Simple PhotoCell Timing System
(S.P.T.S.)
THEORY OF OPERATION
(c) Bryan Brown & Art McEwen 1997-1999
All rights reserved, not to be copied without permission.
A simple CDS PhotoCell available at most electronic
supply stores for less than $1.00 US
A PhotoCell is really a "VARIABLE RESISTOR" driven by light intensity. In high school, we all learned about Ohm’s Law, namely V=IR or Voltage Equals Current Times Resistance. Accordingly, if we change the resistance it affects the voltage. Since light intensity affects the resistance in a PhotoCell, then the voltage in the circuit is also affected. Your computer measures voltage changes in the numerous circuits contained in the parallel port (eg. LPT1) and these voltage changes affect the bits in the data byte address for the port (i.e. "0"s or "1"s).
With PhotoCells, the brighter the light source, the less the resistance (typically 6k -18k ohms). Similarly, the darker the light source, the more the resistance (typically 1M ohms).
Once the PhotoCells are installed in the track, with an adequate light source above, your computer will detect a certain voltage in the circuit according to the resistance in the PhotoCell. When a slot car passes over the PhotoCell, it creates a brief shadow of darkness which increases the resistance and alters the voltage. This momentary change is detected by your computer, the data bits are set and the Scuderia McEwen program will detect that change and initiate scoring and timing logic. Simple!!
There are no other complicated electronic components. No IC’s, diodes, capacitors, transistors, rectifiers, voltage regulators, external power sources etc. The true beauty of this technology is that it is simple, works with the natural light spectrum (so you can see what is going on) and it can be tested easily with a simple Multimeter to which most "do-it-yourselfers" have access.
Occaisionally, we encounter a computer port that has an extremely high internal resistance or PhotoCells with resistances that too low to work with the computer port resistances (ie "Trigger Point"). In either case, a simple resistor is added, either in series or in parallel, to the circuit to bring the PCell within the working range of the computer port/pin.
The voltage required in the PhotoCell circuits is supplied by the computer. There is no need for additional or external power (a great feature of S.P.T.S.).
Do NOT feed any additional voltage into the LPT port or you may damage your computer.
Last Updated July 21/2001
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