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?

 

[IRstuff]

Why is that a problem? A phototransistor with a resistor to 5 volts would provide the correct voltage swing.

Is this a school project?

TTFN

 

[hman123]

No, this not a school project. I am trying to put together a device to count boats on a lake. I am 40. I have been researching op amps and not sure how to set the circuit up. Do I need an op amp at all. Thanks

 

[hman123]

Just found out from a manufacturer of phototransistors that from 110 yds there will not be enough energy to use the device. Suggested silicon photodetector. So I am back to the small current and op amp circuit to create the 5 volt drop.

 

[cbarn24050]

You can get photo diodes with an amplifier built in, however its more usual to use a modulated light beam to give more sensitivity and daylight rejection.

 

[analogkid2digitalman]

Off topics, sort of, but what wavelength and power of laser do you intend to use? You may want to consider the safety aspect of laser radiation entering someones eye's.

There is also the aspect for potential laser beam drift (depending on the mechanical/electrical construction of the laser) that at 110 yards may be significant enough to displace the beam off of your detector if it is of small size.

Modulated beam and narrow bandpass filter at the detector will improve signal/noise ratio.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[analogkid2digitalman]

If needed, you could use a beam expander/collimator (telescope) at the laser end to expand the beam and then another at the detector end to compress the spot back down to minimize the effects of beam drift.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[hman123]

I have considered a lens to focus the light on a detector. I have not settled on a laser but have gotten close. A 5mv laser. I have found detectors with filters to help the sunlight but have not heard of one with amp. I am having trouble with how to "trigger" the counter when the beam is broken. There would be such a small state change or current. With a really small currnt can you use an op amp to have the Vout change from 0.5 volts to 5 volts?

 

[analogkid2digitalman]

Yes, an op amp or transistor can be used, look at the 2nd page of this link to get an idea of circuit.

http://usa.hamamatsu.com/assets/applications/SSD/si_pd_circuit_examples.pdf

I am sure you can find other schematics with a search on the web.

Ideally, select a photodiode that has good sensitivity to the wavelength of your laser. You mention a 5mW laser, that is more than enough to cause eye damage from a raw unexpanded beam if in the visible to NIR range

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[buzzp]

Ultrasonic sensors are capable of detecting at these ranges. See

http://www.sickusa.com/live/master/default.asp

.
This would eliminate the reflector required for photoelectric sensors. They have a simple NPN or PNP output so you can get the voltage swing you want. Just another idea.

 

[ScottI2R]

hman123,

"Class 3a lasers
Class 3a lasers are those that normally would not produce injury if viewed only momentarily with the unaided eye. They may present a hazard if viewed using collecting optics, e.g., telescopes, microscopes, or binoculars. Example: HeNe lasers above 1 milliwatt but not exceeding 5 milliwatts radiant power; some laser pointers.

Class 3b Lasers
Class 3b lasers may cause severe eye injuries through direct or specular exposure. Examples: continuous lasers not exceeding 500[mW] for any period greater than 0.25; pulsed visible lasers not emitting more than 30[mJ] per pulse; pulsed IR or UV lasers not emitting more than 125[mJ] during any period less than 0.25."

Please be careful in your endeavor. You may also be interested in ANSI Z136.1 published by LIA. Use the following link:

http://www.laserinstitute.org

Intentional direction of laser light into free air is tightly regulated for good reason. Please beware that liabilities exist.

Hopes this helps somewhat with your considerations.

Scott

In a hundred years, it isn't going to matter anyway.

 

[IRstuff]

If you must use lasers, a safer wavelength is 1.5 or so. There are diode lasers in that wavelength as well as detectors and APDs. You can run substantially higher powers and still be eye-safe AND there's relatively less solar background

TTFN

 

[ScottI2R]

IRstuff,
I agree with you on the background but isnt the eye still a window for 1.5um? ND-yag @ 1.06um is. Just asking as most my experience is in industrial (material processing) and we work with 1.06 and 10.6 (CO2) quite extensively. Seems I do recall a good atmospheric window for XFl lasers in the 1.5um to 3.0um range...Maybe that was 5um... Have to check my notes....

Scott

In a hundred years, it isn't going to matter anyway.

 

[IRstuff]

No,

ANSI-Z136.1 classifies 1.06 um radiation as retinal damaging, while 1.5x um as corneal damaging.

"They've" done in lots of rabbit eyeballs to get this bit of data.

TTFN

 

[itsmoked]

You should immediately can the laser idea....

I have done many projects like the one you are working on. You have no idea how hard this will be and how flakey the results will be. Note: It will be very hard, and very flakey.

This is the same as seeing someone wading into the water beyond shouting range while seeing dozens of 20 foot saltwater crocs sliding into the water with great interest.


A perfect match for this job is radar. It avoids all the huge hassles with the sun. It won't need any reflector placed just so. The projected energy can be less. The fog won't bother it. Energy consumed can probably be less. It will be easier to environmentally protect. Spiders and wasps won't bring the system down. The initial alignment of the optical reflector is a very large problem!! Water spatters and mirror dirt will bring down the optical system too.

Do look into radar. Please.. I beg of you.

 

[hman123]

The lake was just an example to get a visual as to what I am trying to do. The conversation got lost in addressing the human eye. This application will not be used where an eye can be damaged. But the example application is exactly what I need...a long distance counter system used away from a/c current. The ultrasonic modulated system was a good idea but $900. Trying to keep the whole system under $250. I have not checked radar so I do not now if it can be used to produce a "line" (i.e laser beam) that can be counted if broken and the cost.

So a 5Mw laser with a collecting lens (5" diameter with focal length of 3 inches)that focuses the beam on a filter/silcon detector tied to an op amp (to create the 5 volt difference in state) which goes to the counter. All run on 12 rechargable batteries. But this answer has the 20 foot croc problem!!

Thanks for everyone's input!!

 

[ScottI2R]

Hman123,
I guess I may qualify for taking your question off course. For this I apologize. I work with lasers so laser safety is always a major concern for me. I hope you find a satisfactory solution to your problem and I wish I could have contributed more towards it.

Hey, I dont think anyone will care if you take out a few croc eyes though! (Except maybe the croc!)

Scott

In a hundred years, it isn't going to matter anyway.

 

[IRstuff]

crocs have nictating membranes for out of water use, so they're probably more robust than ours.

TTFN

 

[Yaad77]

hi,
it will be great if someone give me a suggestion please;
if there are two op-amp in an IC,e.g MAX4203, can i use one filter(a resistor2.2R & 1uF capacitor) to +5V and another filter for -5V, rather than using two filters for a each op-amp +5V and -5V.
Many Thanks
Aziz

 

[jsolar]

itsmoked is correct, but perhaps understated. Unattended optics are very difficult. You may find that coding gain will be needed if you are not anywhere near 30 dB SNR. If you have SNR to spare, you might want to use a reflector and coherently detect the return. This puts all your electronics in one location. Radar is much more difficult and licensing is an issue except for some frequencies.

The op-amp circuit is simple. A Radio Shack book or vendor catalogue will help. But still, an early step is to determine your requirements on detection SNR and path loss and interference. Once you have those, you can determine the need for coding gain (analog or digital). Even the simplest form of coding gain, like ON-OFF (OO) keying or perhaps sending a set of AM tones,could get you out of trouble with pass loss drift.

Best Regards,
John Solar