.boxed { border: 1px solid green ; }

# Building LED “Candles” for Grade 10 Optics

Richard Taylor, retired Physics teacher, Ottawa Carleton District School Board
Richard@teya.ca

“Brrrriiinngg! Hello, Richard? I was just cutting up some candles to use in the grade 10 optics labs and I got thinking about those cute little LED things you made. Do you think you could make some more for me?”

Certainly I remembered the grade 10 optics labs! Carefully prepping all those candles and optical benches, carefully priming the students to use the candles. Turn out the classroom lights and then the chaos begins. Matches breaking, fingers and hair getting singed, wax spilled on lab benches and floors (amazingly slippery!), images too dim to be seen, and which way up is that candle flame image anyway?

Several years ago, I came up with a better idea: mounting a red and a green LED on top of a 9 V battery. The LEDs produce about the same amount of light as a candle, and having the two different colours makes it easy to see whether an image is upright or inverted. After building 15 more of these, I thought it was about time to share the instructions so that other teachers can build their own.

The circuit is a very simple circuit of a 9 V battery, two LEDs connected in series, a switch and a resistor. My original idea was to mount all the parts in a Lego block glued on top of a 9 V battery snap connector, but I rebelled at the thought of destroying another 15 Lego blocks and this time used small blocks of wood instead. You will need a soldering iron and a hot glue gun; I also found it useful to use two-part epoxy glue for some of the important joints.

So far, so much grade 9 electricity. However you must be aware that LEDs behave in a completely different manner from incandescent lights:
• LEDs are diodes and only allow the electricity to flow in one direction. Pay attention to the lengths of the two wires: the long wire must be connected to the positive, the short wire must be connected to the negative.
• LEDs don’t follow Ohm’s Law. They do nothing if the voltage is low, then suddenly start glowing when the voltage reaches a critical value (usually around 2 Volts). If the voltage is increased further, they don’t change much until suddenly they stop working altogether. If you suddenly apply a high voltage (like 9 V), they might even explode! (No kidding, this happened to me. Luckily I wear glasses.)
• That’s why LED circuits almost always contain a resistor. The resistor can reduce the voltage that gets to the LED, and more importantly, the resistor reduces the current in the circuit. LEDs, by themselves, don’t have much resistance and will allow dangerous amounts of current to flow.
• You can search for an on-line calculator to figure out the resistance of the appropriate resistor, but these calculations usually give the minimum resistance. A larger valued resistor will often work just as well and will have the advantage of making your battery last longer. I could have used a 250 Ω resistor, but chose to use a 750 Ω resistor instead.
Here are the parts that I used. For the most recent batch (November 2022), I ordered bulk quantities from Digi-Key Canada:

I particularly looked for parts that were in stock and shipped directly from Digi-Key. My order arrived within a few days.

The wooden block was cut from available scrap material and drilled with the smallest drill I had available. Previously, I had used Lego blocks, also drilled. Any small rectangular block could be used if it will fit on top of the battery connector and will hold the parts. The spacing of the holes should match the spacing of the wires coming out of the LEDs. Make sure you leave enough room at the top to glue the switch. I didn’t buy batteries because I had just finished changing the batteries in all my household smoke detectors. The used batteries still have plenty of life left in them for this non-critical application.

Mount the wooden block on top of the battery connector. I roughened the plastic of the battery connector and used epoxy glue because I wanted the connection to be very strong. It takes some force to attach and detach the connector and the battery; the wooden block will inevitably be used for leverage and won’t last long if not attached securely. Super glue might also work, but I haven’t tried it.

Insert the LED wires into the holes. Pay attention to the length of the wires coming out of the LEDs. Make sure that the side-by-side wires coming from the two different LEDs are opposite polarities. I find that it also helps to put the red on top and the green below like traffic lights. Keep track of which wire is top and bottom - you’ll be cutting these wires short soon, and it’s easy to forget which is which. I also epoxied these LEDs in place at the same time as I was attaching the block to the battery connector. Note also that on the wires coming from the battery connector, red is positive, black is negative.

Cut and bend the wires from the LEDs and also the wires coming out of the resistor. If you don’t have dedicated wire cutters, most pairs of pliers have wire cutting blades near the fulcrum. Scissors could also be used, but it will dull and damage the blades. Later, I found it easier to glue the switch and the resistor to the wooden block to hold them steady before soldering. This picture shows a different switch.

Twist and hold the parts together and keep them as steady as possible while soldering all the wire connections. WARNING: when the LED wires are hot they are VERY delicate. Any movement of these hot wires will destroy the LED (yes, I destroyed two of them in my initial prototyping). Cut and strip the insulated wires coming from the battery connector. Connect one to the lower LED, connect the other to the resistor. This is where you have to keep track of the LED polarities.

After the solder joints have cooled, test the completed circuit. If it works, coat the entire back surface and all the components with a thick layer of insulating hot glue. At first, I tried wrapping the edges with black electrical tape, but this was more trouble than it was worth, so most of them I just did free-hand.

If it doesn’t work, decide whether your time needed to take things apart is worth \$1.77. You did order extra parts, didn’t you?

Below is the finished product, working. The camera can’t show the colours of the bright LEDs.

Below is the optical bench setup. The LED “candle” at the bottom has the red LED on top and the green LED on the bottom. After passing through a convex lens, a real, inverted image is formed on a screen. You will have to darken the room to be able to see these images.

Feel free to copy and personalize this idea. There are many variations that you could try. If I were to build more, I would choose a brighter green LED. It’s always a good idea to test the parts just by touching or clipping the wires together before making more permanent connections, but remember my warnings about the LEDs. You may have to learn some new skills to assemble these, but by the time you build 15 of them you’ll be very good at it. And then you can teach your students how to build the next set.