Laser distance module

Discussion in 'The Projects Forum' started by bogalark, Apr 24, 2013.

  1. bogalark

    Thread Starter New Member

    Apr 24, 2013
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    HI,
    i need help for making a laser distance module which can detect a distance of 200 feet or 60 mts...i have gone through few available laser diodes and photodetectors but could not select any...
    can anyone please do help me what are the components required for making it...
     
  2. aws505

    Member

    Mar 11, 2013
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    You're basically looking at making an interferometer. Unfortunately, this may be a bit harder than you're imagining. The interferometers I've used previously have been for highly scientific in nature -- usually used for measuring plasma density. For a simple range finder, I imagine that you could use a laser diode, pulsed at a predictable rep-rate, a mirror on your object and a photodetector back near the transmitter. You'll probably want to put an optical filter on your photodetector so that you're not picking up stray light. Once you pick up laser pulse at the detector, you'll need to make a phase comparison between the detected pulses and the transmitted pulses. This phase difference, along with a bit of math, can yield your object's distance from the transmitter.

    Unfortunately, I'm not quite sure how to do the phase comparison -- perhaps somebody else will have an idea. I can provide a rough schematic for the transmitter later, though. It would be pretty straight-forward and be driven by a 555-timer, I imagine.

    Best of luck!
     
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  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I believe you can now buy cheap "laser tape measure" from the hardware store for about 30 bucks, and just pull the laser distance measuring module out of it.

    It's been talked about a bit.
     
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  4. aws505

    Member

    Mar 11, 2013
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    Seems to be a much easier solution. Although, out of curiosity, how do you think this is implemented? Would the system be, more or less, as I've described? If so, how is the phase comparison done? Off the top of my head, I suppose there's a few ways: Maybe using the transmitter pulse to start a counter and using the received pulse to reset the counter and then translate the count to a distance using some summing amplifiers, etc. Given the amount of math that is involved, perhaps a small microcontroller isn't a bad idea?
     
  5. mattbullet

    Member

    Nov 29, 2008
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    An interferometer uses a beam spliter and mirrors to detect
    tiny changes (sub wavelength) in one leg of the optical path.
    I think you are looking for something to measure the time of flight
    of your reflected beam. Then the distance can be calculated.
    I would rather hack an existing device. Did I read $30.°° for a new one?
    Matt
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    A small micro will most likely not work (at least for a time of flight measurement). Speed of light is close to 1ft per ns, so 200ft gives you about 200ns between the time you send the pulse and when you receive it. I bet they use some specialized chip for to get any accuracy.

    Another approach is to measure the parallax of the projected beam, but that requires interfacing a camera and accurate and rigid mechanics to work properly.
     
  7. aws505

    Member

    Mar 11, 2013
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    Hmm. Good back-of-the-envelope calculation. Maybe they just use a beam-splitter to combine the received beam with the transmitted beam and then pass that beam into a photodetector? I think that the beam out of the beam-splitter would only have a DC term, though, and a high-frequency term. It may be hard to pull out the phase shift from that... Unless you heterodyned the probe beam, but I feel like that's getting too complicated.

    I can't imagine that their laser has that good of coherence length, though. I can't imagine that a $30 tool has an expensive laser in it.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I guess that is sort of what interferometry is about, but you´d need very precise optics and mechanics to be able to determine the distance.
     
  9. aws505

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    Mar 11, 2013
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    Yup. There has to be an easier way. Let me draw up a schematic of what I imagine this looking like... Don't you hate it when things get in your head?? ;)
     
  10. aws505

    Member

    Mar 11, 2013
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    Alright, so this is what I was picturing earlier today. Basically, there's a 555-timer (Timer 1 in the schematic) with a small duty cycle causing an LED "laser" to burst output at a high frequency so that it looks steady state to our eyes. Each time the LED begins a transmission cycle, another 555 timer (Timer 2) starts charging a capacitor (COut) through Q2 and Q4. As soon as the laser pulse is received back at the circuit, it is detected by the "Receive" phototransistor (or photoresistor, as in the circuit) and Timer 2 stops charging COut and the voltage on COut is kept steady(ish) by RKeep. At this point, a microcontroller would take a measurement of the voltage on COut and do some math to get time-of-flight. As soon as the transmission period ends, COut is allowed to discharge through Q3.

