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    Resources for teaching with the Parallax Boe-Bot

Is the Parallax Boe-Bot faster than the speed of light?

Consider Chapter 07, Activity #1 on page 222 of “Robotics with the Boe-Bot, ver. 2.0” by Andy Lindsay (essence of the code listed below).

‘Robotics with the Boe-Bot - TestLeftIrPair.bs2
    irDetectLeft VAR Bit
    DO
       FREQOUT 8, 1, 38500     ' Send 1 mSec of light of freq 38.5 kHz out an IR emitter on pin 8
       irDetectLeft = IN9      ' Check an Ir sensor on pin 9. Ir present = 0, Ir absent = 1
       DEBUG…
       PAUSE 100
    LOOP

An Ir emitter on the front of the Boe-Bot broadcasts Infra Red (Ir) light forward. If the light strikes something it will bounce back and be detected by an Ir sensor. The Boe-Bot logic will then know there is an object ahead.

It takes ~ 0.25 mSec for the Boe-Bot to execute each command. The line irDetectLeft = IN9 does not do the sensing until 0.25 mSec after the FREQOUT 8,1,38500 pulse ends. But 0.25 mSec is a long time compared to the speed of light (3x10^8 meters / sec). During that 0.25 mSec the pulse of light has gone from the emitter to the wall, bounced off the wall, traveled back past the Boe-Bot and then another 10^4 meters ( ~ 6 miles). The pulse of light will be long gone behind the Boe-Bot by the time the BS2 executes the command to sense the pulse of light.

How can the sensor detect light that is long gone? Or, is the Boe-Bot sensor faster than the speed of light?

The answer is available, free, as an explanation illustrated by oscilloscope demonstrations. To get it, make a link to www.BoeBotTeacher.com from your .EDU web page. When the link is made, send a Feedback notice to get the answer file Emailed to you.