Page 294 - Hacking Roomba

Chapter 13 — Giving Roomba a New Brain and Senses 275

In order to measure a variable resistor like a photocell or potentiometer, you would usually cre-
ate a voltage divider with another fixed resistor and feed the output of that voltage divider to an
analog input of your microcontroller. The BS2 has no analog inputs, but it does have an inter-
esting function called RCTIME that records the amount of time it takes a capacitor to charge or
discharge through a resistor. The charge time to go from a digital LOW to HIGH is related to
the resistance value.The capacitors in parallel with the photocells and the resistors leading to the
BS2 pins are part of this timing circuit RCTIME needs. Because capacitors, resistors, and photo-
cells all have widely varying tolerances and are temperature sensitive, the values returned from
RCTIME cannot be used as absolute values, but rather as a mark of relative change. For details on
how RCTIME works, see the Basic Stamp language reference available from the Parallax site.
Listing 13-2 shows how to use RCTIME to measure the photocells. Only the front photocell
measurement is shown, but the back one is just the same. The code spins Roomba around every
so often, checking light levels. When it finds a darker area, it drives straight toward it. Even
with such a simple algorithm, Roomba will almost consistently find the darkest area of a room.
It never stops moving though, and still runs the BumpTurn algorithm so it avoids obstacles.

As a final bit of fun, you can now chase Roomba around with a flashlight.

Listing 13-2: Additions to BumpTurn to Make RoombaRoach

‘ the pin where our light sensitive resistor is
LSRF PIN 1
Main
‘ ...everything else as in BumpTurn, then...

‘ check light levels, find the dark
FOR i = 1 TO 4
oldresF = resF
GOSUB Read_LightF
IF resF > oldresF+10 THEN ‘ its getting darker
GOSUB Go_Forward
ELSE
GOSUB Spin_Left
PAUSE 500
ENDIF
NEXT
GOTO Main
Read_LightF:
HIGH LSRF ‘ charge cap
PAUSE 1 ‘ for 1 ms
RCTIME LSRF, 1, resF
DEBUG DEC ? resF ‘ print out value to debug port
RETURN