Simple PhotoCell Timing System
(S.P.T.S.)
ALL ABOUT PHOTOCELLS

(c) Bryan Brown & Art McEwen 1997-1999
All rights reserved, not to be copied without permission.

CDS (Cadmium Sulfide) PhotoCells can be purchased at any electronic supply store. They are common electronic components. In North America, Radio Shack carries them in their catalogue and in all of their stores. We started experimenting with a PhotoCell from a 120 Volt night light, which turns out to be a good source for PhotoCells if you do not have ready access to an electronics store (simply open up the night light and snip out the PCell). Night lights are available in most hardware stores and supermarkets.

Radio Shack sells them individually as part number 276-0196 or in packages of five as part number 276-1657. We refer to Radio Shack part numbers simply because most people have ready access to Radio Shack/Tandy retailers and/or their catalogues.

CDS PhotoCells come in a variety of different specifications. The PhotoCells in Radio Shack's five pack are all different an vary from pack to pack. The different specifications in our packs all worked to some extent in the S.P.T.S. application but some worked better than others. We did find that one of the PhotoCells in the five pack did not work effectively with our light sources, so if you have difficulty with one, just buy another and experiment. PhotoCells are inexpensive, they cost less than $1.00 US each.

The LPT port on every computer is different. Even the different pins on the same port have different resistance levels. It is important to match the PhotoCell to the Port/Pin. With most Ports, just about any PCell will work straight away. However, with some of the newer computers and ports, where the resistance levels are extreme, some PCells and Port combinations will not work together.
Your first step should be to identify and measure each PCell that you purchase. Since all PCells seem to look alike, an easy way to identify them is to solder different colour combination wire leads on to the two terminals of each PCell. This saves a lot of confusion down the road.



Measure and record the resistance of each PhotoCell
with a MultiMeter before installation and/or connection to computer.



You can test the resistance of each PhotoCell with your Multimeter. Set the Multimeter to the resistance scale and connect a lead to each side of the PhotoCell. You should immediately get a reading from the ambient light in the room in which you are working. If you cover the surface of the PhotoCell with your finger (or some black tape) you will get the "Dark" reading. This should be about 500K ohms. Remove the black tape and you will get an ambient light reading, which will be somewhere around 5-50K ohms, depending on the brightness in the room (sunlight etc.). If you take a table lamp and move it in towards the PhotoCell until it is within about 3-4 inches (10 cms) you will notice the resistance drop dramatically on the Multimeter. This "Bright" reading will likely be in the 1K-3K range. By moving the light source towards and away from the PhotoCell, you will see immediately the effect on resistance. If you set the light at a fixed distance and wave your hand between the light and the PhotoCell, you will see the needle on the Multimeter dip dramatically, this is a simulation of your slot car passing over the PhotoCell on the track.

A PhotoCell is a "VARIABLE RESISTOR" whose resistance changes with the amount of light that hits it's surface.
More Light = Less Resistance
Less Light = More Resistance

Since lighting conditions are highly variable, I found that I was getting different readings each time I measured the PhotoCells. To remedy this, I developed my own "standard" for measuring resistances of the PhotoCells, thus replicating the conditions that you might expect on a slot car track. I do all my testing on my hobby desk, not at the track itself. For measuring the "Bright" resistances, I locate the PCell 12 inches away from (and aimed directly at) a 60 watt table lamp. For measuring the "Dark" resistances, I use the same light source but cover the PCell surface with a flat piece of cardboard. This allows some ambient light to hit the PCell just as it would when located in the track and with a car overhead. Covering the PCell with you finger or a piece of black tape gives a reading that is too Dark and therefore unrealistic for your application.

Here is an inventory of my PCell collection showing the significant variations of resistances.


My PCell Inventory (Resistances in Ohms)
No.Identification"Dark""Bright"
1Red(Small) 70,0002,800
2Yellow(Small) 60,0003,000
3Green 20,0001,300
4destroyed n/a n/a
5Red (Large) 9,000 700
6Yellow(Large) 9,000 500
7Orange 2,000 280
8Purple 5,000 370
9Blue/Gray 5,000 430
10Brown 5,000 430
11Night Light 20,0001,400
12Red/Black 3,000 450
13Red/Gray 6,000 500
14White/Blue 5,000 420
15White/Gray 1,700 270
16White 3,500 330

Measuring resistances in PCells is not an exact science. There are a lot of variables and you wont always get the same readings.

Once installed in your track, the working resistances will vary slightly since the installed PCell will be receiving a different amount of light than you gave it during this measurement exercise, both during the "Bright" and "Dark" conditions. Not to worry though, most computer parallel ports are quite tolerant of these variations in resistance. Also, it is a simple matter to modify the resistance levels of the PCells with the simple addition of a resistor. (Changing PCell Resistances)

Click Here for an Illustration of the relationship between Trigger Points and PCell Resistances.

Last Updated July 21/2001

Return to SPTS Instructions