Hi,
I'm starting a timing system using arduino for drag racing. I've already made a prototype using leds and push buttons, but when for real I will need to use laser instead of push buttons (and also lamps instead of leds). My main problem is change the push buttons to lasers: I started trying LDR on analog ports, but reading is very slow. Any ideas?
Have you tried using a phototransistor on a digital port?
I don't know of any part numbers offhand, but googling for some of those "talk on a light beam"-type science projects should turn up some examples.
Ran
i use a laser from a laser pointer aiming at a photoresistor to detect the presence of an object blocking the beam. the photosensor is connected to an analog pin. when the value from the resistor exceeds a certain value i know that the beam is hitting the resistor directly. I would give that a shot and see how it works... you may need to play around with the resistor value threshhold to see what number will work. This will depend on the amount of ambient light... you don't want to set this limit to low or it will respond whenever someone turns on the lights...
good luck!
a timing system using arduino for drag racing
whenever someone turns on the lights.
Indoor drag racing that's is something I would like to see. ;D
If you put the photo detector at the far end of a tube then the laser can get down but the ambient light is not so much of an issue.
If you put the photo detector at the far end of a tube then the laser can get down but the ambient light is not so much of an issue.
Indeed... that's a good idea! Although there isn't much room to work around the area where my sensor is, i may give that a try. Thanks for the feedback!
The main problem is that I don't want to make analogReads, because they are very slower than digital reads. And my idea is to know if is there any kind of sensor that reacts only to laser light, else I would need to make a enclosure for protecting from the daylight.
So my main doubts again:
Ran mentioned phototransistors but no specs, any idea? Any help is very welcome. Thanks
is it possible diferentiate laser light from daylight
Well, it's coherent and polarised, but I would count on that distinguishing it sufficiently at reasonable cost.
Maybe you could modulate it.
There isn't much high frequency A/C daylight. 
Ok, I really not very concerned about identify if it's a laser or daylight (I can use the tube idea), but digital reading is crucial! Anyone?
Hi,
I have a part of a circuit I built that may help you. Unfortunately I can't seem to find any way to attach images to my reply 
Yeah for crucial timing applications you'll definitely want a phototransistor measured digitally.
Take a look at the attachinterrupt() library, it should work perfectly for you.
Also, note that most phototransistors respond to a very narrow frequency band, and lasers also put out a very specific frequency, so you'll have to find a pair that matches.
Saiello, if you wish, mail me with the image.
Also, note that most phototransistors respond to a very narrow frequency band, and lasers also put out a very specific frequency
For safety, please keep to visible wavelengths.
A google search for "laser voice communicator" turned up several results that should give you some ideas.
Most of the designs are a lot more complex than you need, but there are some nuggets in there for you.
A complete voice-transmission system: http://home.comcast.net/~rwilliams309/Laser_Pointer_Voice_Transmitter.html. The key point is that he found a solar cell fast enough to transmit voice, so it should be good enough for timing. The output voltage will be low, though, so you'd want to use a simple opamp comparator to get a 5V digital output.
This "laser alarm" http://www3.telus.net/chemelec/Projects/Alarm/Laser-Alarm.htm is also over-complex, but the builder identifies a phototransistor (2N5277) that worked with a visible laser pointer.
I also came across another circuit that used a BPW17N phototransistor
And here's a model railroader that did something very similar to what you want to do: http://www.msnbc.msn.com/id/4938258.
Ran
Hi Anibal,
I've sent you the diagram, it is the input stage to my tachometer circuit. The output can be fed directly to one of the digital pins. You can use a phototransistor instead of the photodiode, but the photodiode would be better because it is less sensitive to ambient light and will allow the output to switch much more quickly ( at least 2MHz ). You can play around with the Threshold to further decrease the sensitivity to ambient light as the laser will still easily overpower the photodiode when they are facing eachother.
Ran, Saiello: ok, now I think you gave all the clues I need. Time to prototype and test. Lot of thanks!
Just a final note, you can use practically any photodiode/phototransistor with any laser. Photodiodes/phototransistors have their peak sensitivity roughly around 900nm wavelength and those cheap visible red 5V laser diode modules produce their light at 635nm/650nm. This means that the PD's/PT's will not be as sensitive to the laser's light, but in practical terms it really doesn't matter..!