Photo diodes vs Photo transistors

DIY timing systems
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terryep
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Photo diodes vs Photo transistors

Post by terryep »

Electronic finish lines usually use photo transistors "PTs" to detect when a car breaks an infrared beam. PTs are simple to use and inexpensive but can be easily saturated (heavily switched on) with stray IR even though a window. Once saturated, a car becomes partially or completely invisible! This is especially worrisome with cars that have reflective coatings and or pointed or lifted front ends. When using a finish line in the daytime you should check it by swinging a pencil through each beam quickly. This will make sure the sensors are not saturated.

Some finish lines are specially built for outdoor use. They use photo diodes "PDs". PDs generate an electric current proportional to the ambient IR plus the signal, they don't saturate. By using a pulsed signal and filtering at the pulse frequency the signal can be extracted minus the ambient IR. Cost is higher since you also need a trans impedance amp and filters etc. The cool part is you don't need to build all this circuitry yourself you can buy it in one device. Panasonic remote control receivers have all you need built into a 3 pin device, see block diagram, and costing only $1.47.
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Below is a schematic for a 2 or 3 lane finish line. The 12F629 is a Microchip flash micro $1.75ea. The IR remote receiver is a PNA4701M $1.47. It has a photo diode front end, trans impedance amp with AGC and a filter/decoder tuned to 46.7kHz. The 12F629 pulses the IR LEDs at 46.7kHz and de bounces the PNA4701M outputs. The 12F629 inputs switch to outputs to flash the winning lane LED when a car is detected. With these inexpensive components there is no need to use PTs and have to worry about using your finish line in the daytime. The micro needs to be programmed, I'm happy to supply a listing of the code to anyone who wants to see it.
Terry
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Last edited by terryep on Tue Mar 29, 2005 10:54 am, edited 1 time in total.
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kcobb
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Re: Photo diodes vs Photo transistors

Post by kcobb »

Terryep,

This is just totally awesome stuff! Thanks very much. I would be interested in the code. I've got a few interns at work that will just jump into this!

Thanks!
Kevin
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terryep
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Re: Photo diodes vs Photo transistors

Post by terryep »

Thanks Kevin, I'll e-mail you the listing tomorrow. For those who use PC computer parallel ports you can also use these devices and you can use a 555 timer to pulse the IR LEDs. Also, you will need the 5V version of IR remote, PNA4602M 38.0kHz. Remember that these devices are sensitive so don't allow the beams to cross! :roll: Joke, don't allow the beams to bounce around. Paint the area black so pulses don't reflect up/down and back up into the sensor.
Terry
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Re: Photo diodes vs Photo transistors

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As I said before these devices are sensitive. :shock: In a remote application they are rated at 7 meters (22 feet). If you allow the IR to bounce off the bridge (structure over the track) back down to the track there will be enough bouncing back up into the sensor to fail the pencil test. You can blacken the underside of the bridge and you can blacken the track under the bridge but you can't blacken all the cars!

I tried black electrical tape under the bridge to soak up some IR but there was still too much bouncing around (18mA IR LED drive). I then set out to find the best IR absorbing material. I used an IR filtered photo diode connected to a micro-Ammeter and a quartz halogen desk lamp to measure the IR reflecting off of several black materials. I found that electrical tape and flat black paint were not very good at all. Black felt was a little better than electrical tape. Some black foam materials were good to excellent. I found a black self adhesive rubber tape that was good (5:1 better the electrical tape).

Back to the finish line with rubber tape. I did my testing with white paper over the track to provide a "worst case" IR bounce. If I could pass the pencil test with white paper, any car should be ok. Still not good enough. A fast pencil one inch over the track sometimes could get through undetected. Obviously I needed to reduce the range! Remember 7 meters? Estimating 5 meters at 50mA drive I settled on 1.8mA for a potential sensing distance of 1 to 2 meters.

Back outside to make sure it works in the sun. White paper reflecting sunlight into the sensors and also directly at the sensor holes. Still works! 8)
Terry
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