Connect with us

I can't see how this will work

Discussion in 'General Electronics Discussion' started by KTW, Mar 7, 2015.

Scroll to continue with content
  1. KTW

    KTW

    273
    15
    Feb 22, 2015
    I found this schematic somewhere and in looking at LED#2 on the right with the photoresistor wired in parallel how could it possibly control the LED when the LED is wired in circuit with the power source?
    I would think the light would stay on day and night.

    LED will switch ON in the dark.jpg
     
  2. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The current is limited by the resistor.
    In the first case, the LDR goes conductive when lit and allows current to pass to the LED.

    In the second case, the LDR goes conductive when it is lit and passes the current around the LED so it does not light.
     
  3. KTW

    KTW

    273
    15
    Feb 22, 2015
    This one doesn't work like that Duke, stays on regardless of brightness, do you see a mistake?

    circuit1.JPG circuit2.JPG circuit3.JPG
     
  4. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The LDR is not perfect, it needs to go to a low enough resistance when lit.
    Measure the voltage across it and see if it comes down to less than the voltage which is needed to light the LED.

    Edit: A transistor could be employed to make the effective resistance much lower.
     
  5. KTW

    KTW

    273
    15
    Feb 22, 2015
    I'm seeing 2.88 volts with the LDR uncovered and a slight increase to 2.9 volts when covered.
     
  6. KTW

    KTW

    273
    15
    Feb 22, 2015
    I have a feeling it's missing something.
     
  7. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    Get a small npn silicon transistor.
    Connect the LDR between base and collector.
    Connect the assembly where the LDR was with the emitter towards the negative supply.
     
  8. KTW

    KTW

    273
    15
    Feb 22, 2015
    That's not working either, I have another schematic with a variable resistor controlling the LDR so I'll pick one up and try that .I'll keep you posted, thanks for the suggestions.
     
  9. KTW

    KTW

    273
    15
    Feb 22, 2015
    I have it working with .9 ohm resistance before the LDR and it's close to what I want but it's still operating in too much light.
    Is there anything I can use to drop the resistance a little more?
     
  10. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    I do not see why the amplified LDR should not work. We need information to see what is happening.

    Measure the resistance of the LDR at zero light, at the level where you want the LED to change and the resistance at maximum light.
     
  11. KTW

    KTW

    273
    15
    Feb 22, 2015
    In bright light the LDR reads 1.9K
    No light 160K
    Where I would like it to operate is between 125K and 160K.

    [​IMG]

    I replaced the 330R with a pot down to 1 ohm and it's better but not what I want.
     
    Last edited: Mar 16, 2015
  12. Colin Mitchell

    Colin Mitchell

    1,417
    313
    Aug 31, 2014
    You really need a schmitt trigger
     
  13. KTW

    KTW

    273
    15
    Feb 22, 2015
    Thanks Colin, I'll look for a schmitt trigger schematic where I can incorporate a 555 timer.

    Kevin.
     
  14. Colin Mitchell

    Colin Mitchell

    1,417
    313
    Aug 31, 2014
    Try this transistor Schmitt Trigger

    SchmittTrigger.gif
     
  15. KTW

    KTW

    273
    15
    Feb 22, 2015
    I can try that, what do you think of this one?

    [​IMG]
     
  16. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    If P3 out is high there will be no current limit for the lower LED - POP. Add another 470Ω resistor.
     
  17. KTW

    KTW

    273
    15
    Feb 22, 2015
    Ok, will do.
    Thank you both, very helpful.
     
  18. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Or move the one resistor to go between pin 3 and the junction of the two LEDs.

    Bob
     
  19. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    I think not, if there is no resistor in the LED chain, then the current from the supply will be unlimited.
     
  20. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    You are right, what was I thinking?

    Bob
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-