Modifying a solar garden light for 12 volt

Discussion in 'The Projects Forum' started by malabarmcgyver, Jan 21, 2010.

  1. malabarmcgyver

    Thread Starter New Member

    Jan 21, 2010
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    I am attempting to utilize the solar panel and/or LDR of solar garden light to turn off a 12 volt dc circuit.

    I am reutilizing the case of the garden light to insert a 12 volt, 60 mA circuit and would like to use either the solar panel and/or light detecting resistor to turn off the circuit during the day. I tried using the LDR first with an NPN transistor and NPN Darlington but it would only decrease the current from 60 mA to around 30 mA. I then starting working with the 3 volt, 30 microamp solar panel with a PNP transistor, but couldn't drive the transistor into saturation with the panel.

    Any ideas?

    Thanks in advance.

    Jeff
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Is solar panel 30 uA or 30 mA? Or maybe not part of garden light. Check resistance range of LDR. Did you want ckt to turn off with light?
     
  4. malabarmcgyver

    Thread Starter New Member

    Jan 21, 2010
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    Retched,

    That is almost exactly the LDR circuit that I used. I couldn't get the base current high enough to saturate the NPN. The LDR range was 20K to 600K. I used two 10K pots.

    Bernard,

    The solar panel is 3 volt and at best was 120 microamps. I want the LDR or solar panel to be the component to turn off the 12 volt 60 mA circuit when ambient light is present.


    I haven't done this stuff in years, so I suspect that I am just doing something stupid. By using both the LDR and the solar panel I can get the transistor into active mode and significantly decrease the collector current by shielding the light source, but if I get either into saturation, then shielding the light source has no effect.

    Thanks!

    Jeff
     
  5. malabarmcgyver

    Thread Starter New Member

    Jan 21, 2010
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    I have a 2n3906 PNP with 12 volt positive at the emitter. I have the load, which is 4 LEDs, connected with anode to the collector and cathode to ground, and I have the 3 volt solar panel with the positive output to the base and the negative to ground.

    The circuit works opposite of how I think it should. Under lighting, with about 2.3 volts to the base, the load at the collector is energized at about 45 mA collector current, which is about 80% of full current of the load, so it is near saturation. When I shield the solar panel, the collector current is zero or cutoff.

    Can someone explain what I am doing wrong?

    Thanks,

    Jeff
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Maybe this is not too far removed with what you tried.
     
  7. malabarmcgyver

    Thread Starter New Member

    Jan 21, 2010
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    After several hours of unsuccessful testing, and trying other methods, I finally discovered my major error. The data sheet for the 2N3904 indicated Emitter, Base, Collector across the T0-92 style transistor. I was reviewing my invoice from Mouser Electronics and in the description, it states that the collector is the middle pin although the Fairchild data sheet doesn't even list this as an option. When I hooked it up correctly, it works perfectly.

    Thanks for your time and consideration!

    Jeff
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    I think Mouser knows nothing about electronic parts.
     
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