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It is usually recommended that you not exceed 20mA but the spec does say (IIRC) a max of 40mA from any one pin. However you still have the problem that the Arduino pins produce 5V and the laser uses 3V.

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So could I get away with using a 77 ohm resister? 2 v / .026 A = 76.98 ohms.
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As I read this tread, it look like an interesting experiment.  I will use a laser to measure the RMP of a fan <-- another idea...

@astrodad

So you are planning to use a Laser Diode ( those keychain laser ) and use a 77 ohms a limiting resistor DIRECTlY from a digital pins. Hum... bad idea.

I do have a Laser keychain here. When I look at it, it has a push switch, a smt resistor of 51 ohms and the laser. I measured mine. I have : Vdrop of laser : 1.8 V  I = 10 mA   V = 3 V   

I don't if your laser has a small resistor inside.

So let your mine as an example.  Typical calculation of a limiting resistor of a Led.

   5 - 1.8 / 10 mA  = 320 ohm resistor or 330 ohms.  Safe for the Arduino - 10 mA

In your case :    5 - 2 / 26 mA = 115 ohm or 110 ohms or 120 ohms  but using 26 mA for a digital pin make me feel - worry.

I still prefer you use a transistor to control the laser. A 2N3904 should do it. You can use 3 V. And don't forget the resistor at the base of the transistor of a value of 1 K to 4.7 K.

If you insist  a digital pin way, go for 120 ohms. Not 77 ohms.

Here a picture of my laser.  One note : I notice this type of keychain laser at the dollor store in the pets section .


* laser_diode.jpg (89.38 KB, 1024x768 - viewed 17 times.)
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could make your own system out of 2 laser pointers and 2 Light dependant resistors.
or search ebay for 'doppler radar sensor' and make your own speed gun.
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@TechOne

Thanks for the suggestions and the clarification on the calculation. I'll take your suggestion of using the 2N3904 transistor. Of course, I have to learn how that works, but that's part of the fun.

@pYro

I thought about using the radar sensor as well but chose not to do that because 1- from what I've seen, the programming would be tough for my eight-year-old to grok and 2- he really likes lasers  smiley-grin
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Grab a 50 - 100mw green laser off ebay, the beams are clearly visible, not just a dot. These lasers are usually way overrated ( mW ) and pwm can greatly reduce the actual output to a few mW, while leaving a nice beam. Got 2 off ebay for $9 each.
For pwm you will need a laser without a driver unless the driver has its own pwm input.
Diode lasers are not like LED's apart from the fact that they are an LED, They will instantly die from the slightest misuse, or at the least have their output dramatically reduced.

Some general diode laser info, may be helpful
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Grab a 50 - 100mw green laser off ebay, the beams are clearly visible, not just a dot. These lasers are usually way overrated ( mW ) and pwm can greatly reduce the actual output to a few mW, while leaving a nice beam. Got 2 off ebay for $9 each.
For pwm you will need a laser without a driver unless the driver has its own pwm input.
Diode lasers are not like LED's apart from the fact that they are an LED, They will instantly die from the slightest misuse, or at the least have their output dramatically reduced.

Some general diode laser info, may be helpful

Thanks for the suggestion. The green laser would certainly up the cool factor. Of course, I only have a vague idea of what PWM means. I looked on ebay for 50 mW lasers and they were all $15-20 each! And the ones I saw, I think, didn't include a PWM input, but then again I'm not sure.
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@astrodad

Can you post a picture of the laser you are using. I am curious to see it.
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The more powerful the laser the farther/more reflections away you can blind someone. It's all fun and games till then.

You could make foot-step switches like baseball bases and forgo the whole beam interrupt thing for all that. They would be easier to set up for one.

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@astrodad

Can you post a picture of the laser you are using. I am curious to see it.

Sure, I'll take care of it when I get home. It's actually an LED/Laser combo but I've taken off the button for the LED lights.
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The more powerful the laser the farther/more reflections away you can blind someone. It's all fun and games till then.

You could make foot-step switches like baseball bases and forgo the whole beam interrupt thing for all that. They would be easier to set up for one.



That's a good point. Using the foot-step switches would be easier, but I wonder if it's as accurate.
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By using PWM (Pulse width modulation) you turn the laser on and off really quickly, with a certain on and off time you can control how much time per second the laser is actually on. A 100mw laser controlled using pwm can have output as little as a 5mw laser over a timed duration.

Green lasers are good for this as the IR laser pump shouldn't reach its maximum output power with very fast pwm values.

A foot switch circuit will suffer relatively the same latency as a beam detection, as signals are interpreted by the Arduino.
Use an interrupt telling the arduino to process data rather than periodically checking the state of the sensors for a change.
Also the float type may not have enough resolution to see the delay in pressing a switch compared to blocking a beam. And you'll need both setups to check the difference anyway.

Use tubes over the laser and receiver to block any reflections that may reflect off the receiver at strange angles, and have the Arduino turn it off once triggered, until restarting the timer.

Get some laser goggles for the wavelength of your laser for additional safety. Won't beat the safety of a foot switch though.
« Last Edit: January 21, 2012, 12:37:17 am by pYro_65 » Logged


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If you use means to block background light you will find that the same light and sensor that works at night will work in the day. It will just take time and material to make and line the pieces up.

I don't say to not use lasers. Just that if you do, be careful as even reflected they can hurt. That's not just the runner(s) either. Take care and the high likelihood is that nothing bad will happen. So Take Care.
 
Foot switch accuracy? If you always step on it with your lead foot and the bags/plates are at least 30 feet apart then the accuracy should be as high or higher than when a beam is broken by an arm swinging across as opposed to torso breaking the same beam. The best accuracy tool is how far apart the measures are taken and perhaps do your runners get to start and up to speed before they break the first beam/step on the first target.

Without telling your son, ask him what might affect accuracy and how he'd go about it. You might get to know what he has picked up from school and life. My feeling is that he has surprised you already.
 
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I know it's low tech, but how about replacing the laser beams with plain old bits of string? I'm sure that with a little ingenuity you could come up with a switch that was operated by a string being pulled or broken. Maybe not as impressive as the laser beams, but quite a lot safer and more robust I would have thought.
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could make your own system out of 2 laser pointers and 2 Light dependant resistors.
or search ebay for 'doppler radar sensor' and make your own speed gun.


If you get doppler radar on a running person, you will get the speed of the part(s) the beam hits and returns from.

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