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Why we don't use Photocells

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Photo beam Problem 1

A competitor (who uses photo beam detection) advertises a resolution of 0.00001 seconds and guarantees no ties. They completely fail to mention that the resolution is that of their timer not that of their sensor (nor of their display). While their sensor is probably comparably as fast, the drawing below shows the fallacy of the claim.

In the example we’ve used pointed partial cars and a small track segment for drawing simplicity. You can get around the pointed car problem by not having pointed cars but pointed (or rounded) cars are more aerodynamic. By the way the competitor also claims, “The smaller the sensor, the greater the accuracy. “ This is nonsense unless you are trying to detect a very small object. The output of any sensor (that is smaller than the width of the car) switches from ‘no car’ to ‘yes car’ at some fairly well defined point. By way of analogy, it doesn’t matter how big the faucet is, at some point water either comes out or it doesn’t.

Note that Car 1 (solid line) traveling in the center of the lane will break the beam almost 3/8" sooner than Car 2 (dashed line) traveling on the edge of the lane. Which one is the winner?

This to-scale drawing also points out an interesting track detail. Whether the track is center-guided or edge-guided, the car usually has about the same size lane to travel. It is not possible to say the car has the same size lane in all cases since there are at least two commonly published track designs that use different size center guide rails.

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Photo beam Problem 2.

Virtually all systems (NOT SUPERTIMER II) use Infrared (IR) Beams. This is done partly to restrict interfering light and partly because IR transmitters and receivers are commonly and inexpensively available. But the warmth you feel from sunlight is from the IR part of the spectrum. IR is everywhere. Both flash bulbs and video lights have high IR energy especially when compared to the relatively weak IR photo beam. So the sensor is usually placed in a tunnel and an optical filter is added to help the problem. But a tunnel and filter only reduce the potential interference, they don’t eliminate it. By way of analogy, you usually can’t hear a car on the street from inside your house, but you can if the car backfires or if it’s a truck. The house filters the sound but it doesn’t (can’t) eliminate it.

In the drawing below, a flash camera or video light hits the sensor while the car is crossing. Note that the car has broken the ‘normal’ beam but that the sensor still ‘sees’ the extra very bright light. Will you get an answer? Of course. Is it correct?

By the way, the fairly obvious solution of putting the beam below the track and the sensor above helps but doesn't cure the problem since IR light bounces fairly well.