 150 mA hours, you are still looking at 30 hours of use. If each test takes 5 seconds and you test 24 LEDs EVERY DAY, it will last 4,500 days. That's well over ten years. The shelf life of almost all batteries is well under ten years. You aren't likely to run this battery down; it will die of old age.

Of course, you COULD put an LED in it and leave it there. Put it on your desk or workbench as a desk accessory or conversation piece. Then you'll need to replace the battery a lot more often.

This is a very simple tool. But as is often the case, the simplest turn out to be the handiest. I hope you enjoy building and using it.