laser motion sensor

Thread Starter

swgold

Joined Apr 1, 2015
14
I am trying to find a motion sensor that can detect a drop of water from a distance of about 10 feet or more without a reflector behind it. It is the big hurdle in a project I am working on. Does anyone have any ideas or places I could go for something like this? I am assuming a laser would be needed for something not only that small, but that far away. I have looked into ultrasonic with no good leads. I am open to any suggestions. I am by no means set on a laser.
 

joeyd999

Joined Jun 6, 2011
4,477
Is the location of the drop the same at all times?
What are the ambient lighting conditions?
Can you place a non-reflective medium behind the drop?
 

Kermit2

Joined Feb 5, 2010
4,162
the naturally curved surface of such would make even a laser reflection difficult to capture. Better to make a surface where such drops land send a signal.
 

Thread Starter

swgold

Joined Apr 1, 2015
14
OK, great questions. For now, lets assume the drop will be in the same location. But ultimately, it will not. Ambient lighting conditions will be very dark, if any existing light at all. And I am trying to get away from a reflective medium behind the drop. It would defeat the experiment. Ideally, there would be nothing in back of it at all.

As for detecting the drop on the ground. It is an option, but ultimately it would not be ideal. I am trying to stay away from that for now.
 

joeyd999

Joined Jun 6, 2011
4,477
Ok...so, using a laser, the light reflected from the drop will be minimal, but likely detectable using "extraordinary" means.

Under these conditions, I would consider synchronous detection. This would be done with a laser that could be modulated (say, 10kHz or so) and a lock-in amplifier. Of course, you'd need a photodiode front-end and some optics to gather the reflected light.

The nice thing about this is, even if there is some static reflected laser light, or other ambient lighting, you should still be able to detect the drop.
 

wayneh

Joined Sep 9, 2010
17,153
Is there, or could there be, anything in the water? Turbidity is often measured by comparing light intensity both on axis with the source (normal absorption) and the light scatter at 90° off-axis. In a dark room, I think a drop lit from the side would appear to flash a bit even without suspended particles. But a little bit of suspended material to give turbidity would certainly increase the effect.
 

Thread Starter

swgold

Joined Apr 1, 2015
14
Joeyd999 just gave me a night of research to do. I have been under no illusions that the detector would have to take "extraordinary" measures to find a drop of water. I knew this was going to be an uphill battle. Just getting ideas for detecting without a backside reflector is a big start. I figured the drop would have to be the reflector itself. I assumed the photodiode would be a laser just because of the need for intense reflectivity. That would also allow for a lit room since I could have a wave specific optic to see the reflection. As for the modulator, I am not sure I understand the need? Would the receiving optic need a switch that says light/no light? So therefore a frequency?

As for the suspended solids in the water, I am sure there will be some, but minimal. Preferably, I would like to think the water would be really pure.
 

Thread Starter

swgold

Joined Apr 1, 2015
14
OK, I see. So that takes care of the erroneous light. I will look around, but do you know if there are any off the shelf, wave specific lock in amplifiers? If I got an LED at a specific wavelength, is there a matching LIA? And could it be possible that there is a photo receptor that happens to include an LIA at a specific wavelength? That would solve a whole lot of my problems.
 

wayneh

Joined Sep 9, 2010
17,153
Detecting the droplet might not be so bad if you can use optics to focus on where the droplet might be, like a short-range telescope. But you said in #4 that you want to anticipate the position not being fixed.
 

Thread Starter

swgold

Joined Apr 1, 2015
14
That's right. I am interested in finding the drop in a plane ultimately. I am currently looking up the reflectivity of a water drop at a specified distance. So far, I am finding far too much information on Doppler and weather which is not my intent.
 

Bernard

Joined Aug 7, 2008
5,788
Part of the problem can be solved by using a bar lens to spread the laser light into a flat sheet. How large is the drop plane ?
 

Thread Starter

swgold

Joined Apr 1, 2015
14
I do plan on doing that. But for now, I am trying to see just how far away the drop can be detected. So I am focusing on a drop in a single place right now. I have been trying to find out just how reflective a drop of water is. It has been difficult to find. By determining the reflectivity of the drop (at a determined distance), I can determine the sensitivity needed for the receiving optics. Spreading out the plane is going to be the next step.
 

Thread Starter

swgold

Joined Apr 1, 2015
14
Last post was referring to Bernard. But I think you might have a point wayneh. I was hoping on a more calculated way to do it, but I might just be buying some equipment and experimenting myself.
 

Alec_t

Joined Sep 17, 2013
12,248
Rather than look for light reflected from the drop, which is likely to very weak because of the drop shape, have you considered detecting refracted light (think rainbow)? Gut feeling tells me that would be more intense, hence easier to detect.
 

Kermit2

Joined Feb 5, 2010
4,162
another approach might be to always sense the laser and have the sensor producing a steady state signal, which would deviate when the beam is interrupted in any way.
 

wayneh

Joined Sep 9, 2010
17,153
Yes, I noted earlier that off-axis light scattering is probably a good option.

You know, I've played around with a laser pointer (when the cats weren't using it!) and one of the things that's fun is to cause the beam to reflect off shiny objects, like a piece of crystal for instance. To do this by hand, you have to pan the beam around to find the sweet spot.

What if the beam was always panning in a plane, on purpose. An object in the scan would cause scattered flashes as the beam passes by. If that beam was modulated at 20kHz for example, the receiver could filter out everything not at the modulation frequency. So a pulse or pulse train of a few ms of a 20kHz signal at the receiver would indicate the presence of a droplet.
 
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