Advice needed for a light trigger circuit

Discussion in 'The Projects Forum' started by Vrmithrax, Jun 2, 2010.

  1. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Greetings circuit gurus...

    I have an interesting little project that I need to work out, and I thought I would pass it by some experts before I begin fumbling my way through it.

    I have a need to make a fast response, compact trigger mechanism that will detect a light source (in this case, a laser beam) and will light up an infra-red LED whenever the laser is detected. The infra-red emission will be detected by some of our other equipment, and is (theoretically) going to be used to calibrate some alignment in our machinery.

    I have already done some basic proof of concept testing, using a simple photodetector/relay/LED circuit, but I need something that has much faster response and does not have wear issues (relay contacts). I also need a setup that is as compact as possible, with the smallest power requirements I can manage (for battery size).

    The 2 components I do have (because we use them in other areas) are:

    TSAL7200 - IR Emitting Diode (Vishay)
    Forward Current If(AV): 100mA
    Supply Voltage Range: 1.35V to 3V

    BPW34 - Photo Diode (Vishay)
    Dark Current: 2nA

    At this point, I don't know if I will need an adjustment to set an ambient light threshold, but I am assuming I will need something like that to prevent false triggers on anything but the laser input. I also know I will need a standard LED light up when triggered, so that we know the unit is firing at the correct time.

    Anybody got some suggestions for a workable circuit? I'm dusting off some of my electronics basics books, but it's been years since I've even though in circuits (other than basic power circuits attached to DAQ systems), so I am flying a little blind here...
     
  2. bertus

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  3. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Thanks bertus, looks like a good place to start... I'm just a little worried about efficiency and power requirements (it may sit in place for long periods with no laser activation).

    I'll have to take a look at that laser timer trigger and work out how it operates so I could reverse the function - I need the IR diode emitting only while laser is hitting detector, which is a bit backwards from this example.

    Plus, I am pathetically rusty on electronic circuits, so I feel completely clueless! :)
     
  4. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Have a follow-up question... Anyone familiar with the ins and outs of photo transistors? I was playing with an incredibly simple "photo transistor, resistor, LED" circuit, using some stuff I had around the shop here, and it works somewhat (good response time, at least). I just need to get the proper photo transistor and probably put a light filter ahead of it to cut all but the wavelength I want for triggering (green laser in this case). But, I don't really have a good source to just go pick up some parts in this area - Charlotte, NC seems fairly lacking in good places for electronics hobbyists or enthusiasts to get their parts (at least, as far as I can tell, only been here a year now).

    I am thinking of using the photo transistor to kick the IR LED (TSAL7200) and a standard LED on whenever the laser hits the photo transistor. The goal is to use as low power as possible, the guys that want this done would like to use 2 AA or 2 AAA batteries to power this sensor...

    Any of you circuit gurus have a suggestion for a circuit along those lines? Trying to sort it all out before I start ordering some parts for testing and assembly... Might have to build a handful of these for different alignment locations.
     
  5. bertus

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    Apr 5, 2008
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    Hello,

    You can make the IR detector ambient independend by modelating the IR source and detecting the same frequency at the detector.
    If the beam is broken , the detector will not recieve the expected frequency and see a broken beam.
    For the modulation a NE555 can be used.
    For the detector side the NE567 tone detector can be used.

    Bertus
     
  6. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    It's actually only an IR output. The input is a scanned laser product that we use, that happens to have an IR sensitive detector array already on it. So, as the green visible laser moves across the sensor (photo transistor, etc), the IR LED needs to fire to indicate that the laser hit the target - the detector logic in the laser will use this information to triangulate and calculate positioning data. Unfortunately, this means modulation is not an option - the laser product is a finished unit that cannot be modified.

    We typically use reflective crystal targets to bounce the laser back to the detectors, but in the situation I am working on, the distance is too far for a good reflection - but an IR emission can be detected at this distance.
     
