If you're using a laser + receiver then you want to blink the laser at 38Khz and use three-pin IR receiver module to detect that signal. It's the type of IR receiver that would be used in TV's, etc. with a remote control, but you want a specific subtype that will allow a continuous 38KHz signal. Vishay describes their modules as "fixed gain" in their product matrix. They are really quite simple to use; when the receiver sees the light signal it sets its output pin voltage low and when no signal is present it sets its output pin high.
So on your receiver you need to have a row of X many receivers (~9 or so) to cover the physical distance you want and a way to invert the signal to turn a LED on/off when the laser beam is shining on that corresponding receiver. I believe this is the type of method that is used by the type of laser receiver used in your picture.
On the laser end you need to blink that laser at 38Khz and this can be done with an Arduino or more cheaply with just a 555 timer. Any cheap pen laser will do that without any issue and give you multiple tens of meters of range. A red laser is preferred as that is closest to IR frequency. In your case you need a wide, flat beam and you can find laser pointer modules that will put out a line (I'm sure you've used non-spinning laser levels that cast lines on walls, etc.), but I can't comment on how the reception distance will change in that respect. If the laser line is visible to you then it will be plenty visible to the receiver.
Thanks, I think you are on my track.
Would these be right? http://www.ebay.com/itm/IR-Receiver-Module-38-kHz-TSOP4838-FREE-SHIPPING-/260837712263?pt=LH_DefaultDomain_0&hash=item3cbb23a187
I might use a IR laser, but I only found one on 300mw, sounds kind of dangerous when I can't see the beam and hard to aim, but effective...
I got a few green lasers, they can be seen ok in daytime, and had a narrow beam. I can find red also, but is there some "ir" sensor that see green (well not ir any more, but I guess you know what I mean)?
I would want a bunch of receivers in a line, the ones to buy is about 250mm long. But if I only get 1 to work, the rest will work to.
If I get a laser that have a horizontal line, I don't have to have a moving part and that is good . Ordered one from ebay just now, I have a water level that have a line, but that is very faint, hope this will be stronger. A problem is that they are made for use indoors and are very wide angled, outdoors at distance beam get very very wide and weak. I would like beam to be 1 meter wide 50 meters away... That's why I plan to make laser move, professional lasers rotate and the beam is precisely 90 degrees, I cant find that precision in my budget. Just hope I don't loose to much precision by that...
You say that if I can see it, the receiver will see it easy, is that if I see it at night or daylight? I want 300 meters
Some comments to rest of answers:
No I can't dig deeper and fill, it will pack and I need bottom to be +-15mm now and after 10 years or I get sediment trapped and a non working drainage. But I do plan to do the last 2-5cm by hand, but it will be narrow and deep for shovel.
A water level on string, or hose type is good for making something medium sized like a garage or so, there it would work just fine.
Here we talk of something like 3000 meters of digging out in the fields.
Tolerance is important and I have been working as a measuring engineer (is that the word?) for 7 years at building sites, but is now a project manager instead and don't have access to the old toys To get some one to do it will not work as it will be needed all the time.
I could rent the equipment, but I will use it all spring, summer and autumn so it will be lots of money on rental.