A Self-Powered Illuminated Pumpkin

 

Need: To have an illuminated pumpkin in front of my house without the need to go out and light a candle, or worse yet have to worry about installing/replacing batteries.

 

Objective: Utilize the acidity of a pumpkins flesh to make a galvanic battery and use the electrical energy to power LEDs to illuminate the pumpkin.

 

Design: The electrical side of the electric pumpkin is shown in Figure 1.

 

Figure 1: An Eagle schematic representing the pumpkin circuit.


The parts list for the items in Figure is:

         (1) Pumpkin

         16 feet 30-gauge enameled copper wire. 8 feet red, 8 feet green.

         (2) Red LEDs

         (6) 1x1 copper plates

         (6) 1x1 zinc plates

 

The real design challenge was to determine the most effective battery layout. Initial testing with a zinc and copper plate proved the soundness of the concept. The plates have a 0.65v open circuit potential and a 100uA short circuit current. The next step was to increase the voltage and current by arranging the plates in parallel or series. Stacking the plates in series in the same chunk of pumpkin failed to produce the expected increase in voltage. Photo 1 shows a test where the plates were arranged in parallel in order to increase the current. This test also failed to produce a significant increase in current. The reason is that the plates are effectively shorting each other out because they are in the same electrolyte.

 

 

Photo 1: Arranging plates in the same piece of pumpkin is not an effective way to increase the voltage or current of a pumpkin battery.

 

The solution to this problem would be to electrically isolate the pumpkin batteries from each other. However, instead of having each pumpkin provide 1 cell, battery shaped chunks of pumpkin were cut from the rear of the pumpkin and a pair of plates installed in each chunk. A variety of materials were used to isolate the chunks from the body of the pumpkin, but it was found that plastic sandwich bags were most effective and convenient. Photo 2 shows the rear of the pumpkin and 3 of the 6 cells.

 

 

 

Photo 2: Battery shaped chunks were carved out of the pumpkin. Each chunk contained it own zinc and copped plates and are electrically isolated from each other by plastic sandwich bags.

 

The leads from all 6 batteries were brought out of the rear of the pumpkin and attached to a prototype board. Table 1 given the open circuit voltage and short circuit current of each cell.

Cell

Voc (volts)

Isc (uA)

1

0.67

187

2

0.64

182

3

0.85

135

4

0.65

175

5

0.65

125

6

0.64

97

 

Table 1: The electrical properties of each pumpkin cell.

 

Photo 3 shows the LEDs illuminated by two different battery configurations. In the left photo, each LEDs is illuminated by its own bank of three LEDs. In the right, all 6 batteries are arranged in series and both LEDs run off the common supply. The right most configuration yielded more satisfactory results and was solder in place as shown in Photo 4.

 

 

 

 

 

Photo 3: Left, each LED is illuminated by an independent bank of by 3 cells in series. Right, both LEDs are illuminated by all 6 cells arranged in series.

 

 

Photo 4: The battery leads are soldered together and protected from shorting out by shrink insulation.

Photo 5: Ill never have to replace batteries as long as my pumpkin lasts!