Laser pulse Help

Discussion in 'The Projects Forum' started by idmiller, Jun 18, 2013.

  1. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    Hi Everyone!

    I'm trying to create a circuit that will pulse my astro laser on and off. I have most of the circuit complete. So here is a bit of brainstorming I have done already.

    Requirements:
    1. Easy power replacement. 9v Battery
    2. Fewest parts possible
    3. Multiple power rail.
    4. Least physical modifications possible
    5. Pulse Generator 555
    6. Small dimensions

    Goal:
    To use a variable speed pulser to turn a laser on and off as I don't want to keep turning the laser on and off my self.

    Parts:
    Enclosure
    9V Battery
    4.2v Battery
    Etched PCB
    100k Pot
    330o resister
    555 timer
    4.7uf cap
    Transistor of some kind.

    Completed:
    555 Timer circuit
    Laser modification
    Enclosure

    My design skills have never included transistors so i'm unsure if this is even possible. but here is a rough diagram of the circuit. would like help picking the right component to fit in the "bubble" as well as the thought process you took to get there, as I would like to learn :)Thank you

    circuit2.jpg
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,005
    745
    i would use an Irf840 fet
     
    Last edited: Jun 23, 2013
  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    You need a logic level MOSFET, since a 555 powered by a 9V supply won't be enough to fully trigger the IRF610.

    You'll also want a pulldown resistor on the gate, so that it's pulled low enough to turn off the MOSFET when the 555 goes low, and so that the gate goes low by default if the timer circuit fails for some reason.
     
  4. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    I was thinking about a 2n3904 would that work?
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    How much current does your laser require?
     
  6. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    That I am unsure of... I know the battery is 3.6v (full charge is 4.2) and it's rating is 1200mAh. It is claimed that the battery will last 20 hours between charges. the output rating of the beam itself is 5mW.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Is that 20 hours of continuous on time? That would mean an average draw of 60mA, and if it's not pulsing then that should be pretty steady with no peaks far above the transistor rating. If it is pulsing during those 20 hours, then we need to know the duty cycle.

    The 2N3904 is rated to 200mA, so it should be fine for this. You'll want a current-limiting resistor on its base, 100Ω or so. That will limit base current to ~6mA, or 10% of the laser current. That'll be plenty to turn the transistor fully on.
     
  8. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    Well finished the circuit, and it works and does what I would like for it to. BUT it's very dim. on the silly scope i'm watching 2.9vdc at it's slowest pulse a little more than 1.5hz. to battery voltage at it's max 306hz. but even when it's outputting battery voltage to the laser the laser doesn't change it's intensity.

    I added a 100 1/2w ohm resister to the base of 2n3904 as suggested. I'll updated the print. Wish I could read wattage... I'm just wondering if the 100ohm 1/2 watt resister is too much.

    Or does it need something else?

    This is exact circuit. it has been updated to working model.

    circuit2.jpg
     
  9. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    update.... if I change the 100ohm 1/2w resistor to a 10ohm 1/4w I get the power I desire. but the resistor and the transistor get very hot. how and what should I do to get them cool.
     
  10. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,005
    745
    Like i said before use a FET IRF840 , you wont need the gate drive resistor fet
     
  11. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    Ok. I'll look into it... by any chance is there something at radio shack that might do the trick?

    Looks like the only thing I have local is MOSFET IRF510 Transistor is this an option?
     
    Last edited: Jun 23, 2013
  12. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,005
    745
    that will do, or get it on Ebay.
     
  13. idmiller

    Thread Starter New Member

    Jun 16, 2013
    12
    0
    Thanks all it works just like I had hoped!
     
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