Illuminating The Light Bridge
This article illustrates the workings of a typical infrared light bridge.
A required part of any lap counter using optical sensors is a source of light to illuminate the sensors and bias them ON so that they will go OFF when a car blocks the light as it crosses the counter. This can be as simple as a desk lamp positioned over the track to a sophisticated light bridge. Trakmate and Slottrack use infrared sensors so the light source must emit a sufficient amount of light at the correct frequency (wavelength). An incandescent bulb will typically emit enough IR to do the job. The disadvantage is that most of the light is wasted as it is not the correct wavelength to be picked up by the sensor. An incandescent light also emits quite a bit of heat and location can be a problem. A fluorescent source (4 foot tube or compact fluorescent) usually will not put out enough IR light to trigger the sensor. Some warm-white fluorescent lights will put out enough IR to get the job done. However, itís hit or miss if they work.
Infrared LEDs are an excellent source of light for the sensors. The advantage is that they put out sufficient light with little or no heat. The disadvantage is that you cannot typically see if the LEDs are working. Infrared LEDs can be obtained from Radio Shack to electronics catalogs to E-bay. My favorite source is E-bay using 850nm LED in the search field. Typical LED specifications to look for are as follows:
Emitted Color : Infrared
Color : Water Clear
Forward Voltage (V) : 1.5~1.6
View Angle: 15-30 degree.
Typical Infrared LED Specifications
The frequency of the light emitted by the LED is critical. The Trackmate and Slotrack sensors are sensitive to light having a wavelength of 850nm (Nanometers). Infrared LEDs are typically available in 850nm or 950nm. While the 950nm LEDS can be made to work typically they have to be run at or near their maximum rating to trigger the sensors. The 850nm LEDs can be run at or below their nominal rating with success. As an example I made a bridge using 950nm LEDs. The LEDs in this bridge had to be run at their maximum rating (100mA) to be successful. The bridge had to be located within a few inches of the sensors. With 850nm LEDs the bridge could operate at less than 15ma and a maximum distance in excess of a foot! LEDs have a life expectancy that is dependent on how they are operated. When operated at or below their nominal current ratings an LED can have a life in excess of 100,000 hours. When operated at their maximum ratings that life is significantly reduced. My first bridges were made using 950nm LEDs and had a life of 3-5 years. I expect the latest bridge using 850nm LEDs to have a life expectancy in excess of 15 years.
The following photo shows a bridge made for a 6-lane track in Wisconsin with two 850nm LEDs per lane being bench tested under power. Note the red glow coming from the water clear LEDs. This is typical of 805nm LEDs.
6-Lane Light Bridge Under Testing
While not as pretty as a an etched printed circuit board the above bridge is rugged and will get the job done. This bridge will be located beneath a second structure and, as such, esthetics was not a requirement. The bridge can be built into an aluminum channel and look rather elegant if desired. My home track bridge consists of a section of aluminum channel. After assembly and testing the channel was filed with flowable silicone and covered with black mylar. The result speaks for itself. As with the above bridge this bridge uses two LEDs per lane. The bridge is also under power and a red glow can be seen from the water clear LEDs.
CRRís 4-Lane Light Bridge
I prefer to build a light bridge using two LED strings with one LED in each string illuminating each lane. The LEDs are located directly above the sensor. Each string of LEDs is wired independently of the other and each string is designed to trigger the sensors independently. This design provides some redundancy as the bridge will still work if one LED string fails. The only shared component is the power supply. The following diagram shows how each string is wired.
Schematic of a Single String of CRRs 4-Lane Light Bridge
Each stringís dropping resistor is sized using a Microsoft Excel spreadsheet. This spreadsheet takes into account all important factors. To use the spreadsheet, fill in the yellow fields and the blue fields will automatically update to provide the correct resistor value and rating. Use a resistor with a minimum rating that exceeds the wattage calculated. The following example sizes the dropping resistor for the 6-lane bridge in the above photo.
Power Supply Voltage 17.8 VDC Dropping Resistor 410 OHMs
LED Voltage 1.6 VDC 0.163 Watts
Number of LEDs 6
LED Current 20 mA
Spreadsheet Results For The 6-Lane Light Bridge
The spreadsheet recommended a 410 Ohm resistor having a rating in excess of 0.164 Watts. The as built 6-lane bridge uses 480 Ohm, Ĺ Watt resistors as they are standard, readily available values. While the current may be less than the desired 20mA, performance is excellent and this bridge will trigger my trackís sensors at a distance in excess of one foot. Since the bridge will be located approximately two inches above the sensors this design will provide more than enough light!
When factoring in power supply voltage use a measured no-load voltage as opposed to the supplies nameplate voltage. Typically filtered DC power supplies have a no load voltage that is 150% of their nameplate rating. In the case of the 6-lane bridge the 12 VDC power supply (nameplate) used has a no-load output of approximately 18 VDC. Use a filtered or regulated power supply to eliminate pulses in the lights that can occur if an unfiltered power supply is used. Use a voltmeter to determine if the supply is filtered. If the voltage does not decay to zero for a few seconds when the supply is unplugged then the supply is filtered.
The 6-lane bridge in the above photo has a third string that consists of a single blue LED. This LED is a pilot light and will illuminate when the bridge is powered. Itís not necessary but was included as this bridge will be mailed to the site and installed by others. When viewed directly the 850nm LEDs emit a bit of visible red light under low ambient light conditions. This allows you to see if they are working! Unfortunately you have to remove the bridge and rotate it to look directly into the LEDs to perform this test so its not real practical once the bridge is installed. As seen in the above photos some digital cameras will display the light coming from an 850nm LED. The 950nm LEDs do not emit visible light and cannot be viewed in this way.
One last point, super glues and most silicones will fog the LED or sensor lens making them less sensitive or worse. If this happens the LED or sensor typically cannot be salvaged and must be replaced. I take great care when assembling a bridge or track to not contaminate the LEDs or sensor lenses with silicone or glue. A bit of care in design and installation goes a long way!
Hope this sheds some light inside the light bridge. Do not hesitate to ask if you have questions.
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