Solar Powered Owl (Stumper)

Discussion in 'General Electronics Chat' started by PickyBiker, Aug 18, 2015.

  1. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    I have an Owl that turns it's head about once per minute when the sun shines. The circuit in it died and I want to make a replacement. It's an interesting challenge because there is no battery, just a 3V solar panel and 10,000 ufd capacitor to activate a tiny solenoid about once a minute. Here is the basic original operation:
    It starts with the big cap at 0V
    The sun begins to charge the cap
    After about 1 minute, the solenoid fires
    The solenoid firing discharge the big cap to near zero and the process starts again.
    The first cycle was longer than a minute, but subsequent cycles are somewhere around 1 minute each while the sun shines​

    I have tried a couple different ways to accomplish this, but so far nothing works. Basically I was trying to do this with an RC circuit driving a MOSFET which drives the solenoid.

    Ideas for how to do this are welcome
     
    Last edited: Aug 18, 2015
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    First. A picture of the capacitor and surrounding parts would help. There has to be at least one more element you haven't mentioned as the solenoid would not work with just the three parts you listed
     
  3. MikeML

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    Oct 2, 2009
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    There has to be a "breakover" negative-resistance element like a Diac or an old unijunction transistor somewhere. Basically as the capacitor charges there can be no conductive path to the motor other-wise the capacitor wouldn't charge. Only after a certain voltage is reached would the diac conduct, dumping the charge in capacitor into the solenoid.
     
  4. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    Yes, you are correct. The missing part is the circuit I am trying to design. A picture of the circuit board isn't going to help because it's sealed.
     
  5. Kermit2

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    Could it be something as simple as a 3 volt zener in series with the solenoid?
     
  6. MikeML

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    Any idea what the open-circuit voltage of the solar panel is?

    ps. I get to fly my avatar any time I want; do you get to fly yours?
     
  7. MikeML

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    no.
     
  8. Alec_t

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    Sep 17, 2013
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    A zener won't do it, but a unijunction transistor (or a simulated one using an NPN and a PNP) ought to.
     
    HarryH likes this.
  9. ian field

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    Oct 27, 2012
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    With the simulated programmable unijunction, you can set the breakover voltage to almost any voltage you want with a low voltage Zener.

    Just put the Zener across B/C of either one of the transistors.
     
  10. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    Here's what I am going to try next,
    YUP!
     
  11. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    Here is what I am going to try next. Owl.jpg
     
  12. ian field

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    Most TO220 style MOSFETs need about 6 - 8V on the gate to turn on.

    AFAICR: some of the TO92 types like 2N7000 and BS170 will switch at 3V or less.
     
  13. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    BTS141 switches at 1.3 to 2.2

    I think that would work
     
  14. ian field

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    No need to shout!!!
     
  15. PickyBiker

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    Aug 18, 2015
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    Sorry, I didn't mean to shout! I copied the BTS141 from a web page and it was large bold. When I typed after it, the format stayed.
     
  16. ian field

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    Don't forget to add up all the volt drops between the capacitor and gate.

    Someone else suggested the 2 transistor implementation of the programmable unijunction - I still think you'd do better with that.
     
  17. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    I think I'll research a PUJT solution, perhaps triggering an SCR.

    Thanks for all the suggestions.
     
  18. ian field

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    The transistor implementation can potentially be an SCR.

    The programmable unijunction is basically an SCR with the gate at the anode end instead of the cathode end.

    The transistor job is actually equivalent to a silicon controlled switch (SCS) which has both gates brought out on pins.
     
  19. MikeML

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    I have an idea, but I need to know the following:

    What is the open-circuit voltage of the solar panel?
    What is the resistance of the solenoid?
     
  20. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    In bright sun the open circuit solar cell voltage is 5.94V. The dc resistance of the solenoid is 3.4 ohms.
     
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