How to set condition from sensor for drive LED without microcontroller ?

Discussion in 'General Electronics Chat' started by mmc01, Jul 8, 2014.

  1. mmc01

    Thread Starter New Member

    Jul 27, 2010
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    I use LDR sensor for turn on LED when dark and I use PIR Sensor for check motion. If somebody pass the sensor then turn on more LED. And set all LED turn off when have no light. It like And Gate circuit. So, I try to use circuit like this.

    I use LED 12V 1W 4 light so it is 4W. I test LED by connect to power supply it very bright. But when I connect LED to transistor 2N2222 in circuit it bright a little. how to make LED very bright ?

    [​IMG]
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    because the led is powered through the resistor R5, which lowers the current, so it dims more.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    It would be more typical to put the LED in series with the collector-emitter of the transistor, so that all current thru the LED also passes thru the transistor. That will however change the logic if you use the NPN. You can retain the same logic if you use a PNP. R4 will need to be a lower value to allow more current to flow try the transistor. Plan for the base current to be as much as 10% of the LED current.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Q3, when on, shorts out the LED. Is that your intention?
    You refer to 'more LED' and 'all LED' but you show only one. How do you plan to drive the other LEDs?
     
  5. mmc01

    Thread Starter New Member

    Jul 27, 2010
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    Actually I want turn on 4 LED series when dark and not have motion . And turn on more 4 LED when dark and have motion. I connect 4 LED to output LDR OpAmp circuit. And connect output from op amp to transistor input circuit like this image.

    I use PIR sensor like this http://www.adafruit.com/products/189 . I measure output from PIR it's 3.3V when have motion and will be 0.8V when have no motion. But output from opamp is 12V. So I don't know how to design circuit for And Gate. I use this circuit with battery 12V So want to make low power. I'm not sure TR will work all time or not. It may waste the power. This circuit LED not very bright .May be I have to use opto coupler in circuit. Can you help me how to design circuit for make LED very bright.

    [​IMG]
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    The Q3 looks like a basic voltage amplifier setup.

    The problem is that you don't need/want voltage amplifier.

    You want Q3 to be a switch.


    [​IMG]
     
  7. mmc01

    Thread Starter New Member

    Jul 27, 2010
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    Thank you but it alternate on/off LED. how to invert output on/off for LED
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the LEDs are white LEDs, 12V may (depending on the LED Vf) only be enough to drive 3 in series.
     
  9. mmc01

    Thread Starter New Member

    Jul 27, 2010
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    It work with this circuit from shteii01 but LED output alternate on/off. how to invert output on/off for LED .


    [​IMG]
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I think this will do what you want, if I've understood the requirements:
     
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