This electronic sensor system is best used with a computer tracking software such as Grand Prix Race Manager (GPRM) by Randy Lisano. My research found this to be the best software for the job and one of the best prices. It makes races run faster and more organized, this makes it more fun.

These plans incorperate the derby timer electronics and my own design for the sensor bridge and componant location.

I have previously built them into the base of the track itself and many people build them into the finish line over the track. I chose to build my electronics into a project box this time for maximum flexibility and ease of storage. The nice thing about this design of the circuit is it allows you to keep the Circuit box near the computer. Meaning the only long cable you might need is a very easy to find and relatively inexpensive one, a Cat 5 cable. It is peanuts compared to a long serial or parallel cable.

Solenoid Start Gate. I have made a schematic for it. Click here.

The layout is for a four lane system, but it can be easily scaled down to fewer lanes or increased for more.

Constructing the Lane Sensor Bridge

You can use a standard project box from Radio Shack to mount all main components in and remote mount the IR LED's and Photo transistors in a Sensor Bridge. Make sure the box is big enough the more room you have the easier it is to work on the circuit and the neater it will be. I have also made a box from Plexi glass, or you could even use a plastic container from the dollar store.

IR LED Mounting- You need to build a frame to suspend the IR LED's over the track lanes.
I prefer the aluminum bridge for it's low profile and strength and ease of use, but it can be made of wood.

Aluminium Bridge: Use 3/4 inch Channel Top and Bottom and 3/4 inch SquareTube for the uprights.
These pieces can be found at you local Home Depot or Lowe's etc. You will need LED mounts for this option and your holes will have to be the right size for the mounts you choose. you can also glue them in place with silicone or hot glue. Sections of aluminium are bolted together using 8-32 machine screws and nuts.
Pictures of the Aluminium Sensor "Bridge".

Measure the width of your track and where to centers of the lanes are. Cut one piece of channel to the same width as the track and cut two more peices 1 1/2" longer. Deside what hieght above the track you want the bridge then add 1/2" each for the top and bottom channels and add the thickness of your track as the bottom of the bridge runs under the lane surface and cut the uprights. Most organizations have a max hieght of 3" for their cars so 5-6" uprights allow for a little extra room.

Cut the ends of the lower channel to allow the uprights to fit into the sides. Drill holes for the bolts.
On one of the uprights you will need to round the corner to allow for it to rotate up so you can slide the bridge into position on the track. Also drill holes for the wires in the other upright.

(insert pictures)

Make sure to stack the aluminium channel used for the IR LED's and the Photo transistors together and then drill the holes through both pieces at once. This way the holes will line up properly, but care should be taken to drill the holes perpendicular to the material. Use a drill press or a drill with a built-in level.
Insert the LED's into the mounts (if you are using them) and snap into place in the channel. Use silicone or hot glue to secure them in place if disired.

Next attach the resistors and wires to the LED leads and use heat-shrink tube or tape to cover the connections to prevent shorting.. Connect the IR LED's with the short lead (cathode) wired to ground. Two Wires one for +5V, one for ground.

Photo transistor. Mounting - You can mount the photo transistors just as you did the IR LED's. Insert the photo transistors, attach the wires to the leads and add heat-shrink tube or electrical tape. Wire the photo transistors with the long lead (emitter) to ground (you should use one common ground for all four, five wires in all). Attach the CAT 5 cable jack to the upright of the bridge.

Now with the track, drill holes in each lane at the finish line, a bit larger than the holes in the "bridge" and then depending on your track design, attach guides to hold the sensor bridge in place or cut holes for the sensor bridge to slide through under the track.

Housing the Circuit Board

External Connections - Drill holes in the side panel, to mount connectors for the external components (power jack, serial port, lane sensors, start sensor) Connect the wires from the connectors to their corresponding locations on the circuit board. Also, if using display LEDs, drill holes for those and secure in place or remote mount them using a differant connector.

Mount the stand-offs to the back panel, so the circuit boards will be centered in the enclosure, and then mount the circuit boards to the stand-offs.

Attach the Front Panel - Use screws to attach cover, do not glue together incase you need to access the system if there is a problem in the future. In the case of the plexi glass you can also use silicone, as it is easily cut.

Start Sensor

Almost any switch will do. Originally I used a commercial momentary push button (found at any electrical supply, and some Home Depot's) mounted on a small 2 inch angle. I felt that because I was having the start gate land on the switch a small cheap switch just wouldn't do. Also it is only 5VDC so I did not enclose it.

I prefer a magnetic switch similar to one found in an alarm system, but has N.C. contacts that open when the magnet is brought close to it. Screw or use two sided tape to secure the switch in place with the magnet on the drop gate. Attach a wire to the switch with a standard 1/8" phone plug on the other end (or what ever connector you use) to plug into the system.

Any questions I might be able to answer, feel free to send a message to Dave

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