Friday, December 23, 2011

The Lights Project - LEDs and Traffic Signal Lights

Day #8!

Before we go any further, and since we're on the subject of cardboard and lights, you need to see this

Well, let's begin, shall we?
Today we're continuing with circuits-from-scratch, and introducing two new ideas:
  1. LEDs instead of bulbs
  2. Double lights with a toggle switch
Now, the next part of this post might sound like a Science lesson, and it is. You can skip it and just scroll to the projects at the end if you like. I thought I'd include all the technical stuff (written in English, not geek) because I know some of you homeschool your kids, and might want to try these projects for real, and might appreciate the theory so you know why you're doing what you're doing.

Let's pretend we're in class and you have questions and I'll try and answer them, OK? 

Q: LEDs? What are they?
A: Light-Emitting Diodes. A diodes is a circuit component, like bulbs and switches and motors and buzzers and whatnot. They are things that let current pass through them in one direction but limit (or stop altogether) current in the other direction. Like a valve, sort of. Some diodes, in addition to doing that, also emit light when the current is passing through them.

Incidentally, LEDs cost about $1 and upwards. 

Q: Whaddya mean, "in one direction"?
A: See those two sticky-out things on the bottom of each LED? One of them is shorter (called the cathode) than the other (called the anode). When you connect them to, say, a battery, the cathode needs to be connected to the "-" terminal of the battery and the anode to the "+". If you switch this around, the LED will not light up. Another way of saying this is that LEDs (and diodes, in general) have polarity, like batteries do.

Q: So they emit light. Then they're like bulbs, right?
A: No. Bulbs emit light because they have little wires in them called filaments, which get hot and glow. LEDs don't- instead they have semiconductors in them that produce energy in the visible part of the EM spectrum. I won't go into their operational theory, but you can google it if you're interested. Also, bulbs let current pass through them in both directions i.e. they have no polarity. Finally, LEDs can emit light in different wavelengths, which translate to different colors. When you buy an LED, they'll have their color stated on the package.

Some LEDs are can change colors. Here's one:

That single little thing can turn red, green and blue. 

Notice it has four leads (the sticky-out thingies at the bottom), not two. You'll still use two at a time - and depending on which pair you connect, you'll get a different color. Here is my lousy videoclip showing this LED changing colors, using a homemade toggle switch (more on that later) with one position more uh... flimsy than the other:

Q So why not just use bulbs? 
A LEDs are brighter, last longer, have fun colors and faster response, to name a few reasons.

Q: Fancy schmancy. So you just attach wires to these LEDs then, and hook them up to a battery?
A: Yes and no. LEDs are delicate. They are easily damaged by current that's too high/large. They need strong enough power to light up at all, yes, but too much and the thing goes kaput. We always connect something called a resistor in the circuit to suck some of that power away. When it's used to limit the current in a circuit to an amount that's safe (in this case, for the LED), it's called a limiting resistor. You don't need this with bulbs. 

This is what a typical resistor looks like when you buy it in the store:

Here's how you use it. Look at the back of the LED package. 

Look first for something called "supply voltage". That has to do with the battery you're using, which supplies energy to the circuit. This LED does 3.2V, which is about 2 batteries (each battery is 1.5V).
Look next for forward current - that's the current that this LED can take.
The last value is the voltage drop across the LED. In most cases, it's about 1.6 to 3 V, depending on the color of the LED. 
If you enter these values into the fields of the calculator, it tells you what resistor to use - in this case 68ohms. 

Of course most stores will NOT have 68 ohm resistors, just as fabric stores will not sell buttons in packs of 23, or zippers in lengths of 5.75". These packs of five 100 ohm and five 10 ohm resistors cost a little over $1 each. You have to get the closest value (like 100 ohms - higher than 68 is safer than lower) they have and either make do, 

or combine them using Physics principles, to get more exact values.

I made a 60 ohm limiting resistor using two 100 ohm ones in parallel and one 10 ohm one in series. I'd add a second 10 ohm in series to make it 70 ohms - even closer to 68 ohms - but couldn't get it in this photo. But don't worry about the Physics. Just ask the folks in the store to help you buy what you need and assemble accordingly.

Q: That's a lot of technical stuff. My head hurts. I think I'll just use bulbs. Can I?
A: Sure. And if you want color, just take a sharpie and color the glass!

Well, that's the end of the Science lesson! Let's get on with some making!

We're making signal lights today. There are so many ways to make traffic lights to play with. Here is one way to make them completely out of paper and cardboard:

But there is something magical about flipping a switch and changing the color of lights (at least, to my kids), so we'll do that today.

Using just one bi-color LED 

and that toggle switch,

you'll get a light that switches between red and green.
That's how bright LEDs can be, even in daylight.

A toggle switch is just a two-way switch that lets you alternate 
between two positions. You can buy them for about $3.50

but sometimes they work only with higher voltages than what we're using with our little batteries. So we'll use the old paper-fastener and paper-clip ones.

Here is the circuit layout:

Or you could use two bulbs (or two LEDs) and make the more familiar double signal lights:
(That's a blue LED, btw; I didn't have a green one.)

The circuit layout for that is very similar. Here it is for two BULBS:
Don't forget that if you use two LEDs instead of bulbs, you'll need the limiting resistor.

Are you curious about what the traffic signal lights are perched atop of?

It's a train station!

Here's an in-progress shot of the hollow posts that hide all the wiring.

It's part of Trainville, our cardboard train playset. You'll get it to see it tomorrow in our last Lights post. See you then!


  1. thanks a lot for the detailed explanation, for someone who snored in physics classes, this will help me a lot!

  2. Lier, you are definitely the Einstein of crafters whereever. Thanks for the tutorial. Although I'm quite familiar with resistors and LED's, I loved it how you sketched everythink so clearly.
    Loved the train station too. Adroable!

  3. Just wanted to let you know that I found this post to be amazing and very interesting. Thank you.


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