CHAPTER 14 ■ VARIABLE RESISTORS
Because of the balancing potentiometer (R1) and the striking contrast of the line on the floor, the
photoresistors don’t have to be matched exactly. Truth be told, the four photoresistors for Sandwich
were picked at random from a previously defective-removed lot. And Sandwich performs beautifully.
Subsequently built Sandwich clones had more carefully matched values, but show no differences in
performance.
In order of testing, Table 14-3 shows the values I found for otherwise identical-looking photoresistors.
Table 14-3. Sample Photoresistors Values
Label Resistance Pressed Against Light bulb Resistance Under Dark Desk
A 123 W 260 kW
B 130 W 338 kW
C 115 W 400 kW
D 124 W 370 kW
E 105 W 440 kW
F 128 W 470 kW
G 110 W 430 kW
H 115 W 434 kW
I 119 W 550 kW
I would pair photoresistors ‘C’ and ‘H’ on one side of a line-following robot and photoresistors ‘G’ and
‘D’ on the other side. The sensor order would be ‘C,’ ‘H,’ ‘G,’ and ‘D.’ ‘C’ and ‘H’ have a combined range of 230
W to 834 kW, whereas ‘G’ and ‘D’ have a combined range of 234 W to 800 kW. In the center of the robot, ‘H’
and ‘G’ have similar ranges. On the sides of the robot, ‘C’ and ‘D’ also react similarly.
You don’t have to go to all of this effort. But imagine the bad luck of a robot comparing photoresistor ‘A’
with photoresistor ‘I.’ Both photoresistors react the same to light lines, but the resistance of photoresistor ‘I’
would be double that of photoresistor ‘A’ when looking at the same dark surface.
None of the photoresistors in Table 14-3 are bad or damaged. A bad photoresistor I ran across
ranged from 4 MW in light to greater than 40 MW in darkness. That’s extremely different than the other
photoresistors. If you find a part like that, dispose of it properly before it gets into one of your creations. In an
assorted grab bag, expect as many as 10% of the photoresistors to be faulty.
Resistance-Balancing Potentiometer
Referring back to Figure 14-15, R2 is a 20 kW potentiometer. As the dial is turned, it splits the resistance
supplied to each branch that leads to each pair of photoresistors. R2 works the same way it did in the
brightness balanced LED’s circuit (see Figure 14-9).
In the middle of its range, R2 provides 10 kW resistance to the left side and 10 kW to the right side.
Jamming it in one direction provides 20 kW resistance to one side and 0 W resistance to the other (or vice-
versa). Table 14-3 shows that the maximum anticipated difference in bright ranges isn’t very much, so R2
provides plenty of resistance to equalize the photo-resistance of both sides in bright conditions. The robot
has headlights to run in dim conditions so dark balancing isn’t necessary.