RC Car Detector
Two (or more) emitters to get more IR on the target or to get coverage over a wider area without having to aim at the receiver or if you don't know where the receiver might be.
At the risk of offending the electronic purists (again), this looks like a perfect application for some inexpensive lenses to increase the range of reliable detection. I don't know about the emission pattern of your emitter, but the spec sheet for your sensor shows a very broad reception angle - 60 degrees off-axis at 50% sensitivity. Since you have very well defined source and detector locations, you can easily greatly increase the range of reliable detection with a pair of lenses. Notice that companies like Banner and Honeywell use lenses on most of their emitter-detector pairs, even many of those intended for fairly short ranges. Plastic lenses tend to have better IR transmissibility than glass, but I'm not sure it makes much difference at 880 nm. A thin plastic Fresnel lens (from a bookstore or a kids science supply store) at the sensor can give you a greatly increased range.
One reason for using a beam interrupter, rather than looking for the reflection off the car, is that you have much better consistancy of detection in situations where the cars are non-cooperating (do not have retroreflectors mounted on them), if the cars can be at highly variable distance from the emitter-detector (a few cm. to more than a meter), and can be of variable IR reflectivity. Opposed emitter/sensors with lenses at opposite sides of the track will give you the best reliable detection range.
Another way to reduce interference from ambient light (the world is very bright from reflected sunlight at 880 nm.) is to modulate the emitter and have matching narrow band filtering at the sensor. That can also reduce heating of the emitter and allow higher peak emitter currents.
You might be interested in the Opto Diode Corp. OD-50L "Super High Power IR Emitter," with 50 mw, 880 nm. optical power output at 500 ma (much more if pulsed), 500 mw/steradian radiant intensity, 7 degree half-intensity beam angle, $11 each (
www.optodiode.com).
awright