  7. bertus

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  8. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Don't have a drawing handy, but the setup is this:
    A laser product using scanning motors (think laser light show) scans lines across an area. In normal situations, we use reflective targets at specific locations to bounce that laser back to the source, where detectors pick up the reflection and make note of the exact scanner position at the time of occurrence. That scanner position data is then used to calculate the exact position of the scanner in space.

    The problem is, the reflectors we use do not reflect back enough light at extreme distances (around 50 feet away). We could use larger reflectors, but the accuracy suffers greatly (too much area to scan and data to process).

    The reflection sensors on the laser scanner are very sensitive to IR light, as it turns out. Which is why I am looking for this circuit - I want to fire an IR light when the laser scans over a light sensor, which has the same effect as having the laser reflected back - the laser scanner doesn't really know the difference, it just knows that a signal was received. So, as you can imagine, the response has to be as quick as possible between sensing the laser light and firing the IR emitter.

    I have made this work using a basic photocell and relay setup to test the proof of concept. It worked perfectly using the TSAL7200 LEDs that I acquired for the test (high IR output). The long-term lifespan of the relay contacts, and the somewhat slow response time of the photocell/relay combination, were what worried me, and pointed me towards a more elegant (and efficient) solution. Which is where we are now, trying to evaluate options.
     
  9. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Another update... Been playing with a super-simple test circuit:
    3V supply (2 AA batteries)
    photo transistor / LED / resistor series

    It works, but LED output was low, so I threw a transistor onto the output of the photo transistor to boost the signal to the LED. Good response, but too universally sensitive. Also tried the same circuit replacing the photo transistor with that BPW34 photodiode we have in stock (using the transistor as a switch) - it worked but was still firing at low light levels, like the photo transistor, and would probably need a second transistor to boost its output...

    So, now I'm to the tricky part for me, where my lack of knowledge is causing me to flounder a bit. Any suggestions on how to set a threshold for triggering? An adjustable pot to set a threshold would be ok, or a method to use a second input to create an ambient light comparator... The laser is scanning across the target rather quickly, and is a tight enough beam that I could have a second ambient sensor close by, if needed, but I think that might over-complicate the whole situation.

    Any words of wisdom?
     
  10. bertus

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    Hello,

    Can you draw (and post) a schematic for the two situations?
    This will make us understand better how you connected the parts.

    Bertus
     
  11. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Here are the basic circuits, I hope I drew them correctly. As I indicated, they are VERY basic, just using them as a starting point to test theoretical operation (and try to brush the rust off the electronics theory part of my brain).

    As you can see, I don't need much, I just need that LED to kick on when the correct light hits the detector. However, the main flaw with these circuits is that I need the LED to kick on only when the desired light (laser in this case) hits the detector, and when that happens the LED should be at full output... Right now, the LED flutters in ambient light, and I don't think it ever hits full output.

    Any suggestions would be appreciated. I wouldn't be surprised if I am totally barking up the wrong tree, so feel free to cut these circuits to shreds if necessary - I won't take it personally.
     
  12. bertus

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    Hello,

    You do not have a switch voltage in your circuits.
    Perhaps an opamp may give you a switchpoint.

    [​IMG]

    In this table you will fins siutable opamps for your low voltage apllication:

    [​IMG]

    This table comes from the attached pdf om opamps.

    Bertus
     
  13. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    I really appreciate all the feedback, Bertus... I'm slowly getting an education here.

    Hope you'll forgive my ignorance at the moment, but opamps are a bit out of my realm of electronics familiarity. Looking at the diagram you provided, am I to assume that the variable resistor will give me an adjustment to set my switch voltage level? And will the output from the opamp be full strength/current, or will I require boosting to get full power to the IR LED? I have to drive that LED at its max rating to get good signal on the detectors when they are positioned at max distance away.

