Page 151 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

6.29 Electrical function of a CdS photoresistive cell

also accurate in classifying height. Why would one choose a fuzzy
logic method over a digitized model function? The fuzzy logic
method has simplified mathematics and learning functions.
To implement fuzzy logic in a PIC microcontroller, one assigns a
numeric range to a group. This is what we will do in our next project.
Building a fuzzy logic light tracker

130 The project we will build now is a fuzzy logic light tracker. The
tracker follows a light source using fuzzy logic.
The sensors needed for the tracker are two CdS photocells. These
photocells are light-sensitive resistors (see Fig. 6.29). The resis-
tance varies in proportion to the light intensity falling on the surface
of the photocell. In complete darkness the cell produces its greatest
resistance.
There are many types of CdS cells on the market. One chooses a
particular cell based on its dark resistance and light saturation
resistance. The term light saturation refers to the state where
increasing the light intensity to the CdS will not decrease its resis-
tance any further. It is saturated. The CdS cell I used has approxi-
mately 100K-ohms resistance in complete darkness and 500 ohms
of resistance when totally saturated with light. Under ambient
light, resistance varies between 2.5K and 10K ohms.
This project requires two CdS cells. Test each cell separately. There
may be an in-group variance that may change the scale factor used
in each cell. In this project, I used a 0.022- F capacitor, with the
scale parameter set at 255 for both cells in the Pot command.

The schematic is shown in Fig. 6.30. The CdS cells are connected to
port B, pins 2 and 3 (physical pin numbers 8 and 9). The photocells

Team LRN
Chapter six