CHAPTER 23 ■ THE MOTHERBOARDWow! 96000 W is much greater than the book’s default 150 W. Remember, this proposed change is to
handle the extreme case where all of the sensors are looking at a dark surface. Usually, at least one of the
sensors sees a bright line. The potentiometer resistance provides more than enough voltage drop during
normal operation. If you’re at all concerned about making such a major change, particularly given that it
may alter the normal performance, you have some choices:
- Leave R1 with the 150 W resistor. You can always turn up the headlight brightness.
- Compromise and install 10 kW. (Probably the best choice.)
- Install a 100 kW resistor but be prepared to solder a 10 kW or smaller value resistor on
top of it. This trick works because electricity is lazy. It will take the less-resistant path.
(You’ll end up with a total resistance of around 9 kW.) - Leave the 150 W resistor and replace the LM393 comparator with a LMC6772BIN
comparator (Digi-Key $2.77). The LMC6772 can compare voltages over the entire
range, and thus won’t be affected if the sensors all see darkness.
Point-to-Point Soldering Versus a Printed Circuit Board
At this point, you have built and tested the line-following circuit on a solderless breadboard with either the
default or custom values. There are several methods of creating a soldered circuit. You can obtain a blank,
solderable breadboard (see left side of Figure 23-2), insert the components, and solder wires from each
component lead to the other. This is called point-to-point soldering.
Figure 23-2. The top of a breadboard (left) and the top of a printed circuit board with the line-following
circuit (right) before soldering
The other method is to obtain (or make) a printed circuit board (PCB, see right side of Figure 23-2)
where all the wiring is built onto the surface of the board. In that case, the parts need only be inserted into
the correct holes and soldered into place.
Point-to-point soldering is faster and cheaper for one-time small circuits. After all, there’s up-front effort
required to design and etch a printed circuit board yourself. Also, since ordinary wires in a point-to-point
board connect all of the parts, you can arrange (or fix) the circuit somewhat more easily by desoldering and
resoldering the wires.
Obviously, buying a pre-printed circuit board with the line-following circuit is much easier and faster.
There’s a lot less soldering to do. Not only that, but if the circuit has been previously verified as being correct
on the board, then you don’t have to be concerned about miswiring the circuit.