CHAPTER 15 ■ COMPARATORS
In a schematic, the dual comparators would not be drawn as a chip package. Instead, each comparator
would be drawn separately as symbols near the parts they are comparing. The schematic (“package-free”)
technique reduces the number of lines crossing over the drawing. Pin numbers are usually not shown in
schematics, which make the illustration independent of the particular chip package eventually implemented.
Unlike schematics, wiring diagrams present the layout as it looks in real life. Final pin numbers and
package grouping are indicated. (While I was at it, I changed the resistors to look a little more realistic,
instead of symbolic. I beg the forgiveness of any old-school electrical engineers.)
A schematic is better when you’re designing the functionality of the circuit, without regard to the
physical layout. A wiring diagram is easier to follow when you’re actually assembling the circuit.
Labeling Parts
The rules for part labeling are reasonably flexible. Because the circuit in Figure 15-5 connects to another
circuit containing resistors, I chose to number the new resistors where the last circuit left off (R7, for
example). For ease of reading, I switched to using “LED” instead of “D” to label the LEDs. I also numbered
the LEDs to match the resistors to which they’re connected (LED7 and R7).
The LM393 is labeled as IC1 for integrated circuit #1. It’s called an integrated circuit because the
comparators are nothing more than an ordinary circuit of resistors, wires, diodes, and transistors squished
into a single part. Got it? It’s a circuit that’s been integrated into a small package.
By the way, for proper connections, don’t forget to pay attention to the notch in the chip.
Drawing Connected and Unconnected Wire
Figure 15-5 is the first circuit presented in this book that has been complex enough that a few lines cross over
each other. Sometimes the wires are connected to each other; sometimes they’re just passing over.
A solid circle or dot appears on the wire near the arrow going to TP2. The dot indicates that those wires
are connected.
Figure 15-5. Wiring diagram of the brightness comparator circuit