1 Chapter 6: Light Sensitive Navigation With Photoresistors Presentation based on: "Robotics with the Boe-Bot" By Andy Lindsay Parallax, Inc Presentation developed by: Martin A. Hebel Southern Illinois University Carbondale College of Applied Sciences and Arts Electronic Systems Technologies 7/6/04

2 Photoresistor A light sensitive resistor The more light falling on it, the lower the resistance will be.

3 Digital Photoresistor Input A voltage divider can be used as a simple ON-OFF sensor. As the sensor darkens, the voltage at P6 will decrease. Voltage < 1.4  State is 0 (low) Voltage >1.4V  State is 1 (high)

4 Vo = 5V x 2K/(2K+R)

5 "RoamingWithPhotoResistors.bs2"

6 Analog Photoresistor Input Capacitors are devices which can store electrons much like a tank of water (though the 'leak' much easier.) Units: Farads, typically  F

7 Circuit charges quickly through 220 ohm, discharge rate is based on resistance of the photoR.

8 RCTime RCTime – Measure how long it takes a pin to change from a '1' to a '0' (or, how long the cap takes to discharge. RCTime Pin, State, Variable to store duration Duration is number of 2uS increments HIGH 6 PAUSE 3 RCTIME 6,1,timeLeft