Step 1: Program the Arduino and Hook It Up
The code for this is based on an example from Tom Igoe's Making Things Talk. It's pretty simple: the Arduino monitors a Sharp GP2D12 Infrared ranger, and if it detects movement nearby, it turns all the LEDs up to maximum brightness. While it's waiting for someone to step close to it, the Arduino flickers the lights.
Pins 9, 10, and 11 go to LEDs that are shown in the next step. Analog pin 0 goes to the proximity sensor. Power and ground go to the breadboard that houses the LEDs and the sensor.
The attached file, Pumpkin.pde, contains the Arduino code you need to upload to your Arduino.
Step 2: Build the Circuit
At this step, you're just completing the connections made in the previous step. However, you'll notice that instead of three LEDs, I've got two perfboards with four LEDs each and also one LED that lives on the breadboard. The perfboards are an implementation of the multiple LED schematic from Arduino playground. The connection is almost exactly like hooking up an LED to the Arduino, except that you have an extra connection to +5v, shown in the photo notes, in addition to the usual digital pin and GND connection.
Step 3: Carve Your Pumpkin and Install
Now it's time to carve the pumpkin. Once you've made plenty of space, put the parts in, Use toothpicks to hold it all in place, double-check all your connections, and fire it up! Your pumpkin should be all blinky now. I suggest powering it from a MintyBoost so you can put it pretty much anywhere!