Appendix F: Balancing Photoresistors · Page 321
Appendix F: Balancing Photoresistors
In this appendix, you will test the photoresistors to find out if they respond similarly to
the same incident light levels. If the measurements they report are different for the same
incident light levels, you can modify your programs to scale the values reported by your
photoresistors. The values will then be similar for similar incident light levels, which can
help the Boe-Bot recognize different incident light levels more reliably. This technique
can in turn assist the Boe-Bot in exiting dark rooms, even with mismatched photoresistor
circuits.
RC circuits with photoresistors can report different values for the same light level for
many reasons: The stated value of the capacitors is 0.01 µF, but the actual value of
capacitors can be very different. Many common ceramic capacitors are rated with a
tolerance of +80/-20%, meaning that the actual value of the capacitor could be up to 80%
larger or 20% smaller than 0.01 µF. This means that your measured decay time could also
be between 80% larger and 20% smaller. The photoresistors themselves can also behave
differently if they come from different manufacturing batches or if they have smudged or
chipped light collecting surfaces.
Testing for Well Matched Photoresistor Circuits
This next example program displays the decay time of both photoresistors in the Debug
Terminal. It makes it easy to judge the differences between the two readings for similar
light levels.
For best results, eliminate sources of direct sunlight: In general, uniform lighting
conditions improve the Boe-Bot’s performance with photoresistors. Draw the blinds to
eliminate sources of direct sunlight. Rooms with distributed light sources such as
fluorescent lights or ceiling lamps work well.
Example Program: TestPhotoresistors.bs2
√ Enter, save, and run TestPhotoresistors.bs2.
√ Cast a shadow over the Boe-Bot’s photoresistors with a white sheet of paper.
Find a level of shade that gives you readings between 20 and 100.
√ Record the values of both time measurements in the first row of Table F-1.
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