CHAPTER 23 ■ THE MOTHERBOARD- The brains and LEDs modules are the brightness comparator circuit with transistors
- The motor module is the motor and Schottky diode circuit
A couple of minor changes were made to make the robot “kit friendly” or “production friendly:”- Two 470 W resistors (R1 and R9) were replaced with 150 W resistors to make it less
complicated for someone to offer this line-following robot as a kit. Now, only 1 kW
and 150 W resistors are in the circuit. The reduction in values shouldn’t pose a
problem, since the resistor values are still large enough to limit current extremes.
■ Note Recall that resistor value selection can be an art. Earlier in the book you had to choose a resistor
value that either leaned toward making an LED brighter with shorter battery life, or saved power but made the
LED dimmer.
The change presented in Figure 23-1 represents another potential resistor value selection methodology: You can
consolidate on common values in a design so that components can be purchased in bulk. Schools or companies
producing multiple copies (or kits) of this robot can save money by purchasing greater quantities of just a few
resistor values, rather than lesser quantities of many resistor values. Additionally, the person assembling the
robot has fewer opportunities to insert the wrong value resistor, because there are fewer resistor values from
which to choose.
- A 20 kW variable resistor (R10) has been added to the headlights to allow their
brightness to be adjustable on the robot. The value of 20 kW was chosen so that
R10 and R2 are matching parts. Again, this makes it “kit friendly.” However,
because you are unlikely to ever dial down the headlights to be that dim, a 2 kW
value would have been preferable if not for the kit advantages.
Feel free to test these modifications on your solderless breadboard before committing them to a
soldered circuit.
Tweaking For Better Performance
Before we continue, let’s discuss the opposite strategy for selecting resistor values for R1 and R9. Instead of
designing for a kit, what optimal values should you select if you want to tailor the resistors for the unique
components sitting in front of you now?
You would want to find a value for R9 that permits your headlight LEDs to be as bright as safely possible
and select a value for R1 that is correlated with your sensors. These customizations help ensure that the
sensor voltages for your specific robot are in the range that IC1 can compare. That means your robot will be
able to accurately follow a line at faster speeds.
Because different types of white LEDs vary significantly in voltage drop and brightness, and because
cadmium-sulfide sensors vary significantly even in the same batch, there is no way to select a “perfect” value
in a book. Yet, the kit values work just fine for most people.