I set up a basic circuit with a battery, an LED and a photodiode. I shine the laser on the photodiode and the LED glows brightly. When I take the laser away the LED dims but slowly.
Speed shouldn't be a problem. Check the specs, and I'll bet the photodiode works at MHz rates. In fact you might have to slow it down, or take several readings in a row to make sure it doesn't get falsly triggered by a bird, or flying piece of dirt, etc. I think we need to see a schematic... You've got a laser and a photodiode, so what's the LED got to do with it? It probably will take some experimentation. You have to make sure the sensor is not turned-on by sunlight (or other light), but that it's always turned-on by the laser, and turned-off when the laser is blocked. At outdoor events, of course the lighting will change through the day.And in a tough environment like this, you might want to have a 2nd redundant system running in parallel, or at least one or two people with stopwatches.
Yes, LDRs are slow compared to photo diodes and transistors.As DVDdoug pointed out, how do you intend overcoming the possibility of false triggers or missing triggers due to natural light which contains both visible and infra-red light?You will need to place the photo detector in a tube painted black (pvc pipe is ok) so as to limit the detectors field of view.Experiment with the diameter and length of the tube.Simply using a laser (or other light source in the visible or infra-red spectrum) is not sufficient and you will have to modulate the laser with a carrier (similar to what is done with remote controls) then demodulate that carrier and use it's absence as your trigger.
.....can you maybe just explain "in simple terms" why the laser light on its own won't work? I don't understand what u mean when you say modulate with a carrier? Sorry I'm new to this so please bare with me.
In my experience, some days you can have a lot of people 'fall down' and miss a lap or two
I think you will need to do some math and work out 'average' bike length?Wouldn't you get multiple interupts from leading edge of front tyre, trailing edge, frame (including rider, etc), rear wheel; before you get next 'clean' signal when bike has past sensor?
What will discarding triggers based on a timing window going to solve?Optical interference from natural light is not limited to time, it has nothing to do with the "beam" being interrupted by a riders hand, foot, leg or any other part.