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Thread: Experimental Brightness Attenuator / Scan fail

  1. #1
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    Default Experimental Brightness Attenuator / Scan fail

    OK, a while back one William R. Benner wanted to see whom could get the closest to a 69 cent scan fail. His requirement was one opamp, low cost parts, and easy to adjust.

    Here is my latest entry.


    Note, this is not a absolute scan fail, it is a simple design, and attenuates the laser instead of 100% switchoff. It will pull a 5V analog input to six tenths of a volt, in other words most lasers will not go 100% off. This could be improved by replacing Q2 with a N channel Fet.

    Operation, it looks for large voltage swings in the galvo feedback signals, yet tends to ignore small, noisy, signals.

    I'm still toying with it in the simulator, but some of you may find the input block "interesting" for conditioning analog signals.

    The circuit is not "calibrated" for any given situation, that is up to the builder.
    for example, the diode used to set the threshold may be switched to a potentiometer, and altering the resistor (R5) in front of the two coupling caps changes the input frequency response somewhat. You may of course replace the back to back caps with a "NPO" non polarized electrolytic.


    One of these days I may try turning Q1 and Q2 into a monostable, but for now, its a nice toy in the simulation software.

    Note, corrected attached drawing.

    Steve
    Attached Thumbnails Attached Thumbnails QUASIscanFail2.bmp  

    Last edited by mixedgas; 10-25-2010 at 12:34.

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    Nice, good job!
    I don't have the time to try it for real but I hope someone else does it and reports back!

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    The schematic looks very clean, nice job.

    Is it possible to add a horizon control with a potentiometer so you can have the scan fail on below the horizon and still can use static beams above the horizon.

  4. #4
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    Default Advancd version for mccarrot.

    Quote Originally Posted by mccarrot View Post
    The schematic looks very clean, nice job.

    Is it possible to add a horizon control with a potentiometer so you can have the scan fail on below the horizon and still can use static beams above the horizon.
    As that sort of defeats the purpose of "Bills challenge" I'm going to give that to you in a different format.

    LT Spice is a free download and a breeze to use, although it has some limitations. You can even convert one axis of a ILDA file to a wav file using Laserboy, and import it as a waveform. Have not tried it that yet, I test things with 2 sine or square waves, plus noise, beat together in a mixer circuit and input into the DUT circuit. If the waves are audio and say 1.00 Khz and 1.020 Khz, you can get nice simulations for test.

    LT SPICE DOWNLOAD:
    http://www.linear.com/designtools/software/ltspice.jsp

    While I could of just added a HC series logic gate, I kept with the M^2L spirit.
    Note M^2L is "mickey mouse logic", logic done with crude yet effective, cheap, parts.
    Use TL084 for the opamps and LM741 for the compare circuit.

    Paste the following text into a text file called scanfail2.asc and copy it to the LT Spice directory.

    Steve

    Version 4
    SHEET 1 1596 880
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    TEXT 296 864 Left 0 ;STATIC BEAM OVER RIDE
    Last edited by mixedgas; 10-26-2010 at 15:44.

  5. #5
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    Thumbs up

    Thanks for sharing!!!

    I never used this program, so I'm a little bit in a learning curve at the moment, learning how to do good simulations.

    I'm more used to breadboards and a scope.

  6. #6
    mixedgas's Avatar
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    Quote Originally Posted by mccarrot View Post
    Thanks for sharing!!!

    I never used this program, so I'm a little bit in a learning curve at the moment, learning how to do good simulations.

    I'm more used to breadboards and a scope.

    Yeah, the sim mode learning curve is steep, and the fact that the default parts are mostly all Linear Tech chips can be a bit of a drag. It has trouble with a few minor things like linear oscillator circuit startup, but its free,has many users, is actually pretty good, and you can reach the guy who writes it if you have issues.

    Compared with other spice programs on the market, it is easier to use, in my opinion.

    I too do bench models, but when I need to bang out a analog circuit for someone who can already solder, owns a scope, and can decode parts packages and pinouts, it rocks.

    So I figured you'd like to do a little learning.

    Steve

  7. #7
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    see I never understood the point of one of these scan fails... if you have a beam stop to prevent your projection from entering the audience it seems like one extra thing that can go wrong in your show...

    plus sometimes a slow, sweeping static dot can be a really cool effect for certain songs... although pangolin usually inserts invisible "points" even if your just projecting a dot so maybe it won't be an issue

  8. #8
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    Guidelines for crowd scanning 10xMPE are that you have a suitable scan fail device in operation

  9. #9
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    Quote Originally Posted by norty303 View Post
    Guidelines for crowd scanning 10xMPE are that you have a suitable scan fail device in operation
    ya for you maybe, over here doing audience scanning legally isn't worth the trouble/resulting effect

  10. #10
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    Quote Originally Posted by flecom View Post
    see I never understood the point of one of these scan fails... if you have a beam stop to prevent your projection from entering the audience it seems like one extra thing that can go wrong in your show...
    Now what if your galvo PSU breaks down.

    Or one of the galvo's gets a melt down and get stuck (sometimes seen with chinese scanners)

    Or some idiot drive over your ILDA cable shorting only the X Y wires

    Or one of the Galvo drivers dies

    Or one of the galvo cable comes loose due to the 10KW PA system next to the laser.

    Or one of the galvo bearings gets stuk

    or.....

    or...

    or....


    I hope the scanfail will never have to do its job, but if something goes wrong you will be happy you have it.

    Also if there is problem where a audiance member gives you a claim its good to proof you have a scanfail in place, this give you a advantage in court...

    finaly the guidelines advise you to use a scanfail.



    On only time you do not need to use a scanfail is when you mask of the audiance area with a cover, so in case of a problem there is zero chance a beam hits the audiance.
    Attached Thumbnails Attached Thumbnails MPE..JPG  


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