Robotics with the Boe-Bot text v2.2

Appendix F: Balancing Photoresistors · Page 325
Now, we know how to correct the timeLeft variable so that it reports values similar to
the timeRight variable in this narrow range of light levels:
7timeLeft37.1timeLeft
7x37.1y
bmxy
)adjusted(
+×=
+=
+=
A Linear Equation in PBASIC
In most programming languages for PCs, this equation could be entered as-is. The
BASIC Stamp is a very tiny processor compared to a PC. Because of this, it takes an
extra step to multiply by a fractional value. You have to use the */ operator (it’s called
the “star-slash” operator). For the
timeLeft equation, the PBASIC code to adjust the
timeLeft variable can be done like this:
timeLeft = (timeLeft */ 351) + 7
The adjusted value of timeLeft after this line of code is executed is 1.37 times the old
timeLeft, plus 7.
Why did 1.37 become 351? The way the */ operator works is that you have multiply your
fractional value by 256, and place it to the right of the */ operator. Since 1.37 X 256 =
350.72 ≈ 351, the value 351 goes to the right of the */ operator.
You can find out more about the */ operator in the BASIC Stamp Editor by clicking Help and
selecting Index. Type in */ in the field labeled “Type in keyword to find”. You can also look
up */ in the Binary operators section of the BASIC Stamp Manual.
Your Turn – Balance Your Photoresistors with m and b
√ In Table F-1, label the first timeLeft entry X1 and the second timeLeft entry
X2.
√ Label the first timeRight entry Y1 and the second timeRight entry Y2.
√ Use these equations and your X1, X2, Y1, and Y2 values to solve for m and b.
√
)xx(
)yy(
m
12
12
−
−
=
and
22
mxyb −=