Photo detector op amp circuit

[hman123]

I want to build a laser counter (counts when the beam is broken) and I am having trouble with the detector side. I can use a photo transistor (or like) but the counter device I have needs a voltage change of 5 volts to trigger the counter. Someone sugested an op amp. But I need help designing the circuit. Example, when the laser shines on the photo the output would be zero volts (or below 0.5) and when the light goes away (blocked) the photo the output would be 5 volts.(or vice versa) Is this "doable" and how/what is the layout of the circuit?

 

[richs]

Hi-

Well reading the original post:
"(counts when the beam is broken)", I don't think that the author needs to worry about reflectors on the boats.

A cheap refector type telescope with a rather large reflector, doen't have to be good, just large, will have the added benefit of allowing some slack in the mechanical
alignment of the system.

I personally feel that a 5mwatt laser is WAY too much for the intended application. I'd be surprised that you could not achieve this with a laser pointer. Assuming that you modulate the beam (as suggested earlier). You might want to think about a google search for infared TV remote controllers and use their suggested modulation frequency. This will most likely take care of all the sunlight issues that you would run across. This becomes an AC coupled amplifier and the DC from the sunlight would be blocked out.

Depending upon said modulation frequency, you can achieve synthesis of the amplifier using operational amplifiers with sufficent gain-bandwidth products. You might not get away with an LM741, but something a tiny bit more should do the job for you.

Now, note that the amplifier will be generating an AC signal that will be most likely after all is said and done, turned into a pulse train. You might want to look at something like a 555 retriggerable oneshot configuration to turn those pulses into a single pulse so the counter is not fooled by the pulse train coming from the modulated signal.

Finally, might I suggest a wander over to the USGS (US Geological (spelling?):

http://quake.wr.usgs.gov/research/deformation/twocolor/twocolor.html

Which is overkill for your application, however, someone else on this forum may yield some information on a preferred wavelength of light in fog conditions.

One thing that does come to mind however. In your "boat" example, if you have two "boats" side by side crossing the beam, you will miss one of the "boats". How does your theory account for this?

Another site that might be of interest is:

http://www.cablefreesolutions.com/background.htm

They too might have some hints as to optical paths.

Hope that this information helps.

Cheers,

Rich S.