Simple PhotoCell Timing System
(S.P.T.S.)
WIRING

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


Wiring the DB25 plug (Male) which goes into any LPT port of your computer, to the two, three or four PhotoCells is not difficult. You only need two wires for each PhotoCell. The Female end of the DB25 connector is mounted on the back of your computer and requires no attention.

The best wire to use is "shielded cable" as it eliminates the possibility of picking up interference which can cause phantom signals or what we refer to as "errant hits". Most installations do not need this, however, and you can use almost any wire for the job. Standard four strand telephone wire works well for two lane tracks and you can get it in six strand for three lane tracks. For more on "shielded cable", Click Here.


REMEMBER: The Pin numbering is mirror image of the Socket numbering.

 

Summary of Four Lane Wiring.

Lane

Data

Ground

Lane 1

Pin10

Pin18

Lane 2

Pin12

Pin22

Lane 3

Pin13

Pin24

Lane 4

Pin15

Pin20



For One, Two or Three Lane Tracks, you can now use any of the Data Pin/Ground combinations in the table above. For example, if you have a Two Lane Track, you could use Pins 12/22 for your Lane 1 and Pins 15/20 for your Lane 2. It is not longer mandatory to use Pins 10/18 for your first lane (unless, of course, you are wiring up all four lanes). We recommend that you measure the "Trigger Points" for all four data pins and then pick the two (for two lane tracks) or three (for three lane tracks) that are most evenly matched. Sometimes, Pin 10 is significantly different from the other pins. The pins that you use are identified in the Lane Description fields in the PARM.TXT file so that SPTS programs will recognize your wiring automatically.

To connect the wiring between the computer and the PhotoCells:

  1. For each lane, connect the first wire from the Data Pin (eg Pin10) on the DB25 male connector to one side of the PhotoCell selected for that lane.
  2. For each lane, connect the second wire from Ground Pin (eg. Pin 18) on the DB25 male connector to the other side of the PhotoCell for that lane.
  3. If you are using "shielded cable" you must also connect the bare ground wires inside each bundle to the outer casing of the DB25 connector.


Remember that the pins on the male connector are mirror image of the sockets on the female connector. Make sure the 10 pin goes in the 10 socket.

For more on DB25 connectors, Click Here.

Soldering to the PhotoCell requires a delicate approach as you do not want to destroy the PhotoCell. For some soldering tips, Click Here. If the thought of soldering tiny electrical components strikes fear in your heart (as it did with me before I discovered "the secret"), then you can always use a mechanical connection, namely alligator clips (as seen in the picture). The clip solders on the cable wire and then just snaps onto one side of the PhotoCell. It is a good idea to bend the end of the wire from the PhotoCell out at right angles to the cable wire so that the alligator clip has something firm to grasp on to. You could also use a small crimp joiner if you can locate them.

Always double check your wiring before plugging it into the computer. There should be two clear circuits (three for three lane tracks, four for four lane tracks) in the connector and no short circuits. This could cause damage to your computer if there are shorts. Also, ensure that a PhotoCell is connected into each circuit, do not let the data wires (Pins10,12,13 or 15) touch any ground without having a resistor in place (i.e. the PhotoCell) as it will send a significant current (5 volts) through the port and may cause it to burn out (yikes!).

Use your Multimeter to test the circuits.

Before you plug in the DB25 connector to your computer, put the light source that you plan to use in place and test the resistance of the two PhotoCell circuits. You should have a minimum of 500 ohms resistance in each circuit with the light source on at it’s brightest intensity. This will protect the I/O board circuitry.

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.

Last Updated July21/2001

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