Servo - December 2011

should detect the PropBOE connected to your computer. If it doesn’t, check the connection and be sure power is applied to the PropBOE. Its green power indicator should glow.

4. Compile the program by choosing Run->Compile Current->Load EEPROM (or press F11).

LISTING 3 - !ReadADC.spin

OBJ
system : “Propeller Board of Education”
pst : “Parallax Serial Terminal Plus”
adc : “PropBOE ADC”
time : “Timing”

Important! The program in Listing 1 requires the use of several object libraries which add important functionality. These libraries — provided by Parallax — are included with the program download on the SERVO website. If any of these object libraries are missing, the program will not compile or run. Also take note of the indentation used in Spin programs. It’s important. The indenting is how Spin keeps track of program blocks, like if conditional tests and repeating loops.

VAR

The program is automatically compiled and uploaded to the PropBOE. Upon successful compilation, the program runs automatically. Assuming everything has been connected properly, the motors should turn in various directions as the robot goes through its motion routines.

Fully test the robot by first disconnecting the USB cable from the PropBOE and temporarily unplugging the nine volt battery. Place the robot on flat ground and plug the nine volt battery back in. The robot should go through its motion routines: forward, backward, turn right, turn left, and stop. If one or both motors turn in the wrong direction, remove power and flip the terminal wiring from the affected motor on the H-bridge.

The best surfaces for testing are tile, wood, or a kitchen table. Carpet is acceptable as long as it has a very low nap. If the bot appears to struggle as it’s moving along, relocate it to ride over a smoother surface.

long ad0
long ad1

PUB Go
system.Clock(80_000_000)

repeat
  ad0 := adc.In(0)
  ad1 := adc.In(1)
  Display
  time.Pause(50)

PUB Display
pst.Home
pst.Str(String(“ad0=”))
pst.Dec(ad0)
pst.Str(String(“, ad1=”))
pst.Dec(ad1)
pst.ClearEnd

Using Light to Control Your Robot

Listing 2 shows how to control the Beginner Bot using a flashlight by shining the light into the photocell eyes. As with the previous program example, Listing 2 also makes use of object libraries. You need these files — which are listed at the top of the program — in order for the project to compile.

The program tells the microcontroller to read the value from both photocells. A series of if conditional logic tests determine if there’s enough light to follow, and if so, in what direction the robot should travel.

Note the threshold value which is used to set the boundary between dark and light. I’ve set the light/dark threshold to 900 — out of a range of 0-1023 — as a starting point. Try higher or lower values to see what works best with your particular CdS cells.

When both cells receive light over the threshold, the robot drives forward. When only one cell receives light over the threshold, the robot turns in the direction of the light. If neither cell receives light over the threshold, the robot stops.