How does this solar powered robot work?

Discussion in 'General Electronics Chat' started by 8-bit, Mar 25, 2014.

  1. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    I am going to build this 'Solar Bot' but like more circuits I am not entirely sure how it works. I have read a overview but would like a little more detailed explanation. I seem to suffer a very annoying condition where I understand how components work but not how they work together in particular circuits, I hope that comes with experience! :s

    I understand that the capacitors fill until they are full but I cannot figure out how they discharge as far as I know you have to remove voltage from capacitors for them to do so.

    This is the circuit....

    http://bjcovertaction.weebly.com/1/post/2010/08/symet.html
     
    Last edited: Mar 26, 2014
  2. MrChips

    Moderator

    Oct 2, 2009
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    I moved your post.

    A title "Circuit help" is too generic.
    I used a title that provides more information about the topic.
     
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  3. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Thanks. The post continually failed to get submitted in other forums on here and was advised doing it that way.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    The robot has two modes, charge and run, that it cycles between based on the voltage on the capacitor bank. It charges until the voltage comes up far enough to turn on the 3904. This turns off the 3906. When the voltage falls, the state reverses until the caps charge up enough again.
     
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  5. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Thanks for the reply.

    Do the capacitors all charge at once or one at a time, and do they all discharge at once and how is this achieve if there is still a voltage across them. Also I am guessing the transistors turn each other on and off? I have never seen this kind of setup before. Many thanks again for your help and sorry for soo many questions! :)
     
    Last edited: Mar 26, 2014
  6. wayneh

    Expert

    Sep 9, 2010
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    The capacitors are all in parallel and behave as one single, larger capacitor. The voltage rises/drops gradually in proportion to the charge (current x time) added/removed. I don't fully understand the flip-flop functioning but the transistors respond to the voltage on their base pins. The flip-flop has a deadband or hysteresis, so that the system does not just hit some steady-state. Instead, it oscillates.
     
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  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It looks like the flashing LED supplies the oscillation, 2N3906 amplifies, 2N3904 drives?
    Google BEAM bot
     
    Last edited: Mar 27, 2014
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  8. 8-bit

    Thread Starter New Member

    Jan 7, 2014
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    Thanks for the reply.

    I have, several times. I guess my understanding of circuits is still pretty basic or I'm simply not up to it. :confused:
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Oscillators always confuse me but here's what I see for this one.

    If the capacitor bank is at low charge and thus low voltage, the LED cannot light. The voltage across it is less than the required Vf for that LED. So any available current from the panel goes to charging the capacitors. With no current thru the resistor, the 3906 sees full system voltage on its base and it is therefore not conducting. That means the base of the 3904 is low and the 3904 is thus providing high resistance.

    Eventually, the voltage at the LED reaches a high enough level to allow it to conduct. This allows current across the resistor, causing a ∆V. This lowers the base voltage on the 3906 and turns it on. This passes higher voltage to the base of the 3904 and turns on the motor. The LED no longer sees enough voltage to stay lit but I believe the two transistors are latched on. As long as current is flowing thru the motor, the base of the 3906 is held lower than Vcc, and the transistor remains "on". Positive feedback.

    A burst of current passes through the motor until it draws the capacitor voltage down until little current flows. The 3906 stops conducting and the 3904 is turned off as well when the system voltage falls to ~0.7V. Both transistors quit conducting at that point. Then everything stays off until the voltage rebuilds.
     
    Last edited: Mar 27, 2014
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  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I tried to show V fluctuaton on FLED but my scope died, meter too slow to show lower V , prob. 3V with white LED. Started operating just below 3 V; red might drop to 2 V. Output is square wave going from 3V to supply V. In OPs ckt it only flashes once per charge/ discharge cycle.
     
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