CHAPTER 14 ■ VARIABLE RESISTORSPart List for Balanced Brightness-Sensing Circuit
- -- – 9 VDC power supply
- R1 – 470 W resistor (yellow, violet, brown)
- R2 – 20 kW trimpot
- R3, R4, R5, R6 – Roughly 100 W bright to 450 kW dark cadmium-sulfide
photoresistors
Brightness-Sensing Pairs
R3 through R6 are photoresistors (notice the variable resistor symbol with a circle drawn around it and
“light” arrows coming in). Each photoresistor changes resistance depending on the amount of light it
receives. R3 and R4 are paired to sense the right side of the floor beneath the robot. R5 and R6 are paired to
sense the left side of the floor beneath the robot.
A pair of photoresistors can detect a physically wider area than can a single photoresistor. Another
advantage to using a pair is that you can hand–select them to average out manufacturing discrepancies in
minimum and maximum values.
Matching Photoresistors
Individual photoresistors have appreciably different overall resistance ranges even if they look the same
or came from the same production batch. This is especially true if purchased in mixed bags. It’s important
to match similar resistance groups and sort them into their own piles. You can balance your line-following
robot with photoresistors that are slightly different, but not with photoresistors that are radically different.
Photoresistor matching is a simple process. Connect a photoresistor to your multimeter and hold it up
to a light. Write down the W value. Now hold the photoresistor under a table (or other dark area) and write
down the new W value. As you make your way through a pile of photoresistors, you’ll get a sense for how
closely the high and low values can match.
Your primary goal in testing the photoresistors is to knock out any defective ones. These will not change
value or will have really low or really high values—especially compared to the rest of the lot.
Your secondary goal is to roughly match the photoresistors. Again, stick one up to a light bulb, count
to 5, and record the value. Stick it under a dark desk, count to 5, and record the value. Good enough.
Figure 14-15. Balanced brightness-sensing circuit