Doctor Who Pumpkin with Pulsing LED

Background

Dr Who Pumpkin Finished
It’s October 2013 and the building I work in is holding a pumpkin carving contest. I’ve decided to use a circuit I helped a friend with for Dragon*Con a year ago, to light up a Doctor Who themed pumkin.

Unfortunately, my attempt wasn’t a complete success. The rind of the pumpkin was way to thick for the LED to show through as I’d hoped. Lack of time prevented me from correcting the situation by removing rind from the interior. If I try this again next year, I’ll do that. For the purpose of this article, I’ll write down the steps I actually took and then note where I would do something different next time to fully accomplish the goals I set.

Goal

Primary

  • Carve a Doctor Who themed image into a pumkin – specifically one that contains a TARDIS.
  • Install LEDs to light up the pumkin.
  • One LED should be blue and should blink periodically to match what the TARDIS does in the TV show.

Secondary

I ended up running out of time to get these completed for the contest but I’ll go into detail on how to implement it.

  • Add TARDIS landing sound effect, only blink the Blue LED when it’s playing.
  • Trigger the sound effect via a motion sensor.

Materials

Implementation

Cleaning the Pumpkin

The first step with any jack-o-lantern is to remove the guts of the pumpkin. Typically you cut out a round hole in the top, but I opted to cut it out of the button so the top of my pumpkin appeared unblemished.Ā Ā Before I made any cuts I washed the outside of the pumpkin with windex – everything, stem and all. Then I cut out a circular hole in the bottom and scooped out all the seeds with an icecream scoop.

I did some research online on how to best preserve the pumpkin and prevent mold growth – my wife kept telling me to rub vaselineĀ over the cuts, but that just didn’t sound right to me. I found this site, here, where someone actually preformed a full experiment on several popular methods of preservation. I opted to use the bleach water method, so I turned the pumpkin upside-down using a pot to hold it in place without hurting the stem, and filled it with bleach water (1 tsp of bleach per gallon of water, for me it was 2 gallons-ish and 2 tsp of bleach). I let it sit over night (roughly 8 hours).

I put some of the bleach water that wouldn’t fit in the pumpkin into a spray bottle instead. I’ll use this later to spray the carvings for mold prevention.

Next-Time Note: Scrape a a good bit of the inner meat off the rind in the area you plan to do the detailed carving (the image). This will allow more light to shine through.

Stencil & Carve

I printed out the stencil as large as I could on one piece of paper. Then, with an Exacto knife I cut out all the dark portions of the image. From here you could just tape the cut out paper to the pumpkin and trace out the image with a dry erase marker. I decided to project the image onto the pumpkin to hopefully make it look better, since the paper is flat and the pumpkin is round. To do this I used my third hand kit I got from Sparkfun to hold the paper up in the air a bit and then used a mag light flashlight to project the cut-out onto the pumpkin. Then I traced where the light shown on the pumpkin for a more accurate rendering.

Then, with the Exacto knife, I cut out the traced areas. This was pretty tricky, I’ve never done it before. I think if I do this again I may use a dremel to “mill” out the drawing.

Dr Who Pumpkin Finished

Circuitry

To make the top of the TARDIS blink I used a 555 timers and a simple “breathing” light circuit to make the blue LED fade in and out every couple of seconds. I covered the circuitry and the battery in a ziplock bag and inserted it into the pumpkin. I then used toothpicks to fasten the LEDs within the pumpkin and position the in a what that they right areas of the image are illuminated. Here is a diagram of the circuit and pictures of the finished product:

TARDIS Circuit

 

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