I am building a device to find your running speed. When you pass through the first laser trip wire the timer starts and it ends when you pass through the second one. I found some cheap 5mw lasers off of amazon. Should I use a photoresistor, phototransistor, or photodiode for the sensor? I have no idea what would be best. The project would be inside and outside.
Photoresistors are too slow, and lasers are hard to keep aligned. 5mW is also too dangerous.
Unmodulated laser/photodiode setup also can have problems with indoor lighting and bright sunlight. Tripwires are usually made with modulated IR and 3-pin receivers, not with lasers.
This has been done before.
Try the search field on top of this page, or Google "tripwire wawa site:arduino.cc"
Leo..
Would Ir sensors work and stay cheap as it has to cover at least 3 feet? And is 5mw dangerous just because of the risk that a light would shine in someones eye?
If you had Googled that string than you would have seen that bridging >50m (>150ft) with a common IR diode found in remote controls is not that hard.
A 5mW laser (>1mW)) is considered a weapon in Australia.
Leo..
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I once developed an optical proximity switch that used a standard IR LED and phototransistor. Spec was to operate at 1 metre with a white A4 page target in bright sunlight or artificial light. Met the spec and the unit would operate at 100m if you used a retro reflective target. Great advantage would be that all your sensing can be done from the same point.
I have used these.
with great success.
Another detector would be RFID. When the user passes through the first RFID reader the timer starts. When the user passes through the RFID reader again the timer stops.
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