Copyright © Parallax Inc.
Parallax Laser Range Finder (#28044)
v2.0 10/31/2015 Page 17 of 23
Synth.spin is a frequency synthesizer used to generate the clock signal required by the OVM7690.
Included with the Parallax Propeller Tool.
JDCogSerial_Lite.spin provides full-duplex serial communication. Written by Carl Jacobs.
F32_Lite.spin provides IEEE 754-compliant 32-bit floating point math routines used for range finding
and calibration. Written by Jonathan “lonesock” Dummer.
FloatString_Lite.spin and FloatMath_Lite.spin provide IEEE 754-compliant 32-bit floating point-to-
ASCII string conversion routines. Included with the Parallax Propeller Tool.
Basic_I2C_Driver_Lite.spin provides the I2C protocol interface for boot EEPROM communication. The
LRF Module has a 64KB boot EEPROM. Only the first 32KB is used by the Propeller for program
storage, so the remaining 32KB is available for data storage. Each LRF Module’s calibration values
and blob detection parameters are stored in the data area. Written by Michael Green.
Optical Triangulation
The LRF Module uses optical triangulation for range finding, where the distance to the target object is
calculated with simple trigonometry between the center points of laser light, camera, and object. The
design of the LRF Module is based, in theory, on the implementation of Todd Danko’s Webcam Based DIY
Laser Rangefinder (https://sites.google.com/site/todddanko/home/webcam_laser_ranger).
Referring to the figure above, a laser diode module shines a laser spot onto the target object. The value
h is a fixed, known distance between the center points of the laser diode and the camera (78 mm for the
LRF Module). When the distance to the target object D changes, so do both the angle θ and the value pfc
(pixels from center), which is the number of pixels the centroid of the primary blob (laser spot) is away
from the camera’s center point.
