It’s never a good idea to leave a flame unattended—even on Halloween. But that doesn’t mean you have to give up the glow of a traditional jack-o’-lantern. You can create a realistic fire effect by wiring up an electronic blinky light. This one, inspired by our friends at DIY blog Evil Mad Scientist Laboratories, uses a safe circuit that requires no soldering. It incorporates six LEDs with a candle-flicker effect, which will shine all night on three AAA batteries. And thanks to a phototransistor, which detects light, the LEDs will automatically turn off at dawn and back on at sunset. Trick or treat!
- Mini breadboard
- 6 candle-flicker LEDs (2 red and 4 yellow)
- 6 100-ohm resistors
- 2 NPN transistors
- Infrared phototransistor
- 5-kilohm resistor
- Jumper wires
- 3 AAA battery box with switch
- 3 alkaline AAA batteries
- Side-cutting pliers
- Wire strippers
- Assemble the components and jumper wires in the breadboard as shown in the breadboard photo. The LEDs, NPN transistors, and phototransistor (which looks like an LED with a black lens) are “polarized,” meaning the circuit won’t work as intended if you install them backward. For the four LEDs nearest the switch, place their “flat” sides facing away from the switch; the flat sides of the other LEDs and the phototransistor should face toward it.
- Trim the leads and jumper wires with the pliers as you go so everything sits flush against the breadboard face.
- Peel the film off the adhesive on the back of the breadboard and stick it to the battery box, right below the switch.
- Connect the red battery-box wire to the breadboard corner nearest and left of the switch, and the black wire to the far left corner.
- Load the batteries into the box and turn on the switch. If the LEDs don’t come on right away, try covering the phototransistor with your thumb or moving the breadboard into a dark room. The sensor responds to sunlight and bright incandescent bulbs, but not to LEDs or fluorescent lights.
- Set the project inside your carved jack-o’-lantern and place it on an open patio or windowsill. If your carving is fairly open and admits natural light into the pumpkin, the phototransistor should easily detect when the sun sets. But if your pumpkin is dark inside, you might need to leave the lid off to prevent the LEDs from turning on too early in the day.
- Another option is to adjust the circuit’s light sensitivity by replacing the 5-kilohm resistor. Swapping it for a stronger resistor (up to 10 kilohms) creates a circuit that turns on only at brighter light levels. A weaker resistor (down to 1 kilohm) makes a circuit that can turn on in dimmer conditions.
This article originally appeared in the October 2015 issue of Popular Science, under the title “Build a Flameless Hack-o’-Lantern.”