Light sensing buzzer with 741. Could someone test it?

Discussion in 'General Electronics Chat' started by cheegii, Dec 7, 2009.

  1. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    Here i made a light sensing beeper on Multisim v10. Could some one test it? Used 741 op-amp, light sensing component, pots, 2n2222 bjt, speaker 16ohm.

    :confused:
     
    Last edited: Dec 7, 2009
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,343
    Hello,

    Take a look at the value of R7.
    You state it is 330mΩ, wich is 0.33 Ω.
    This will blow the transistor.

    Also R3 can be a little smaller, say 2.2KΩ.

    Greetings,
    Bertus
     
  3. bluebrakes

    Active Member

    Oct 17, 2009
    245
    7
    can you attach the original multisim file?

    so we can have a play/adjust.
     
  4. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    330mOhm is not. That is 330ohm. i corrected it. here are JPG, and Ultisim file.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'm curious why you don't build it yourself?

    After all, it IS your project.

    If someone else builds it, then it is their project, and you will not gain the experience of building and testing it.

    However, I will give you some input on your design.

    You are using an antique operational amplifier as a comparator. The 741 opamp celebrated it's 40th birthday a year or so ago.

    While it is sometimes OK to use an opamp as a comparator, like when you already have a quad opamp in the circuit, and need a comparator and no more board space is available, then it might be OK.

    But to use an opamp as a comparator when there is no reason to do so, is not good. Opamps are very, very slow to respond when operated open loop (maximum gain). Comparators are designed to be run open loop.

    Since you are using an opamp to drive the gate of a MOSFET via a large value of resistance, the rise/fall times of the MOSFET will be slow. This will cause heating in the MOSFET.

    You have used an IRF730 MOSFET in your circuit. This is a high-voltage MOSFET. You should be using a MOSFET that has a much lower Vdss rating, and a much lower Id rating. Gate charge (gate capacitance) is basically a function of Vdss, Rds(on) and Id ratings. If you increase Vdss rating, Id rating, or decrease Rds(on) rating, your gate charge (capacitance) will increase. This means it will take much more time to charge or discharge the gate to turn the MOSFET on or off.

    Since you are using a 741 opamp to drive the gate of the MOSFET via a 5k Ohm resistor, it will take a relatively long time to charge or discharge the gate.

    The 741 opamp is not a rail-to-rail output opamp, so it will not be able to bring the gate of the MOSFET all the way to Vgs=0 (turning the MOSFET completely off) or Vgs=10 (turning the MOSFET completely ON).

    Since your power supply is 9v, you will automatically need a MOSFET that has a logic-level gate voltage. Standard MOSFETs may turn on partially, but not fully. I do not know what parts are in your library, but as a very general rule, International Semiconductor MOSFETs that begin with "IRF" are standard (0v-10v off/on) MOSFETs, and those that begin with "IRL" are logic level (0V-5V off/on) MOSFETs.

    In either case, a MOSFET gate terminal should not ever be allowed to be more than +/-20v from it's source terminal.
     
  6. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    thanks SgtWookie, i tried to make cheapest design. also i tried to make circuit with changeable any cheapest mosfet instead of irf730. maybe i can change 5k and 10k resistor with 5k pot to connect mosfet gate.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    So, why don't you build it yourself?

    I actually have all of those parts on hand, and I won't do it for you.

    I would take a much different approach. But this is your project, not my project.

    If you want it to be built, then you build it.
     
  8. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    i had no time to build it. here are spare parts rare. old town. i mean just check it. maybe some errors in schematic. :)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It doesn't work.

    Now what?

    You better build it yourself. Maybe I made a mistake, or had a bad part.
     
  10. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    ok, thank you very much
     
Loading...