Birthday Cake with L.E.D. Lights

Ingredients
 

• 1 multimeter

• Batteries (9-V or AA or AAA will work, but the type of battery you use depends on the design of your circuit: see steps 1 –3)

• LEDs of the same color, check required voltages

• 1 Electronics breadboard (recommended; can be found at RadioShack)

• 1 frosted cake

• Shoestring licorice (preferably a sticky version, such as Twizzlers Pull-n-Peel)

• Edible silver or varak leaf

Instructions

1. Decide what color and how many LEDs you want to use. Stick to one color and model of LED; it will make the design of your circuit much easier, because all of the LEDs will have the same voltage requirements.

2. Divide the voltage of your battery (Vb) by the voltage requirement of your chosen LED (Vled); these numbers should be provided by the manufacturers. The outcome, or cluster count, needs to be a whole number. If it is not a whole number, you need to pick a different LED and battery combination. We used a 9-V battery and blue 3-V LEDs: Vb/Vled = 3 = perfect. This is the cluster count we will use in the next steps.

3. Draw your circuit and test your design on an electronics breadboard before you lay it out on your cake: LEDs have a polarity; they will work only when electricity is flowing from their positive to their negative terminals. With this polarity in mind, you have two options for wiring multiple LEDs together: in series (from the negative terminal of one to the positive terminal of the next), or parallel (from positive terminal to positive terminal and from negative terminal to negative terminal) — see photo 3a. This is where the cluster count becomes useful. The cluster count tells you how many LEDs you need to wire together in a series so that their electrical load will match the voltage of the battery, thus making the LEDs light up but not burn out. These clusters of LEDs are then wired as a group into the rest of your circuit in parallels. This means that the total number of LEDs must be a multiple of your cluster count. So, for the cake we built, the cluster count of three limited us to using a total number of LEDs that was a multiple of three.

4. Roll the licorice in the sheets of silver leaf to completely coat each string: Stretch the licorice a bit or wet it, to make the surface tacky enough for the foil to stick. The sheets of leaf can overlap and be ragged; what matters is that the licorice, especially at its ends, is well covered.

5. Test the conductivity of the wires by touching them at each end with a multimeter set to read ohms and measure the electrical resistance. The resistance should basically be zero.

6. Build the circuit atop your cake: When wiring it is helpful to hook the battery up first and wire an entire cluster of LEDs at once. That way you can see each cluster of LEDs light up as you wire your cake, giving you quick feedback about whether all of your licorice connections are working. If they aren'­t, clean the ends of the licorice, add more silver if necessary, and reset the circuit atop your cake.

Using your diagram, lay out the wiring for your circuit with your edible licorice wire. To make longer sections of wire, overlap the ends of the edible wire, making sure that frosting does not sneak in between the wires. You will need to bend the leads of your LEDs to right angles so that they can straddle your two wires (see picture). Be careful to wire the LEDs with the polarity in the right direction. Usually the longer lead is the positive terminal of the LED. You can check this by holding the LED leads to the + and - terminals of your battery to see if it lights up; don't hold it there for more than a second as you will burn out your LED. If the LED doesn't light up, flip the leads around.

7. Enjoy your glowing masterpiece. But don't eat the LEDs; they still need to be pulled out of the cake, just like candles. The rest of your circuit is good enough to eat—a tiny bit of and only once a year!