Dusk to dawn using an op amp

Discussion in 'The Projects Forum' started by deadlight, Apr 9, 2009.

  1. deadlight

    Thread Starter New Member

    Mar 20, 2009
    Hello all. I have been visiting this site for a few weeks and find it extremely helpful. Now i have come to the point where I could use some help. I am in an associates program and have been given the task to build a simple dusk to dawn switch. The only requirements are that I must use a photosistor, an lm741, and a 9 volt battery. Optionally it could also be able to be fine tuned for the amount of light required to turn it off. (potentiometer maybe?) I have spent weeks on this and cannot get it to work. Any suggestions or schematics would be greatly appreciated. Thank You.
  2. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Use the following magic phrase in Google and you will get many hits. The first few ones all contain circuit diagram & circuit explanation.

    "light activated 741"
  3. Audioguru


    Dec 20, 2007
    A lousy old 741 opamp was designed 41 years ago for a 30V power supply voltage.
    A 9V battery quickly drops to only 7.2V.
    Many 741 opamps do not work with a supply as low as 7.2V to 9V.

    About 35 years ago, Motorola had a datasheet for their better opamp that showed why the inputs and output of a 741 opamp did not work when the supply voltage was low.

    I would use the MC33171 low power low voltage opamp or the MC34071 normal power low voltage opamps instead. Their minimum supply is only 3V.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    By "photosistor," do you mean "phototransistor," or "photoresistor," or "photovoltaic cell?"
  5. Wendy


    Mar 24, 2008
    I'm betting photovoltaic cell.

    The requirements were not his to make, it is an assignment as well as a project. The plus/minus aspect is why I like using 2 9V batteries though.

    Given the op amp doesn't go rail to rail (another way of saying they don't switch to the full power supply voltage) you'll probably need a couple of diodes (or a LED, or a zener) to absorb voltage.
    Last edited: Apr 9, 2009
  6. deadlight

    Thread Starter New Member

    Mar 20, 2009
    Thank you again for all your replies. By photosistor i meant a photo resistor. So far the only thing I have gotten to work, albeit for only a few seconds, was by using a 5 volt relay as the switch itself.
  7. hgmjr

    Retired Moderator

    Jan 28, 2005
    Can you post the schematic of your circuit as it is presently arranged?

  8. wollybyte


    Apr 5, 2009
    1st result in goolge search "741 as a Comparator"