help needed with a simple circuit plz plz

Discussion in 'The Projects Forum' started by arq, May 12, 2009.

  1. arq

    Thread Starter Member

    Nov 2, 2008
    I am builing a simple ldr based light sensor. The purpose of this circuit is to turn on or off the relay depending upon the light. the other side of relay is connected with 220v circuit and lamp as a load. i have built this circuit using simple transistors as well as latching transistors. circuit works well at no load. but whenever i connect the lamp on other side of relay flickering of lamp starts. now i am building this circuit with window comparator as well as simple comparater using LM393 IC. but circuit is not working. IC never saturates. I am new to LM393 IC. can anyone tell what are the possible reasons. plz help someone.
  2. bertus


    Apr 5, 2008

    Can you post a schematic?
    Can the ldr see the lamp?

  3. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Positive Feedback. As bertus commented, the light from the lamp causes the sensor to think it has detected a higher ambient light level.

    Shield your sensor from the lamp or points it to the opposite direction. In simple word, do not allow the lamp to affect your sensor.
  4. arq

    Thread Starter Member

    Nov 2, 2008
    ok thanx. yes ldr is facing the lamp. that is the possible reason. but what about the idea of building the circuit with LM393 ic. I have built the circuit using proteus software. circuit simulation has shown correct and desired results. but circuit did not work actually.
  5. SgtWookie


    Jul 17, 2007
    Post a graphic image of your schematic.

    While viewing your schematic in your simulator, press Ctrl+PrintScreen.
    Then start MS Paint. Paste the screen image into Paint, and save it in .PNG format.

    Upload it here using the "Manage Attachments" dialog in the "Go Advanced" option.
  6. arq

    Thread Starter Member

    Nov 2, 2008
    ok this is cct diagram which i tried to build with LM393 ic. Using Proteus software LM393 IC simulation gives error . so i tried with 741 ic. but actually both 741 IC as well as LM393 IC is not working properly. Supplied voltage is +12 v. So saturation voltage must be equal or less than this voltage. but when i checked output voltage using multimeter it has shown only 1.16 volts. What to do?
  7. arq

    Thread Starter Member

    Nov 2, 2008
    sorry, circuit diagram is here.
  8. mik3

    Senior Member

    Feb 4, 2008
    Comparators have open collector outputs and thus you need a pull up resistor to turn your transistor on. Remove R1 and connect it between the output of the comparator and 12V.