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Thread: Modding from TTL to Analogue

  1. #21
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    Thanks man...
    Yeah in a round-about way I thought I answered that..hehe

    Yes, that could be done , but.
    The reason we still need the original op-amp was for frequency compensation and current
    regulation. This needs to be taylored to insure good performance.

    This requires some feedback components to this op-amp to control spikes and overshoots
    from the diodes current resistor for control of current as well.
    Hence..The idea of adding another op-amp before this driver network...make sense?
    Still, we could do this with the "new" circuit as the old one would need it anyway.
    The fet is a different animal then say that bipolar transistor..as you well know

    I just didnt go too far into detail.. just to keep the concept simple

    You mentioned something that I've been noodling on for some time. Rather than modifying the internals of the PSU to support analog modulation directly, what would prevent you from mounting a power mosfet right at the output of the existing PSU and using that to give you analog modulation? (You could build a voltage divider to knock down the zero-to-five volts of the analog blanking signal to somthing more reasonable - say, zero to 1 volts - and then apply that to the gate of your mosfet.)
    If we tried this...the boards diode driver will try to compensate for the lack of current and will try to make up for it..
    I think that will cause problems... dunno..will have to try it.
    Just lifting the diode and driver rail only might work.
    But to really think about more ..it wont work...It wont be fast enough..
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  2. #22
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    Quote Originally Posted by allthatwhichis
    Quote Originally Posted by eyceage

    What about building an external analog to TTL converter box? It seems like it would be possible to convert an analog input to TTL with PWM, using quasi-CW up to the max frequency of the TTL input. ???

    Does anyone know if such an item exists, and if it would be effective?
    Quit typing in Greek dammit... IT makes my head hurt... :?

    Put another way: Turn it on an off so fast it looks like it's on all the time, but at varying brightness.

  3. #23
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    Quote Originally Posted by eyceage
    Put another way: Turn it on an off so fast it looks like it's on all the time, but at varying brightness.
    Thank you!!!

  4. #24
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    Well, I thought jeejeedr answered that too!

    You see the TTL input only accepts either ON or OFF state...
    hence the reason this logic is used to start with
    You cant make a cow fly very well.. by doing something different.

    There is not much in-between.. to get to the ON state the voltage only needs to get to the threshold area
    of the chip., this is about 2v on most chips... see chart in link

    http://www.interfacebus.com/voltage_threshold.html

    that pulsed signal you want to provide will be just that a pulsed signal

    The blue area is the threshold region..the pink is the area of ON state

    Now this area between threshold and ON range very very small..
    this area could be used for vary-ing the brightness some but not very much
    you would still have pulses in your lines that are supposed to be fully turned ON or anything in-between
    I hope that makes sense?
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  5. #25
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    Meaning you'd get dotted lines at a lower brightnes instead of full lines...

  6. #26
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    Quote Originally Posted by allthatwhichis
    Meaning you'd get dotted lines at a lower brightnes instead of full lines...
    Exactly.. not of much use unless you want them
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  7. #27
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    I've now updated the circuit to show the missing components
    and also the
    wiring for the head

    You will notice that on the driver board there are missing components going to pins 3 and 11 of the DB15 going to the head, there also is nothing connected to these pins on the head, therefore it may be for a different head!

    Jim

  8. #28
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    Good Job..
    now to study that unknown chip..
    Thanks man !!!
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  9. #29
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    Almost.. Just not 5volts only..but I think you meant the same thinking as the rest of us Smile
    But checking the diode for output on a questional unit would also require some fancy tactics with a mirror
    and possibly removing the focusing optics to check the output.
    So what else do I need to check it?

    The 7408 isn't faulty, I get all the appropriate voltages upto the LM358(U8) but the output on pin 1 doesn't change.
    I was thinking of connecting the collector to the emmitor on V3, that would supply the diode with 5v@1.65A (current restricted by R7 0.33R).

    Would I be in danger of popping the diode?

    Jim

  10. #30
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    Hi Jimbo

    Sorry, been busy with the holidays and violet lasers.
    You say pin 1 of the '08 ?
    If so .. you should see the turn on pulse from the timer there.
    If you meant pin 1 of the driver op-amp (u8 ) then just means something else is not doing its thing.
    First .. tie pin 2 and 3 together on the blanking connector. making 5volts or a high at pin8 of u8.
    Make sure you see a high at pin 1 of the '08 if not suspect the timer.
    Check for a high at pin 2 of the '08 (its an AND gate).
    you will see a high at pin 3.
    If not check resistors and soldering. and of course 5 volts and grounds on the IC's.
    pin 6 (u7) should be high (this is the reference supply).

    Anyhow, N5 (the unknonw analog input chip) looks like a dual op-amp to me
    possibly a high speed video amp.
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

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