    Unfortunately, I don't have many options for picking up components locally, my only options are to know specifically what I want and order it online, or go fumble through the pathetically inadequate stock of the local Radio Shacks in the area... Guess I will have to make a Radio Shack run to see what kind of opamps I can find, and hopefully one will be suitable to at least let me experiment.
     
  14. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    I'm also just wondering, after all of the sensing and triggering is done, if I shouldn't run that into a solid state relay of some type, to simplify the process of a full power output to the LED? Are there any down sides that I might need to consider? I don't really have a super-sensitive situation with noise issues, I just need a very fast triggering of a full power LED emission for my application.

    If a SSR is a good idea, anyone have suggestions of one that would fit my application? Size is an issue, I need to keep the entire package as small as possible, so I would need a relay that is small, fast, and it will be running on either 3V or 6V battery setups (depends on if we go AA or AAA, etc).
     
    Last edited: Jun 11, 2010
  15. bertus

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    Apr 5, 2008
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    Hello,

    Do you have the specifications of the IR led to be used?
    You can buffer the opamp with a transistor to have a higher output current.

    Bertus
     
  16. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Right now, I am playing with a Radio Shack IR LED that I picked up:
    • 1.2V forward voltage
    • 100mA forward current
    I have determined I also need to have a standard visible LED light up, so I can know when the output is active (and use it to set thresholds, etc). For that I would probably use another Radio Shack product:
    • 1.8V forward voltage
    • 20mA forward current
    However, I do have a large number of TSAL7200s on my bench, from another product we were experimenting with. The link to the data sheet for these IR LEDs is here:
    http://www.vishay.com/docs/81012/tsal7200.pdf

    I have to experiment between the IR LEDs to find which has the better output. My gut feeling is the TSAL7200s will be the better choice.
     
  17. bertus

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    Hello,

    When I look at the Vf vs If characteristic of the led the Vf will be about 1.4 Volts at 100 mA.
    At 3 Volts powersupply the series resistor should be (3 - 1.4) Volts / 0.1 A = 16 Ohms.
    The nearest E12 series resistor is 18 Ohms.

    For the red led we can calculate it at the same way:
    (3 - 1.8) Volts / 0.02 A = 60 Ohms.
    The nearest E12 series resistor is 68 Ohms.

    When you use a transistor as buffer the base current sould be about 1/10 of the load current.

    When we put the IR led (+its resistor) parallel to the red led (+resistor) they will draw a total of 120 mA.

    The base resistor will be (3 - 0.7) volts / 0.012 = 192 Ohms.
    The nearest E12 will be 220 Ohms.

    [​IMG]

    Bertus
     
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  18. Vrmithrax

    Thread Starter New Member

    Nov 16, 2009
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    Wow, much thanks, bertus... I will be trying to find an appropriate opamp to put this circuit together and test it this week. Hoping Radio Shack has something that will work to at least verify operation, before I begin trying to order a specific opamp online.

    Which reminds me, do you have a recommendation for a specific opamp in this application?
     
  19. bertus

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    Apr 5, 2008
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    Hello,

    Keep in mind that the resistor calculations are for 3 Volts.
    There are not so many opamps for 3 volts as you can see in the table.
    If you want to use the circuit on 6 Volts the resistors need to be adjusted.

    Bertus
     
  20. Solcar

    Member

    Jun 8, 2007
    21
    3
    Bertus has been giving great help.

    I noticed how there was flickering, so that tells me that it might need hysteresis by means of positive feedback in an opamp, or maybe the 74C14 can be pressed into service. The emitter follower to drive the LED is still needed. The Schmitt triggers in that IC provide an upper trigger point to send the gate output low and the lower trigger point to send it high. A nice thing about that hex (6) gate chip is its very low idle current. There are 6 gates available for doing different functions. http://www.national.com/ds/CD/CD40106BC.pdf

    It will be good to keep in mind that a usual advantage of gates is that the voltage level of the inputs is set internally, meaning a voltage divider can be omitted. However, that means that it isn't readily adjustable externally. Good luck on the project.
     
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