phase shift detection circuit

[erique82]

Help! I'm currently working on my laser rangefinder circuit... and i was wondering if there are any pros out there who could suggest how i should build my phase (laser) shift receiver circuit? Basically what this component do is to decode the laser phase shifts to electric signal, which will then sent to a microcontroller. The difference in the phase shift between the transmitted and receiver laser wave can be calculated to measure distance. Again, what components do i need? I read about the FSKs, DPSKs, and I wonder if those are any use to me? if i were to use a phase shift detector circuit from an ULTRASONIC rangefinder, would it still work on my LASER rangefinder? :roll:

 

[evandude]

well let's see, the wavelength of a red laser is typically something like 650nm (assuming it's red)... and the speed of light is 299792458 meters/sec...

so that's 1666666.67 wavelengths per meter, so wavelengths per meter * meters per second = frequency, right?

so the frequency of the light at a given point (the detector) would be something like 4.99654098 × 10^14 Hz.

(feel free to correct me if any of that math is way off, I just woke up :lol: )

Now, there is no way that a circuit designed to measure phase shift at 40kHz is going to work at 500GHz. nor would I imagine ANY electronic circuit that a hobbyist could construct for themselves would be able to do anything useful with that, and even if you could build a circuit for it, I don't quite think a regular detector like a phototransistor would even be able to sense the actual light waveform, rather than just producing an output proportional to brightness.

It seems to me that either A) commercial laser range finders don't use the phase shift of the laser to measure the distance, or B) they use some more creative method to measure the phase that doesn't involve directly measuring it from the wave signal.

now please don't take this as a personal attack on your idea or anything, I'd be interested to know if I'm wrong with this, or how you decide to do it if you go with a different method.

 

[zachtheterrible]

Do a search for laser rangefinder on this forum. There have been many inquiries and I think I remember a detailed article about this subject being posted recently.

 

[erique82]

phase2

thanks. 40kHz is the frequency of an ultrasonic. therefore, i should be designing a circuit which detects laser wave frequency (which is around 150-160Mhz?). there ARE several other methods in laser rangefinding such as time-of-flight which is better than phase shift method in terms of the range and speed. Arguably, phase shift circuit are cheaper, less complex and easier to build (so they say, but i remain clueless) which are the reasons why i wanted to build one. :lol:

having said that, the main challenge right now is to develop an efficient phase shift circuit and decoding the wave signals to electric signals (not important at the time being). I'm not sure on how to go about building this phase circuit, so i do need a few suggestions... especially the components needed. i have limited reference at the place i stay here, so i'm mainly dependent on the net to do my research.

 

[Nigel Goodwin]

Re: phase2

i should be designing a circuit which detects laser wave frequency (which is around 150-160Mhz?).

That's only VHF radio frequencies, light is VERY, VERY much higher than that.

 

[Optikon]

Help! I'm currently working on my laser rangefinder circuit... and i was wondering if there are any pros out there who could suggest how i should build my phase (laser) shift receiver circuit? Basically what this component do is to decode the laser phase shifts to electric signal, which will then sent to a microcontroller. The difference in the phase shift between the transmitted and receiver laser wave can be calculated to measure distance. Again, what components do i need? I read about the FSKs, DPSKs, and I wonder if those are any use to me? if i were to use a phase shift detector circuit from an ULTRASONIC rangefinder, would it still work on my LASER rangefinder? :roll:

Look at how these guys do it. Modulate laser with RF.

https://www-project.slac.stanford.edu/lc/local/systems/special_projects/RangeFinder/RangeFinder.html

 

[Nigel Goodwin]

Look at how these guys do it. Modulate laser with RF.

And look at how those guys price it! - $42,000 :lol:

 

[erique82]

My god, that's way expensive for a rangefinder all right. I'm just trying to build a prototype! With a resolution of around 0.1m, it's not really accurate. there are commercial products that utilizes phase shift rangefinding with prices that range from as low as $399. if i'm not mistaken, it's a a binocular paired with a laser rangefinder... saw that on eBay not so long ago. so the more accurate or precise the laser rangefinder, the more expensive it is. But seriously $42k? I bet that's mainly for astronomy or something. Definitely not fit for the commercial use.

 

[Optikon]

Look at how these guys do it. Modulate laser with RF.

And look at how those guys price it! - $42,000 :lol:

OP said nothing about cost..

I assumed his pockets ran deep...
:lol:

But, it shows you what your up against for a good one!

 

[erique82]

are you guys even helping me out? with my current level, i'm still darn confused. where do i start? can anyone draw me a suitable phase circuit for laser detection? :shock:

 

[Nigel Goodwin]

are you guys even helping me out? with my current level, i'm still darn confused. where do i start? can anyone draw me a suitable phase circuit for laser detection? :shock:

I don't think anyone here has done it?, but generally it's VERY complicated (and expensive) to do laser range finding, and if you're having to ask you're probably not capable of it?.

If you search the forums there's a previous thread, where someone posted a VERY useful link about laser range finding - but it was complicated, as it has to be.

 

[evandude]

are you guys even helping me out? with my current level, i'm still darn confused. where do i start? can anyone draw me a suitable phase circuit for laser detection? :shock:

you're right, we weren't helping you in the sense of giving you circuit diagrams... we just spent the entire conversation on the thread trying to get you to understand how extremely difficult this project is, which you still don't seem to realize! If you think people can just hand you simple circuits, you are going to be disappointed... and from the sounds of your experience level, you probably REALLY shouldn't be doing this project...

 

[magpie]

It's easy.

You pulse the laser, then measure the time between transmitted pulse and received pulse.

Light travels at 300,000,000 metres per second.

Range = C/2t

C = the speed of light. t = time between transmitted pulse and received pulse.