CHAPTER 14 ■ VARIABLE RESISTORS
Power up! Turn the trimpot dial with a small screwdriver and observe the brightness of the LED. If you
have a very efficient LED, the light may remain visible even when the trimpot dial is turned all the way to the
right. If you have a poor-efficiency LED, the light may not be visible even when the trimpot dial is halfway.
Try different values of trimpots. Recall that R1 is protecting the circuit against too much current, so
you’re free to experiment with any value potentiometer.
Brightness Balancing Circuit
Take a look at the schematic in Figure 14-9. Where did the battery go?
Figure 14-9. Schematic of brightness balanced LEDs
The circuit has been simplified by substituting the words “+9 VDC” for the battery. This takes up less
space and focuses attention on the heart of the circuit. Also, this doesn’t falsely lead you to believe you must
use a battery. For example, a wall adapter power supply providing the same voltage would be fine.
What happened to the bottom of the schematic? The LEDs are each connected to three ever-shortening
horizontal lines. Again, a simplification. These are the power return lines. It’s less distracting to show
abbreviated return lines rather than having them cross all over the illustration.
The schematic is cleaned up and simplified by eliminating the battery and chopping all of the power
return lines. For such a small circuit, it may not seem like much, but this commonly accepted technique
really tidies up large schematics.
Building the Brightness Balanced LEDs Circuit
Looking at Figure 14-10, it may strike you that the actual built circuit looks a lot like the schematic
(see Figure 14-9). This is yet another reason for drawing the schematic without the battery or power return lines.