Pack 471 timer with limited sensor range

DIY timing systems
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Don Kern
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Pack 471 timer with limited sensor range

Post by Don Kern » Wed Mar 29, 2006 9:14 pm

I built the four lane Pinewood Derby Finish Line Detector based on Pack’s 471 plans as modified in Randy’s GrandPrix Race Central. However, when I tried to use IR LED's & IR photodiodes (Radio Shack part # 276-142 IR LED/Detector pair) if they got to be more than 2" apart the transistors did not go into saturation anymore. The 5 Vdc power supply puts out 4.97 Vdc and the value of the ballast resistors (R1>R4) is 47 Ω with a measured 4.7mA and drop to 1.29 - 1.31 Vdc going into the IR LEDs. The max separation test was done before mounting the LEDs below the track. The track our Pack is using is aluminum that mounts the IR Photo LEDs approximately 1 ¼ inches below the track surface. I built a wood ‘finish line bridge’ that has a 4 3/8 inch clearance above the track with D1>D4 recessed about 5/8 inch in the wood, thus having separation between IR transmitters (D1>D4) and IR receivers (T1>T4) of approximately 6 1/4 inches. The system will not work as built. I thought in building that I may have overdriven the LEDs at some point. Thus I tried replacing one with a fresh LED (276-143), same results.

Any comments on why such a short range on the IR LEDs?



I am replacing an old “LaneBrain” that uses tubular incandescent bulbs as the light source and phototransitors (276-145) as receivers. If I can not get the IR system to work I am considering buying some cheap mini-flashlights with 1.5v bulbs and mounting them in the “finish line” bridge. This has a couple of advantages, the system would stay compatible with the “Lane Brain”, these flashlights have an adjustable beam and the bulbs are readily replaceable and available. Any comments on this would be appreciated.



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gpraceman
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Re: Pack 471 timer with limited sensor range

Post by gpraceman » Wed Mar 29, 2006 9:58 pm

You may have the sensors in backwards. I did find when building that circuit that you could get it to work that way with the IR LED's very close to the sensors. Try reversing one of the sensors and see if that helps in getting better separation.


Randy Lisano
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Awana Grand Prix and Pinewood Derby racing - Where a child, an adult and a small block of wood combine for a lot of fun and memories.

Don Kern
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Re: Pack 471 timer with limited sensor range

Post by Don Kern » Tue Apr 11, 2006 8:22 pm

Randy

Thanks for the reply. The problem was exactly as you suspected, I had wired the sensors in backwards. I am suprised that the system worked at all.



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