Robot Building for Beginners, Third Edition

CHAPTER 14 ■ VARIABLE RESISTORS

As the floor in front of the robot becomes darker, the photoresistors increase in resistance. If instead of
100 W, each the photoresistors reach 6,000 W each, the share of voltage used by each part changes.

1. Add up all of the resistance in the path. We’re going to assume the photoresistors
   are looking at darker flooring (6,000 W) for this example.

R1 W + R2 W + R4 W + R3 W = total W of the path

This example: 470 W + 10,000 W + 6,000 W + 6,000 W = 22,470 W

2. For any resistor of interest, divide its resistance by the total to determine what
   share of the voltage it will use.

R1 W / total W = R1's share

This example: 470 W / 22,470 W = 0.021

3. Determine the amount of voltage being used in the path.

V at beginning - V at the end = total V used in the path

This example: 9 V - 0 V = 9 V

4. Multiply the resistor’s share by the voltage used in the path to determine how
   much voltage is used in that resistor.

R1's share × total V of path = V of R1

This example: 0.021 × 9 V = 0.19 V

Plugging in the values for the other resistors results in:

R2 V = 4.0 V
R4 V = 2.4 V
R3 V = 2.4 V

As less light reaches the sensors (R3 and R4), their resistance increases. As their resistance increases, they
use up a greater portion of the voltage in that path. As such, the voltage at TP1 increases (see Figure 14-17).

Figure 14-16. Voltages used by each resistor on one branch of the balanced brightness-sensing circuit