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FRED SE Circuit Analysis

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  • Richard Caudle <frankendaddy@yahoo.com>
    Anybody got one or an addy handy? Thanks, Richard
    Message 1 of 5 , Mar 2 6:11 PM
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      Anybody got one or an addy handy?

      Thanks,

      Richard
    • Gord
      Why not check out the Daddy of fred at http://wollongong.apana.org.au/~ben/fred/index.html Gord ... __________________________________________________ Do you
      Message 2 of 5 , Mar 2 7:28 PM
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        Why not check out the Daddy of fred at
        http://wollongong.apana.org.au/~ben/fred/index.html

        Gord

        --- "Richard Caudle <frankendaddy@...>" <frankendaddy@...>
        wrote:
        > Anybody got one or an addy handy?
        >
        > Thanks,
        >
        > Richard
        >
        >
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      • Steve Jones
        I am sure you would find one in the archives or could email Ben Hitchcock about the FRED SE Circuit Analysis ... From: To:
        Message 3 of 5 , Mar 3 2:38 AM
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          I am sure you would find one in the archives or could email Ben Hitchcock
          about the FRED SE Circuit Analysis

          ----- Original Message -----
          From: <frankendaddy@...>
          To: <beam@yahoogroups.com>
          Sent: Sunday, March 02, 2003 9:11 PM
          Subject: [beam] FRED SE Circuit Analysis


          > Anybody got one or an addy handy?
          >
          > Thanks,
          >
          > Richard
          >
          >
          > To unsubscribe from this group, send an email to:
          > beam-unsubscribe@egroups.com
          >
          >
          >
          > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
          >
          >
        • nebbian <beh01@uow.edu.au>
          ... If there isn t one readily available then it s probably time I put one up :-) I ll have a go at describing how it works, but there are probably one or two
          Message 4 of 5 , Mar 3 5:10 PM
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            --- In beam@yahoogroups.com, "Richard Caudle <frankendaddy@y...>"
            <frankendaddy@y...> wrote:
            > Anybody got one or an addy handy?
            >
            > Thanks,
            >
            > Richard

            If there isn't one readily available then it's probably time I put
            one up :-) I'll have a go at describing how it works, but there are
            probably one or two subtleties that I've missed. Let me know if
            something isn't all that clear. There's more to Fred than meets the
            eye!
            Note that I built the first Fred long before I understood how it all
            works, it's only recently that I've gotten a full understanding of
            the nuts and bolts of what actually goes on. OK, here goes.

            FRED SE Circuit Analysis:

            Fred robots, when set up in the 'photopopper' configuration, consist
            of two identical and almost completely independant solar engines.
            These SE's consist of three main parts:
            1) FLED based oscillator
            2) Amplification circuitry
            3) Latch
            An explanation of each part follows.

            1) FLED based oscillator
            This is the 'heart' of the Fred engine. It consists of a FLED, which
            is fed through a 33k resistor. When the FLED turns on, it sucks a
            large amount of current through the 33k resistor, and the voltage on
            the anode of the FLED lowers. When the FLED turns off, it sucks a
            lot less current, so the voltage on the anode rises. This means that
            you get a 2 Hz square wave on the anode of the FLED.

            The amplitude of this square wave is dependant on two things: supply
            voltage, and ambient light. The more supply voltage, the larger the
            amplitude. More ambient light means the FLED won't suck as much
            current when it's ON, so the amplitude will decrease. Less light
            means more amplitude.

            The final part of the FLED based oscillator is the 0.22 uF capacitor,
            which connects the square wave to the amplification circuitry. This
            capacitor is there to only allow the falling edge of the square
            wave 'through' to the amplification circuitry. This makes the Fred
            SE quite efficient. If you look at the voltage on the other side of
            the capacitor, it looks like a 2 Hz spike, with an amplitude that is
            dependant on supply voltage, and ambient light. More volts and less
            light means a bigger spike.

            2) Amplification circuitry
            This is the PNP transistor. Its job is to make the spike have
            more 'oomph', because the spike on its own isn't enough to drive the
            motor driver transistor. So the PNP is set up to amplify the spike
            as much as possible, and drive the motor driver transistor as hard as
            it can. The gain of this transistor is critical -- you need a gain
            of at least 150 or so for Fred to be reliable. I use BC327's because
            they have a gain of 300 - 400 (from memory).

            3) Latch
            This is the combination of the NPN motor driver transistor, the 3.3k
            resistor, and the PNP transistor. When enough current comes down
            through the PNP transistor to turn the NPN slightly on, the circuit
            will 'feed back' through the 3.3k resistor and turn the PNP on
            harder. This turns the NPN on more, and you get a sudden cascade
            where the two transistors latch each other on.
            The motor is set up to spin whenever the NPN transistor is on. Once
            the two transistors are triggered, there isn't much that will turn
            them off, apart from the supply voltage dropping below the threshold
            for the circuit (which is somewhere around 1 Volt or so). When the
            supply voltage drops enough, the two transistors can't supply enough
            current to keep each other turned on, and so they both turn off at
            the same time.



            And then the cycle repeats!

            The important parts to remember are:
            1) The amplitude of the square wave (and therefore the 'spike') is
            dependant on supply voltage. Therefore the Fred solar engine won't
            trigger if the supply voltage isn't above 2.7 volts or so.
            2) The amplitude of the square wave (and trigger) is also dependant
            apon how much ambient light there is. Therefore a Fred with full
            sunlight pouring into the FLED might not fire, and will
            therefore 'lock up'. This is why people put heatshrink around their
            FLEDs.


            Now for the complete Fred 'popper':
            Once you understand how the indepandant solar engines work, you are
            ready to consider the dynamics of puting two Fred SE's together.
            These are coupled in two ways:
            1) Electrically (through the supply rail).
            2) Mechanically (the two SE's are physically connected together).

            The electrical coupling means that both SE's will be constantly
            trying to trigger, but the first SE to trigger will make the supply
            voltage drop, and therefore stop the other SE from getting that tiny
            bit of extra voltage it needed to fire.
            In other words, the dark side fires first.

            The mechanical coupling is quite loose, and means that if side 1 is
            in brightness then the side 2 will fire more often. After a while
            side 2 will turn the body of the Fred around, so that side 1
            is 'darker' than it was before, and therefore more likely to fire
            first.

            Bit tricky to explain properly, hopefully this helps.


            Cheers,
            Ben
          • Wilf Rigter
            Nice understandable description of operation! wilf ... From: nebbian To: beam@yahoogroups.com Sent: Monday, March 03, 2003 5:10 PM Subject:
            Message 5 of 5 , Mar 3 5:29 PM
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              Nice understandable description of operation!
               
              wilf
              ----- Original Message -----
              Sent: Monday, March 03, 2003 5:10 PM
              Subject: [beam] Re: FRED SE Circuit Analysis

              --- In beam@yahoogroups.com, "Richard Caudle <frankendaddy@y...>"
              <frankendaddy@y...> wrote:
              > Anybody got one or an addy handy?
              >
              > Thanks,
              >
              > Richard

              If there isn't one readily available then it's probably time I put
              one up :-)  I'll have a go at describing how it works, but there are
              probably one or two subtleties that I've missed.  Let me know if
              something isn't all that clear.  There's more to Fred than meets the
              eye! 
              Note that I built the first Fred long before I understood how it all
              works, it's only recently that I've gotten a full understanding of
              the nuts and bolts of what actually goes on.  OK, here goes.

              FRED SE Circuit Analysis:


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