Forrest M. Mims Jameco Puzzler 7 Solution

What's the Missing Super Capacitor Component?

By Forrest M. Mims III
Puzzler 7 Solution

When electronics instructor Ollie Circuits arrived at the pet store, he headed straight to the aquarium section. There he found just what he needed to complete his do-it-yourself super capacitor, a sheet of activated carbon filter material used to purify water in an aquarium.

How It Works

To make C1, Ollie first cut four 2-inch squares of paper towel and five 1.75-inch squares of activated carbon aquarium filter material. He stacked the paper and activated carbon filter squares as shown in Fig. 2. He then placed the two PC boards as shown in Fig. 2 (copper sides facing toward one another) to form a sandwich-like structure that he held together with a rubber band. After the assembly was ready, he dripped some lemon juice into the four sides of the sandwich.

Figure 2. How the homemade super capacitor is assembled.

Ollie's super capacitor demonstration circuit is shown in Fig. 3. In operation, C1 is charged by switching S1 to position 1 for a minute. This will charge C1 to about 3.3 volts. When S1 is switched to position 2, the LED will glow for 15 seconds or more. C1 can then be recharged for another cycle.

Figure 3. The mystery component in Figure 1 is a homemade super capacitor like the one shown in Fig. 2 and used here as C1.


Many web sites describe the theory and operation of super capacitors, which are also known as ultra capacitors and by various other names. For example, see Electropaedia, Wikipedia,

Going Further

Details about the do-it-yourself super capacitor can be found in reference 1. The circuit in Fig. 3 can be easily modified by connecting B1 and the LED directly to terminals 1 and 2 of S1 and by moving R1 (1K) between C1 and S1. (R2 is not needed in this arrangement.) Just be sure that the LED is connected to C1 through a current limiting resistor.

You can experiment with the design and construction of C1. For example, do the layers constitute separate capacitors in series? Can the more layers be added to increase the voltage C1 can store? Can the capacitance be increased by increasing the area of the layers? What happens when you use an electrolyte other than lemon juice?


1. Forrest M. Mims III, Light Emitting Diodes, Howard W. Sams & Co., 1973, pp. 118-119.

2. Ibid., Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics , 31, 33, 6965-6967, 1992.

3. Ibid., An inexpensive and stable LED Sun photometer for measuring the water vapor column over South Texas from 1990 to 2001, Geophysical Research Letters 29, 20-1 to 20-4, 2002.

4. Ibid., Five years of photosynthetic radiation measurements using a new kind of LED sensor, Photochemistry and Photobiology 77, 30-33, 2003.

5. Ibid., LED Sun Photometry, Optics & Photonics News 20 , 32-38 (2009).

About Forrest M. Mims III