Vriska Serket's vision eightfold, part 3: Testing the circuit
Now that we've decided how the LEDs in Vriska's vision eightfold glasses will be connected, we'll build a test circuit to make sure everything works before we permanently solder the parts in place.
Part 1: Deciding what we want! Planning the project Part 2: Power and light: what's the best arrangement of LEDs and batteries?
Part 3: Building a test circuit on a breadboard (what you're reading now)
General electronics info: LED basics
Last time, we decided we'd use this circuit for our LEDs and power it with four rechargeable AA batteries. Now we'll build it with actual LEDs and make sure they light up.
To build this circuit, you'll need:
- Seven red LEDs
- Three 82-ohm resistors and one 180-ohm resistor (1/4 watt or more)
- A breadboard (a small, cheap one is fine)
- Four rechargeable AA batteries
- A four-AA battery pack (and a snap connector if the pack doesn't have built-in wires)
- 22-gauge solid-core wire
- Wire strippers
We're going build our test circuit on a breadboard, a tool for assembling electronic components temporarily. Inside the breadboard are pieces of copper that connect the holes together, as shown in the picture.
When you push a component's wire (its lead) into a hole in the breadboard, the copper strip inside will connect the lead electrically to the leads of any other components installed in that row. We don't need to add additional wires between our LEDs and the resistors, or touch their leads together; the breadboard will take care of that for us.
You'll notice that the two sides of the breadboard don't connect to each other. That lets you install specialized computer chips on the breadboard without electrically connecting the legs on opposite sides of the chip to each other. (We don't need any of these integrated circuits, or ICs, for this project, though.)The LEDs and resistors can be stuck directly into the breadboard. To provide battery power, you'll need to attach a battery case — it can be the same one you end up using in the final project. Some battery cases include wires, which you can just put directly into the breadboard. Others have a pair of snaps like the kind you see on 9-volt batteries. Electrical wire typically comes in two types: solid core and stranded1. Solid core wire has a single piece of metal inside, and stranded wire has a braid of smaller wires. The two different kinds of wires are useful in different parts of your design. (We'll talk about that more when we solder a permanent circuit next time.) If your battery case has solid core wires, you can stick them right into the breadboard. If it has stranded wire — and most battery snap connectors do — getting the ends into the breadboard will be like trying to stuff a marshmallow into a piggy bank. It can be done, but it might be easier to go ahead and solder on a couple of pieces of solid core wire and cover the joins with heat-shrink tubing. (We'll talk about that more next time, when we get to soldering.) Now let's build our circuit!
1. Insert the LEDS into the breadboard
When possible I like to arrange breadboard circuits so the + (power) is at the top, and the - (ground) is at the bottom. This makes it easier to see what should connect where.
LEDs also have a + and a - side — the - side has the shorter leg. You'll install the LEDs in three pairs, with one leftover single LEDs. In each pair, the + side of one LED needs to be in the same row as the - side of the other LED.
Leave some space between the groups of LEDs so you'll have room to add the rest of the components.
2. Add the resistors
The 82-ohm resistors go with the pairs of LEDs, and the 180-ohm resistor goes with the single LED.
Insert one end of the 180-ohm resistor in the same row as the - end of the single LED, and the other end of the resistor in a row with nothing in it. (It doesn't have to be the very next row.)
Your pairs of LEDs are already placed so that one row of the breadboard contains a single + lead, one contains a + and a -, and one just contains a single - lead. Insert one lead of the resistor into the row with the single - LED lead, and the other lead of the resistor into an empty row.
Imagine power flowing from + to -. It flows through the first LED (from + to -), along the breadboard row, then through the second LED (still from + to -), then through a second breadboard row, then through the resistor.
3. Add power and ground wires
Now we're going to connect the + sides of each group of LEDs together. (This will let us apply the same voltage to each group.) We'll connect them using jumper wires — short snippets of wire.
Cut some small lengths of wire and use the wire stripper to remove the insulation at each end. Make the bare ends long enough to touch the bottom of the breadboard when they're inserted.
There are four rows on the breadboard with a single + LED lead installed. Connect each of those rows to one other single-+ row with a wire. (You'll need three wires.) These will be your power rows.
There are also four rows with a single resistor lead. Connect those rows to each other with jumper wires. These will be your ground rows.
4. Add power
Put the batteries in the battery pack, and snap on the battery snap if you need one. Now insert the power wire from the battery pack into one of the power rows you created on the breadboard, and the ground wire into one of the ground rows.
If you've done everything correctly, your LEDs will light up!
But what if it doesn't light up?
If you put everything together and your circuit doesn't light up, here are some tips to help you find the problem. (Or, more frustratingly, problems.)
1. Are your batteries dead? Or pointed the wrong direction? Or maybe just loose in the holder?
No juice, no light! (Though even drained batteries might at least give your LEDs a dim glow.) If you have a multimeter, check the voltage on the batteries.
The flat end of an AA battery goes towards the spring side of the case. In some cheap battery cases, the spring doesn't always push hard enough to ensure that the nubby end is touching its metal connector. If there seems to be space between the nub and the metal, push the battery up until they touch.
2. Is everything connected correctly?
Have you accidentally plugged anything into the wrong row? If — for example — the ground wire from your battery pack is connected to an otherwise empty row, nothing is going to light up.
3. Are any of your wires loose?
Maybe the circuit is generally correct, but one or more parts aren't seated well enough to touch the copper strips inside the breadboard. Unplug your batteries before checking for loose wires.
4. Are any of your LEDs backwards?
Power only flows in one diretion through an LED. If you have the + and - ends reversed, the LED won't light. Make sure the longer leg is in the direction of power, and the shorter leg is in the diretion of ground.
So in conclusion
Building your circuit on a breadboard confirms two things: that your circuit is designed correctly, and that all your parts work. With a circuit this simple, we don't expect any surprises. However, when you're using baggies of cheap LEDs, you might get a bad one once in a while — and it's good to know before they're soldered in place!
Breadboards can be reused in future projects. It's possible to install the components permanently using hot glue (thinner glues can get between the leads and the copper), but it's probably more of a pain than it's worth. As you'll see next time, soldering is not so hard as it seems.
I usually build circuits on a bigger breadboard that came with Radio Shack's Electronics Learning Lab (along with an assortment of parts and two really nice project books by Forrest M. Mims III). The breadboard has buttons, switches, and other components built in around the edges. You don't need something like this, but it's nice to have.
Next time we'll solder up the insides of Vriska's eye!
1. Sewable electronics often use conductive thread, but let's save that for a later article. (back to article)