Forrest M. Mims III: Experimenting with a Luminescent "Capacitor"

By Forrest M. Mims III

Luminescent materials are analogous to capacitors in that they can be "charged" with light. Thereafter, they slowly release light, albeit with a wavelength range that may not match that of the charging light. The analogy is not perfect, for the discharge cycle of a luminescent material is spontaneous. While a true capacitor may experience some slight leakage, it is designed to discharge through a resistor, transistor, LED, flash tube or other external component.

You can easily experiment with a self-discharging luminescent "capacitor" by using a CdS photoresistor or silicon photodiode to detect the light given off by a luminescent material after it has been charged by exposure to light. This Recipe will get you started.

How to Couple a Luminescent "Capacitor" to a Photoresistor or Photodiode

Various kinds of luminescent products and materials are sold by hobby and craft stores. These include luminescent decals, labels, paint and tape.

I have had good results with "Glow in the Dark Luminous Acrylic," a product of PRO ART®. The paint is water soluble and dries completely in 72 hours. It can be directly painted onto the light-sensitive surface of photoresistors and photodiodes.

UGlu® "Super Bright Glo Tape" is by far the best luminescent product I've tried thus far. This product is supplied in a roll measuring 1 x 60 inches. Under ordinary room lighting, the tape has a pale yellow-green color. After being charged by light, the tape glows for up to 24 hours.

Super Bright Glo Tape
Figure 1. Photograph of UGlu® "Super Bright Glo Tape" luminescent tape before and after being charged by exposure to light.

The intensity of light exposure is directly related to the subsequent luminescence. For example, Figure 2 shows the luminescent tape in Figure 1 after a small flashlight having nine white LEDs was pressed directly against the tape for several seconds. Note how the pattern reveals that the center LED and two LEDs at left elicited a slightly less intense glow than the other LEDs, presumably because they were not as bright.

Luminescent tape
Figure 2. Luminescent tape following exposure to a small flashlight with nine LEDs.

The simplest way to couple a luminescent material to a photoresistor or photodiode is to apply it as a paint to the surface of the sensor. Figure 3 shows a CdS photoresistor that has been coated with a thin layer of luminescent acrylic. While this method is the simplest, the luminescent tape shown in Figures 1 and 2 is much brighter and glows much longer. It's so bright that you can simply place a circle or square of it directly over the light sensor. While this works, the base of the tape absorbs much of the glow, so it's best to apply the tape upside down. This makes it more difficult to charge the material with light.

So experimentation with both methods is best. One possibility is to insert a disk of luminescent tape inside a plastic cap designed to be placed over the photoresistor or photodiode. Before the cap is positioned over the light sensor, the tape inside is charged with light.

Cadmium sulfide photoresistor
Figure 3. Cadmium sulfide photoresistor coated with a thin layer of luminescent acrylic.

Simple Demonstration Circuit Controlled by Luminescence

Many different circuits can be be used to demonstrate how a luminescent material can gradually alter gain, pulse width, pulse frequency and other parameters. Figure 4 shows a simple timer circuit in which the pulse repetition rate is controlled by PC1, a CdS photoresistor whose light sensitive surface is covered by a piece of UGlu® "Super Bright Glo Tape" luminescent tape.

Simple pulse generator
Figure 4. Simple pulse generator having a repetition rate controlled by PC1, a light-sensitive photocell (photoresistor) covered by a piece of luminescent tape or paint.

How It Works

The circuit is a basic 555 astable pulse generator. The capacitance of C1 is large to provide a pulse rate slow enough to be counted. After initial experimentation, a smaller capacitance can be used to provide an audio frequency tone. In both cases, the pulse repetition rate varies with the amount of luminescence received by PC1.

Parts You Will Need

The following parts were used to assemble a breadboard version of the circuit:

IC1 - 555 timer
C1 - 10µF capacitor or similar
R1 - 1K resistor
PC1 - Cadmium sulfide (CdS) photoresistor (select a CdS cell that has a high dark resistance and a low light resistance)
Piezo Speaker - Piezo speaker element

Miscellaneous: Solderless breadboard, 9-volt battery, battery connector clip, wire jumpers, UGlu® "Super Bright Glo Tape" luminescent tape (hobby, craft and home improvement stores).

Note: While the components listed above were used for the prototype, substitutions can be made.

Assemble and Test the Circuit

The test circuit was assembled on a solderless breadboard and tested with a photoresistor to which a piece of luminescent tape was affixed. Figure 5 shows the assembled circuit.

Assemble and Test the Circuit
Figure 5. The circuit installed on a solderless breadboard. Photoresistor PC1 is behind the square piece of luminescent tape.

In room light, the circuit should produce a series of clicks from the speaker. You can adjust the repetition rate by modifying the amount of light received by PC1.

When the circuit is working properly, illuminate the luminescent tape with a bright flashlight for five seconds of so. Then switch off both the flashlight and dim the room lights. The repetition rate of the clicks from the speaker will begin to slow in frequency as the luminescence of the tape decreases in intensity. Figure 6 is an animated gif that shows the circuit when the luminescent tape is glowing brightly and, later, when it is glowing less brightly.

Pulse repetition
Figure 6. The pulse repetition rate slows as the intensity of the luminescent glow declines.

A very slow pulse repetition rate was selected so you can easily graph how it changes over time. Figure 7 shows one such graph together with the function that defines the changing pulse rate.

Excel chart
Figure 7. This Excel chart shows the number of seconds between pulses over time. For example, the 10th pulse, which is marked by the red circle, occurred 52 seconds into the experiment.

Various other ways can be used to chart the data. If you don't have Microsoft Excel or another commercial spreadsheet, Open Office, which is free, includes Calc, an excellent substitute.

Going Further

Luminescent materials provide a fun way to experiment with various circuits. You can begin experimenting with the circuit described here simply by reducing the value of C1 to 0.1 µF or less to provide an audio-frequency tone that will change rapidly as the luminescence applied to PC1 declines in intensity.

About Forrest M. Mims III