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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 01-04-2011, 09:34 PM
JeriBrunson JeriBrunson is offline
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Default Working on a laser detection and trigger circuit

I'm working on a project and have very little circuit design experience or background. I've been through the thread "Fast Horses" and several others that looked helpful, but haven't had time to go through the entire forum so any pointing to relevant threads is appreciated.

Here's what the circuit needs to do:

1. Detect a laser beam
2. Turn on two LEDs that remain on as long as the laser is detected (one is part of the circuit and one LED is external to the circuit)
3. Send a pulse signal to a DAQ card that the laser has been detected
4. Send a pulse signal to a DAQ card that the laser is no longer detected

I'm working with a set 12 DC source, a Hamamatsu S2386-5k to detect light, and I'm trying to get ahold of a green laser pointer for testing. Of the ICs that I have available, I settled on a LM 741, LM 339, and LM 555 as a starting place for a test circuit on a bread board. I can't use the breadboard set up with the actual laser that it will be used for so I'm trying to do as much pre-thinking and incremental testing as I can. I've been working mostly from datasheets and any sample circuits I could find online or in textbooks.

I've attached a picture of a "rough draft", bare bones circuit that I've put together. I don't have any circuit design software so I just drew it up in paint, but I think it's readable.

Any suggestions, ideas, criticism, etc., is appreciated. Thank you.
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File Type: png Laser trigger circuit.png (14.9 KB, 37 views)
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Old 01-04-2011, 09:40 PM
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bertus bertus is online now
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Hello,

What pins at the LM741 are grounded in your schematic?

If those are the offset pins, the LM741 will not work correctly.



When you look at the internal schematic you will see that the input is disturbed when grounding those pins:



Bertus
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File Type: png 741_offset.png (22.3 KB, 71 views)
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Old 01-04-2011, 09:54 PM
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While we're at it, the 741 is a pretty weak chip. You might want to try a different chip or use a spare in the 339 package, seeing as this circuit only does "on" and "off". Then connect the unused inputs on the 339 chip so those amps don't wander all over the place like lost sheep.
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Old 01-05-2011, 03:04 PM
wayneh wayneh is offline
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Quote:
Originally Posted by JeriBrunson View Post
3. Send a pulse signal to a DAQ card that the laser has been detected
4. Send a pulse signal to a DAQ card that the laser is no longer detected
Since you've got DAQ as part of your project, why not feed the op-amp output directly to it? You can use software to perform the "comparator" function and use the DAQ outputs to control LEDs or anything else you want.

Your timer circuit is to provide de-bouncing? If not, I'm not sure I understand its function.

One thing you may want to check early on is that the detector can actually detect the wavelength of your specific laser.
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Old 01-06-2011, 08:50 PM
JeriBrunson JeriBrunson is offline
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Thank you for the help and suggestions!

Re: Bertus and #12

I decided to try another op amp than the LM 741 and picked an OPA 129. I've attached a second circuit diagram with the OPA 129. The shaded part on the LM 339 indicates that I've grounded those inputs.

Right now, the circuit detects the laser beam (green laser pointer) and turns on an LED whenever the beam is shining on the photodiode. That leaves creating a signal to indicate both that the laser has been detected and is no longer detected.

A simple comparator would send a pulse indicating detection, but am I correct in thinking it would continue to send pulses as long as the laser was being detected? Is there a way to have just a single pulse and then hold that state until the laser beam is removed?

Re: wayneh

Currently, this circuit will be packaged into a box that will be just one of several interchangeable modules, all of which feed into a DAQ. That doesn't mean there isn't a way to feed the signal to the DAQ and that seems like a good way, but I'm not sure I have that choice within the constraints I've been given for this project.
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Old 01-06-2011, 09:46 PM
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A single comparator would send a DC signal as long as this circuit is in any particular state. A single comparator with the signal coupled to it with a capacitor would send one pulse when the state changes. (Comparators don't send pulse trains out unless they get pulse trains in.) Under these conditions, you can get a positive pulse when a comparator sees out1 go positive, and you can use another comparator to get a positive pulse when out1 goes negative, by reversing the logic of its inputs.

Good thing you have 3 amplifiers left over because you have what you need. Just add capacitors, resistors, a mid-range voltage level to compare the changes to...am I making sense to you?
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Old 01-12-2011, 05:43 PM
JeriBrunson JeriBrunson is offline
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Thank you #12! I took one of the extras and added resistors and capacitors and I think it is giving me the pulse I need to indicate the laser was on.

I haven't added "laser off" signal yet but will try adding that in next.
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