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Thread: Led run thru/through/threw/thrue a cap

  1. #1
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    Default Led run thru/through/threw/thrue a cap

    Hi
    I was wondering if any of you cool people ever used this way to light a led
    and if it would save power... I haven't tested the usage yet but my horrible thinking
    is that when the picaxe pin goes high it powers up the led and fills the capacitor then when the pin
    goes low it uses the capacitor power (free power) ...or not...but maybe ?
    If I use PWM then both leds are on which is cool .... I'll do more tests just a idea I'm trying

    your friend
    me
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    I don't see it as a power saver - capacitor voltage charges and discharges exponentially losing power both ways as well as varying the current through the LED... could produce some interesting visual effects... ;-0

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    Now, if you hooked those two wires to your bicycle wheel where you have a generator of "free electricity", then you could pedal the bicycle with some of your "free energy" and power many more LEDs and capacitors.

    Plus, you can say it is "all green" and produced with "free stuff" that everyone was going to throw away and save our planet!
    - Tex
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    Hi,

    Yes, the capacitor could be considered as a "power-saving" method, but only if the supply rail is "higher than it needs to be". Let's simplify the situation:

    Suppose the LEDs are the same colour (same forward voltage drop), are driven by a 50:50 duty cycle (square wave) of reasonable frequency and the voltage drops across the PICaxe's pin-driver FETs are similar. Then, the average voltage across the capacitor will be one half of the supply rail, which tells us that the supply rail needs to be at least twice the LEDs' forward voltage drop. It gets more complicated if we consider the ripple voltage across the capacitor, etc., but the principle is much the same.

    If the power dissipated in the resistor (voltage drop) is considered as "wasted" and power in the LEDs is "useful", then reducing the effective supply voltage with a capacitor is indeed "power-saving". But it's also highly restrictive: The average current through the two LEDs must be the same (there can be no "dc" current through the capacitor), so your Red and Green LEDs become Yellow, where you can change the brightness (only) by changing the "PWM" frequency (or brief flashes of alternatively Red and Green if the capacitor is large enough).

    A much more flexible method to control two LEDs on one pin is to choose a supply rail which is a little less than the sum of the (minimum) forward voltage drops of the two LEDs. Connect the LEDs in series across the supply rail (that's why the supply voltage must be less) and drive their junction from a PICaxe pin via a resistor. Now you can drive either LED to "full" brightness, produce all shades of Yellow inbetween Red and Green by using PWM, or reduce the brightness by tri-stating the output (making it periodically an input), but only under program control, not with the PWM hardware.

    Cheers, Alan.

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    I don't see it as a power saver
    @ premelec ... well shoot...I thought was on to something was going to sell it in a kit ..called like the "PFLPS" or for short
    "PICAXE FLASHING LED POWER SAVER" ...joking ...but it does show interesting effects

    @texas sad thing is i don't own a bike ... but I do try to use alot of parts poeple throw away


    btw could i call it recycling if I use peoples junk and then test it the wrong way and it ends up in the garbage anyway?
    maybe i can call it "attemprecycling"


    Wow thanks Alan didn't see your post till now ...I like the simplified explanation ..

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    Quote Originally Posted by Allycat
    A much more flexible method to control two LEDs on one pin is to choose a supply rail which is a little less than the sum of the (minimum) forward voltage drops of the two LEDs. Connect the LEDs in series across the supply rail (that's why the supply voltage must be less) and drive their junction from a PICaxe pin via a resistor. Now you can drive either LED to "full" brightness, produce all shades of Yellow inbetween Red and Green by using PWM, or reduce the brightness by tri-stating the output (making it periodically an input), but only under program control, not with the PWM hardware.
    So the sum if both leds are 2.2 voltage drop would be 4.4
    I guess I should draw up something to be on the same page

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    Hi,

    Yes, but a better configuration would be to put a resistor in series with each LED and take the PICaxe output pin directly to the "mid point". That will give independent control of the maximum brightness/current for each LED, and reduce the "shoot through" current if the supply rail is too high.

    Cheers, Alan.

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    Hi

    @allycat here is my drawing of controlling both leds with one pin
    maybe there is a better way but since when you mentioned shoot through
    I take as there is enough power to turn on both leds with out any power involved from the chip
    which is true ...but as soon as one led is on from control of the picaxe high or low the other led will shut off
    because power takes the easiest path ... <--- i think this is first grade stuff lol but cool for me
    so on leds A and B ...I doubled them in series so AA-res ...BB-res...so now I can have only 2 leds on at any time
    incase I don't want one led bright and another one dim
    hopefully it makes sense
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    After thinking way out of this world .....has anyone came up with a way to run RGB leds on one picaxe pin?
    maybe its not possible
    first thought is the voltage drops on red-green-blue are different so I might have something here .......or NOT
    but I'm going to give it a try ....

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    How about hundreds through 2 pins? APA102... or an external port expander - where are you trying to go with this?

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