    As you can see, the duty cycle and frequency of Timer 1 must be carefully engineered, along with the maximum measurement range. The maximum measurement range would be controlled by the period of Timer 2 along with RCharge*COut. Also, the Receive phtoresistor/phototransistor needs good filtering so that it only detects light from the LED -- it may even be smart to use an infrared transmitter and receiver. In the infrared case, there would likely be a cheap laser LED near the output that would make the product look cool. I know I left out quite a few details, but I really think this is a big step in the right direction. Any thoughts?
     
  11. bogalark

    Thread Starter New Member

    Apr 24, 2013
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    ofcourse, i agree with you, we can find many... but "making one with our own efforts would gain some knowledge and may increase some practical knowledge as it has wide applications",i felt like that so trying out...:)
     
  12. bogalark

    Thread Starter New Member

    Apr 24, 2013
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    this uses the time of flight principle,the instant laser is sent a PIN detector is used to detect the light and send the signal to the timing discriminator,and the same thing is done with the reflected light ,APD is used to detect and then amplify the signal and send to the timing discriminator which are connected to the time to digital converter and give the distance(distance = speed of light * time/2), i am also trying with the circuit...
     
  13. bogalark

    Thread Starter New Member

    Apr 24, 2013
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    it should have mainly current limiting protection or the laser would drain out the complete battery within no time...what you would say about that?
     
  14. atferrari

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    Jan 6, 2004
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    My friend, trained to maintain geodetic survey equipment (made in Switzerlan) explained this to me: the time of flight is compared to an internal (high precision - well measured) loop.

    High quality, high stability optics are the core.
     
    Last edited: May 3, 2013
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  15. aws505

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    Mar 11, 2013
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    Sure; Current limiting on diodes is always a good idea. I believe I've handled current limiting in the schematic I've provided.

    I can definitely see how a high-precision measurement would rely on high-stability optics, but what of the cheapo measurements? I seriously doubt that a $30 laser tape-measure has much in the way of high-precision optics. A little bit of googling seems to suggest that, while more complicated, typical surveying equipment is closer to $1,000's of dollars rather than $10's. Surprisingly, it doesn't seem like my scheme is too far off of a conventional surveyor's EDM, although mine has much less complexity and, assuredly, less accuracy.
     
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  16. wayneh

    Expert

    Sep 9, 2010
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    Be careful in assuming how things work. I think (assume) the cheap, so-called "laser" tape measures are actually sonic tape measures that include a laser to help target the device easily. If I'm right they do not actually use the laser as the mode of measurement.
     
  17. aws505

    Member

    Mar 11, 2013
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    I actually made the same assumption in a previous post, only I had assumed that the measurement was made by an infrared LED
     
  18. GopherT

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    Nov 23, 2012
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    This exactly true. About a year ago, I received one as a gift and, since it worked like $hit, I tore it down to see how they were able to make a laser range finder for about $50. The answer was, they didn't. It was just a laser pointer attached to a sonic (ultrasonic) piece of junk. The laser showed you what the sound was likely to hit - that is all.
     
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  19. THE_RB

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    Laser distance measuring has recently become feasible on the cheap, due to faster micros and smaller/cheaper SMDs etc.

    Here's a laser distance masuring tool;
    http://www.boschtools.com/Products/Tools/Pages/boschproductdetail.aspx?pid=DLR130K

    And even things like cheap robot vacuum cleaners can have laser distance scanners like this one from a Neato brand robot, that scans the laser in a circle to get multiple distance readings from all directions;
    http://www.protechrobotics.com/proddetail.php?prod=3903499
     
  20. bogalark

    Thread Starter New Member

    Apr 24, 2013
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    it really needs high precision, agreed with you sir..but i really wonder how few companies are able to sell the complete module for just 200$,thought of trying one on my own by getting the components, it needs even coding for microcontroller to measure the time to digital measurement conversion too..one of my friend said he will help me out if i am able to get the required components..and if i can explain what exactly i want..so,needed some help..;)
     
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