TTL Diode Circuit help plz :)

Discussion in 'General Electronics Chat' started by JB123, Apr 8, 2009.

  1. JB123

    Thread Starter New Member

    Apr 8, 2009
    I am kinda new to circuits and am struggling with a problem.
    Please see the attached schematic with explanation.

    The problem I am having, is the power source I am using ramps up and down very fast from 1v-3v. I have been boosting this to 8v and powering the lm317 circuit directly, but it takes too long to turn on and off. Instead, I would like to get rid of the 8v booster and use that <1v-3v power source as a TTL/PWM input... I would power the circuit with a 12vdc always-on source.
    I have gotten as far as the attached schematic... Ive tested it by touching the inside isolator pins with a screwdriver, it does work like i wanted and turns on the transistor + laser circuit.
    I need to know what to put before the opto-isolator so it will turn on when above 2v, and off when below 2v... and not burn out.

    Any help is greatly appreciated!
    Thank you
  2. JB123

    Thread Starter New Member

    Apr 8, 2009
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Typically your opto LED needs about 1.2 V to turn on. If you require (say) 10mA to drive the LED then you need a series resistor R=0.8/.01Ω or 80Ω to drop the 0.8V excess from the 2V drive. Perhaps 100Ω (Idiode ≈ 8mA) would be OK. Differentiating the on >2V and off <2V conditions is too close. There needs to be significant margin between on and off states - if the 2V drive source drops below about 1V then OK. Zero V would be better.
  4. JB123

    Thread Starter New Member

    Apr 8, 2009
    Yes there should be a significant diff, i believe its 3v high, and about 1v or less when low.

    Im sorry but im not good enough to understand exactly what you are saying... it sounds like your telling me if i put an 80 ohm resistor somewhere it will ???? ehhh what will it do again?
    you went right over my head, i have no formal circuit design education.
    please dumb it down for me :/
  5. italo

    New Member

    Nov 20, 2005
    you are using a photocoupler to turn-on-off a lm117 current source. LOOKS like you copied from somewhere this mistake. the lm117 can be easily be controlled from the adj terminal by a a low power circuit as opposed to turn off a current source. since it is a current source voltage out means nothing since the current will set the voltage out. and since there are caps involved in the output it becomes a mistery of a circuit . what is the intent i ask?
    Last edited: Apr 11, 2009