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PIR and LDR light circuit

Discussion in 'LEDs and Optoelectronics' started by KTW, Oct 22, 2016.

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  1. KTW

    KTW

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    Feb 22, 2015
    I put this together and it works just the way I want but it may need tweaking if I missed something.
    Anyone see a problem with this as is?
    Thanks.


    PIR and LDR.JPG
     
    Last edited: Oct 22, 2016
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    There's lots of possible problems, but does it work ok?

    At the very least we need to know more about the light panel and the pir.

    Slowly changing light levels could easily make the middle transistor get quite hot when the LDR is triggered on.

    The voltage drop across the two transistors on the right could cause the panel not to reach full brightness.

    You don't have a base resistor for the transistor on the right. Normally this is a requirement, but if the PIR is capable of only a small output current and doesn't mind the near short across it, then that's fine too.

    You don't show a power supply, but I assume one is connected!

    The fact that I (and many others here) may have done it differently (and maybe better) is not an issue if the circuit works and the drawbacks are not an issue.
     
    hevans1944 likes this.
  3. KTW

    KTW

    273
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    Feb 22, 2015
    Thanks Steve, the light panel is one of those 12 volt automotive direct plug in multi led type.
    The power supply is a 12 volt wall adapter.
    I missed the resistor when I sketched this schematic but I do have one between the PIR and the base of the transistor.
    I've since corrected it on the schematic.
    The voltage at the light panel is about 11.6 volts and it's as bright as it gets.
    The original schematic called for a 100k pot before the LDR and when I was adjusting it for brightness I found the sweet spot, tested it and found it to be set at 10 meg.
    It doesn't work with the room lights on and that's what I was trying to achieve.
    I'll check that transistor for heat.
     
    Last edited: Oct 22, 2016
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If you can leave your finger on either/both of those transistors indefinitely, they're not too hot.

    The current through the LEDs vary exponentially with voltage, so a higher voltage drop across the transistors will probably cause a disproportionately lower current. This will tend to be protective.

    Do you know what current (or even wattage) the LED panel is rated at?
     
  5. KTW

    KTW

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    Feb 22, 2015
    The supply is rated at 2 amps, the light Output: 12V @ 0.05A (0.6W/LED )
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    50mA should be fine for those transistors.
     
  7. KTW

    KTW

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    Feb 22, 2015
    The transistors are cool to the touch, the light panel has a built in resistor , thanks for having a look.
     
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