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Subject: Simple photo-cell detector

Date: 07/29/02 at 11:13 PM
Posted by: M. G. Rolls
E-mail: martin.rolls@willims.com
Message Posted:

This one should be pretty easy for the caliber of this forum.

Last year I attempted to repair the finish line sensors and light circuits on the Pinewood Derby racetrack used by my son's Cub Scout pack. I decided to get rid of the micro switch & relay arrangement and up-grade to an optical type of sensor to detect the cars as they pass.

I purchased a couple of inexpensive 7watt, automatic night-lights. You know the ones with the small photocells in them. These were cheap, easy to get and simple to use because they did not required a dc power supply.

I mounted the cells in the “floor” of the track, one in each lane, so that the cars would pass directly over them. Each cell is approximately ¾” below the track surface and at the end of a ¼” ID tube. The intent of the tube mounting is to eliminate, or at least minimize the effect of light sources not directly above the cell. The finish line has a bridge aver it that has two 7watt incandesant lamps in it, one directly over and approximately 7” above each cell. The cells are wired to trip it’s own DPDT, 120vac coil, relay. Each relay turns on a GREEN light to indicate WIN on it’s lane, a RED light to indicate LOSE on the opponent’s lane and to ‘lock-out’ the other relay.

I replaced a resistor on the circuit board the cells were on with a variable one on to allow for some sensitivity adjustment. I got it all wired-up and it worked great!! I was even able to adjust each cell to trip at the same degree of coverage by a car. I was soooo… proud of my handy work.

Much to my horror when the first pair of cars came zooming down the track, one car was a good three inches in the lead and the lights registered it the LOSER. After some quick testing we discovered that the cars were simply passing over one of the cells too fast for it to register.

Can you recommend a photocell/circuit (and where to get it) that will operate on 120vac, provide accurate determination of the winning car, be easy to get my hands on and yes inexpensive. I can easily build the circuit on a small board if I just get some good quality components that will provide consistent performance. And of course some good advise.

Any help will be greatly appreciated.


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