CHAPTER 14 ■ VARIABLE RESISTORSThe trimpot (R2) will probably be marked “203,” which represents “20” followed by three zeros, which
is 20,000 W. Before adding the trimpot to the circuit, dial it to its halfway point. (Attach the trimpot’s first and
second leads to a multimeter as you turn the dial to watch the resistance settle to around 10,000 W.)
After the circuit is complete, check the voltage at TP1 with power turned on. Recall that to test voltage at
a point, you connect the multimeter’s black test probe to the negative bus and connect the multimeter’s red
test probe to the point of interest (in this case, TP1).
If you bring R3 or R4 toward a light bulb, the voltage at TP1 decreases. If you cover either R3 or R4 with
your finger, the voltage at TP1 increases. In fact, you can witness a change in voltage by merely casting a
shadow over the sensors as you move your hand back and forth.
After experimenting with TP1 for a while, switch over to TP2. Sensors R5 and R6 provide the voltage
effect for TP2. Even so, you may notice that covering R3 and R4 produces a slight increase in voltage.
Because R1 is shared by both pairs of sensors, R1’s voltage usage is affected by the resistance of all four
sensors. When R1’s voltage usage decreases because of one set of sensors, the other set of sensors takes up
the slack, thus increasing the voltage at that pair’s test point. It’s not a consequential effect for line following.
However, it is noteworthy that when two paths have a component in common, their values can affect each other.
Inability to Balance the Brightness-Sensing Circuit
As designed, changing the dial on the trimpot (R2) affects the voltages at TP1 and TP2. If you have two
multimeters, you can connect one to each test point and adjust the trimpot until the voltages match.
You can attempt to swap a single multimeter between the test points. However, you’re likely to cast
dissimilar shadows or nudge the sensors to different angles as you try. Balancing the circuit with a single
multimeter is difficult.
Wouldn’t it be helpful to have something that constantly compared both pairs of sensors and turned on
an LED to indicate which sensor had a higher voltage? That’s exactly what you’ll add to the breadboard in
the next chapter.
Figure 14-18. Balanced brightness-sensing circuit built on a solderless breadboard