With Halloween coming up I thought I'd spruce up some cheap styrofoam tombstones by adding LEDs. Aside from the battery these end up costing about $5 or less a piece to put together depending on how/where you buy the parts.
I started off with a pack of tombstones similar to these. You can usually pick up a pack of 4-8 for less than $10 at your local big box store.
I used mostly 10 mm LEDs, like these, to add glowing eyes.
All that was left is a resistor regulate the circuit current and a connector for the battery.
The circuit for these is pretty simple. 2 LEDs for eyes, a resistor to regulate current, and a battery to power it.
NOTE: if you want to know about the science and calculations to pick a resistor for this circuit read on. Otherwise, you can skip to The Build
Since I know my LEDs will drop about 3.3 Volts and need at least 20 milliamps to light up I used Ohm's law to figure out what resister to use. If you're not familiar with Ohm's law it states that Volts = Amps x Resistance. Since I have 2 LEDs that's 6.6 Volts dropped. That leaves 2.4 Volts to be dropped by the resistor. That gives us the equation 2.4 V = 0.020 A x R, or R = 2.4 V / 0.020 A = 120 Ohm Resistor. Conveniently 120 Ohms is a pretty standard resistance value, but this will not always be the case. If the results is not a standard value you'll want to use the next lower value available for LEDs to make sure they get enough amp flow to light up. Since I didn't have any 120 ohm resistors on hand and there can be some variance in the voltage dropped across the LEDs I went with 100 Ohm resistors. This makes the equation for Ohm's law 2.4 V / 100 Ohms = 0.024 Amps or 24 milliamps, which is well within the acceptable range for these LEDs.
I started by pushing the LED legs through the styrofoam in the locations I wanted. Just make sure to spread the legs out a little so they don't try to cross and short out going through.
Then I folded the legs out on the back side to keep them in place and soldered the resistor and battery holder in position. Remember that the shorter leg of the LEDs is the negative leg and the connection is positive battery -> positive LED leg -> negative LED leg -> resistor -> positive LED leg -> negative LED leg -> negative battery. Once wired, just plug in your battery and enjoy. Based on some 9V battery discharge rates I've seen and the current draw for these LEDs a fresh battery should last 10 to 15 hours solid.
Enjoy and feel free to report any problems or suggestions to email@example.com.