Hi!
i need to do the following and don't know what kind of hardware i need:

i need to detect when a laser beam is being interrupted by water.
(this system will be above the water level of a fish tank, and when the fish shoot water, i need to detect it).

i suppose i need some kind of sensor that can also interact with a computer.

any thoughts for what i should be looking for?

thank you for reading!

Tali

 

Lasers normall come configured as beams. if you wanted full coverage of the tank you would need a laser sheet. that would be super expensive.

how about a fine wire mesh over the tank, with another fine wire mesh over that, seperated by a very small distance. if a drop of water gets between them, a circuit is made.

 

thanks!
i cannot put wires because the fish has to see the computer screen above it.
but we were thinking about two mirrors so the beam will hit them in a small angle.
but what about the sensor to capture the beam?

 

thanks!
i cannot put wires because the fish has to see the computer screen above it.
but we were thinking about two mirrors so the beam will hit them in a small angle.
but what about the sensor to capture the beam?

If you're still talking about a "sheet" of laser, then the receiver would probably be more expensive than the laser itself.

this is the kind of technology you are referring to, right? http://www.keyence.co.uk/products/sensors/laser/lvh300_100/lvh300_100.php
that's their largest one and it's not cheap. http://www.plccenter.com/Buy/KEYENCE CORP/LVH300

plus, you would need several of them to cover the whole top of the tank.

 

actually, using a laser focused at a spinning group of mirrors can give you the same effect, you can get a cheap spinning laser level, and use a cheap webcam connected to the PC to detect where the water is intersecting with the laser "sheet".... here is a system that works similar it is also a webcam based laser 3D scanner >> http://www.david-laserscanner.com/.... but you do not need to use 3D, basically if you map out the area on a PC screen and do some kind of pixel mapping, you can easily determine where the laser is "disrupted" by the water.....

 

... we were thinking about two mirrors so the beam will hit them in a small angle.
but what about the sensor to capture the beam?

If you are using visible light, or infrared light below 1000 nm wavelength, then a silicon PIN photodiode makes a good detector. A small area device (about the area of the laser beam, or a little larger for alignment tolerance will make it so that any disruption of the beam will cause a disturbance in the detected optical power. The issue is going to be the alignment. You'll need precise mounting and alignment hardware for the laser and photodiode. Note that each reflection will result in a loss of power, but you should be able to reflect enough times to cover the water surface, and to have enough power at the detector.

Also, note that systems like this can be sensitive to vibrations. Even distant trucks on the road can shake the systems and the use of multiple reflections and small angles make the system extra sensitive. You can make a crude optics bench out of a sandbox sitting on inflated tire inner tubes.

Use of a narrow-band optical bandpass filter (at the laser frequency, obviously) in front of the photodiode will make life a lot easier. An aperture that shrouds the detector, but lets the beam in, also helps remove background light. Modulation of the laser beam with electrical bandpass filter can also help a lot, if the optical bandpass filter and shroud are not sufficient to remove background light.

Overall, this won't be easy, but it is doable.

 

thank you so much for your help!
you gave me some great ideas!
Tali