Simple PhotoCell Timing System
(S.P.T.S.)
CHANGING PCELL RESISTANCES
(c) Bryan Brown & Art McEwen 1997-1999
All rights reserved, not to be copied without permission.
More often than not, PhotoCells that you purchase will have a sufficient range of resistances between "Bright" and "Dark" to be compatible with the LPT port in your computer. This is most often true with
older computers, where you simply wire up the PhotoCells to the DB25 connector and you are away to the races.
Occaisionally, the internal resistance of the computer's port will be extremely high or it will be outside the working range of the resistances for the PhotoCells that you purchased. Not to worry.
THERE IS A SIMPLE SOLUTION!!
You can change the resistances of your PhotoCells simply by adding a resistor to the circuit.
A Resistor in Series INCREASES Resistance
A Resistor in Parallel DECREASES Resistance
Click Here for sample test results showing the effect of adding a resistor, either in series or in parallel.
You must know the Trigger Point of your port in order to match the PCell to it and determine the appropriate resistor to bring the resistances within range. .
In order for PhotoCells to recognize moving slot cars....
"Bright" Resistance Must Be LESS Than Trigger Point
"Dark" Resistance Must Be GREATER Than Trigger Point
