Simple PhotoCell Timing System
(S.P.T.S.)
"TRIGGER POINTS"

What is a "Trigger Point"?

The "Trigger Point" is the amount of resistance required to turn the pin status from "DARK" to consistently "BRIGHT" (ie not flickering) when resistance is applied between the Data Pin (eg. Pin 10) and a Ground Pin (eg. Pin 18) on the LPT port.

You can see the pin status in CHITI.EXE, our Installation Helper program, which you can download from this site. As the resistance approachs the "Trigger Point", you will see the signal flickering between "BRIGHT" and "DARK" in CHITI.

The "Trigger Point" varies from machine to machine and from pin to pin within the same machine (refer to the "Trigger Points" for my computers below).

It is important to match the resistance of the PhotoCell to the resistance of the "Trigger Point". A PhotoCell that works fine on one pin may not work on another pin.

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

Some PhotoCell/Pin combinations will not work right away because they do not meet these rules.
For example... PCell#1 (with Bright Resistance = 2,800 ohms) will not work with Pin 10 on "Lucifer" (with Trigger Point = 640 ohms) as the Bright Resistance is NOT less than the Trigger Point so it can never get Bright enough to turn the data bit to "BRIGHT"
For example... PCell#2 (with Dark Resistance = 60,000 ohms) will not work with Pin 13 on "Lucifer" (with Trigger Point =315,000 ohms) as the Dark Resistance is NOT greater than the Trigger Point so it can never get Dark enough to turn the data bit to "DARK".

Remember, a PhotoCell is a variable resistor which creates resistance based upon the amount of light hitting its surface. Changes in light result in changes in resistance (until the "Trigger Point" is reached) which results in a change to the data bit which results in the miracle of SPTS computer timing!!

The range of resistance between Bright and Dark conditions of the PhotoCell must cover the "Trigger Point" of the Pin/Port in order for the computer to recognize the signal.

In most cases, particularly with older machines, this is NOT a problem. However, newer machines tend to have some extreme internal resistances and you may find that none of your PCells will work. You can changes the resistances of your PhotoCells by adding a resistor. Changing PCell Resistances

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

How To Measure the "Trigger Point" of a Pin/Port

CAUTION - do not touch bare wires from the LPT port together without some resistance in the circuit (such as a PhotoCell) or it may blow out your I/O board circuitry.

If you do not have an assortment of resistors in your parts bin, you may have to buy "Value Pack" of assorted resistors from your local electronics shop. This should not cost more than a couple of dollars for a package of 50 to 100 resistors of varying sizes from 10 to 1 million ohms.

Connect two pins/wires from the data socket and ground socket that you want to measure on the DB25 connector at the back of your computer (either Pins 10&18, or Pins 12&22, or Pins 13&24 or Pins 15 &20). Be careful that these wires don't touch each other without some resistance between them.
Run the Installation Helper program CHITI.EXE and monitor the status of the data pin you are measuring. When you begin this process and before you apply any resistance, the status of your pin should be "DARK". If it is not "Dark", then you cannot use this pin combination.
Starting with a very high resistance resistor (eg. 1 M ohms) apply the resistance across the two wires. This will probably have no effect as it is likely a much higher resistance than the "Trigger Point".
Gradually reduce the amount of resistance until the pin changes status, from "DARK" to "BRIGHT" in CHITI.
Flickering indicates that you are getting close to the "Trigger Point", you need to reduce the resistance slightly until the pin stays solidly "BRIGHT"
Once you achieve "Bright" status, reverse direction and gradually increase the resistance slightly to see if you can get closer to the "Trigger Point". The closer you get to the actual "Trigger Point", the more sensitive your PCells will be. This is most important for HO scale users as the cars are very small and very fast. The final number is not absolutely critical, as most ports are fairly tolerant.
Write down the final number and don't lose it. You will need it to match against your PhotoCells and, if you need help, I may ask you for this number!

"Trigger Points" for My Computers

I am amazed at the lack of standardization regarding the internal resistances in the parallel ports between different machines and between pins on the same machine! Check these numbers. It's no wonder some machines took some work to get SPTS working properly.

The resistances in the 8088 and the 486/40 are fairly consistent and they were the easiest machines to get up an running. It was simply a matter of plugging in the PCells , adjusting the light source and we were away to the races. But look at the resistances in "Lucifer", my Pentium 90. One pin triggers at 640 ohms and one pin triggers at 319,000 ohms. These are the two extremes, on the same machine!!! Pin 10 required the PCell with the lowest Bright Resistance in my collection and the other required resistors totaling 300,000 ohms. Compare your "Trigger Points" to these.

YMR Track Machine (486-20 mHz) "Kev's Baby"

Pin 10	12,890 ohms
Pin 12	13,420 ohms
Pin 13	13,420 ohms

Original Track Machine (8088-4 mHz) "Old Reliable"

Pin 10	10,000 ohms
Pin 12	8,000 ohms
Pin 13	7,000 ohms

Development Machine (Pentium 90 mHz) "Lucifer"

Pin 10	640 ohms
Pin 12	215,000 ohms
Pin 13	319,000 ohms

Backup Machine (486 - 80 mHz) "New Toy"

Pin 10	1,847 ohms
Pin 12	32,900 ohms
Pin 13	34,400 ohms

HELP...I Can't Match the PCell Resistances to the Trigger Point!!!!

Most installations do not even require an understanding of "Trigger Points". Often the PCells be wired up and SPTS will be operational. However, some computers with extreme Trigger Points and/or the PhotoCell resistances just don't match the Trigger Points.

No problem...you can adjust the resistances of the PCells by simply adding a resistor in series (to increase resistances) or in parallel (to decrease resistances). Go to Changing PCell Resistances for examples. All it takes is some playing around with simple resistors and your multimeter. Its easy and we're always here to help.

Sensitivity Of PhotoCells to "Trigger Points"

The closer the Bright Resistance of the PCell is to the "Trigger Point" of the Pin/Port, then the more sensitive that PCell will be for detecting a fast moving slot car (particularly important with HO scale cars).

Greater sensitivity is achieved through the shorter response time required to reach the "Trigger Point" thereby reducing the period of darkness required to generate the necessary resistance. If your HO cars are sometimes too fast for the sensors, you can remedy this with the appropriate resistor in series with your PCell (to increase sensitivity).