Making a Light Bridge



• Making a light bridge for your photo sensors mounted in the track is quite simple and should cost very little in addition to the cost of the bridge itself.

• Several bridges can be found commercially for slot tracks. I only have two of them (see below) but I am sure the following could work for any other purchased bridge or even home made ones. The only thing to think about is how the LEDs, resistors, and wires will be hidden.

• Good Year Bridge: available from Scalextric and others. One can also find easily a white Plastic pedestrian bridge (I do not know who makes that one).

image002

image004


The next step is to decide which color LED to use. Here is a sample of LEDs that are all bright enough to work with most photocells, all are about $1.00 or less.

o Infrared LED: www.digikey.com (infrared light is invisible to the eye)
• Part number LNA2801L-ND (3 mm LED)

o White LED: www.superbrightleds.com
• Part number RL5-W10015 (5 mm diameter LED)
• Part number RL3-W3030 (3 mm diameter LED)

o Blue LED: www.superbrightleds.com
• Part number RL5-B5515 (5 mm diameter LED)
• Part number RL3-B2030 (3 mm diameter LED). Note that this one might be borderline for some lower sensitivity light sensors

o Red LED: www.superbrightleds.com
• Part number RL5-R5015 (5 mm diameter LED)
• Part number RL3-R5014 (3 mm diameter LED)

• To wire an LEDs is very simple:

image006

Which resistor value to pick depends on which DC power supply is available for powering the light bridge. The following table will give you the value required for the resistor (in Ohms) for typical power supplies:

image008

• In case you do not have a power supply of either 5,9, or 12V, use he following formula to calculate which resistor value to pick: Resistor = 50 x (Vsup - Vdiode) where Vsup is your DC power supply voltage, Vdiod is the diode forward voltage as given in the table above, and the calculated resistor value will be in Ohms. You can also check on this link: http://led.linear1.org/1led.wiz (thanks to Dionisio15 from HRW for the link), and use the calculator there.

• Once you have found the resistor value required, find at your electronics store (like Radio Shack) or any on-line supplier (like DigiKey) the nearest standard value which is just greater than the one you calculated or got from the table. For resistor standard value table see http://www.elexp.com/t_rescht.htm.

• If you do not feel like wiring your own LED, you can always check the SlotTronics web site where you will find pre-wired IR LEDs for your voltage supply with your choice of wire length.

• To assemble your LEDs in the Goodyear Bridge:


o Take a piece of styrene 1/16 thick or any other plastic and cut it at the dimensions of the underside of the bridge.
o Drill two holes (3 or 5 mm, depending on which LED you picked) in that plastic sheet to match the location of your sensors in the track.
o Paint that plastic piece a light grey to match the bridge color
o Mount the LEDs in these holes.
o Bend the leads 90 degrees (with small thin pliers) and solder the resistors (one for each LED) and the wires to connect to your power supply.
o Mount the plastic sheet (with the LEDs, resistors, and wires) at the bottom of the bridge.
o Verify the light out of the LEDs is aligned and centered on your sensors and verify that they trigger when you interrupt the light beam. This can be a little tricky with the Infrared LEDs, but if your plastic sheet is parallel to the track and the LEDs are mounted flush all the way to their flanges in the holes it should work the first time.
o Run the wires on the underside of the bridge into a hole in your table and run the wires to your power supply.
o Now glue everything in place, including the LEDs in their hole (using any plastic glue in moderation)

• To assemble your LEDs in the white pedestrian bridge (shown above)


o Take a piece of styrene 1/16 thick or any other plastic and cut it at the dimensions (length) of the overpass of the bridge. + 2 small 1/8 thick pieces cut at the side dimensions (width) of the overpass of the bridge
o Drill two holes (3 or 5 mm, depending on which LED you picked) in the top of the bridge to match the location of your sensors in the track + one hole towards the stairs on either the side.
o Paint all these white (or other color if you prefer)
o Mount the LEDs in the holes.
o Bend the leads 90 degrees (with small thin pliers) and solder the resistors (one for each LED) and the wires to connect to your power supply.
o Verify the light out of the LEDs is aligned and centered on your sensors and verify that they trigger when you interrupt the light beam. This can be a little tricky with the Infrared LEDs, but if your plastic sheet is parallel to the track and the LEDs are mounted flush all the way to their flanges in the holes it should work the first time.
o Run the wires to the side of the bridge into the third hole and along the side of the bridge into your table and run the wires to your power supply.
o The small 1/8 thickpieces go at each end of the bridge and the large 1/16 pice goes right on top. That should leave plenty of scape underneat for bending the LED leads, with the resistors and the wires. Now glue everything in place, including the LEDs in their holes (using any plastic glue in moderation).

• That's it. Reading this will probably take more time than actually making it ☺

• For an easy tutorial on LEDs see
http://www.kpsec.freeuk.com/components/led.htm