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Three Fives + Larson

The data sheet for the Three Fives Kit (Rev 2.01, Aug 2014) has your analog Larson circuit, but without the low-pass transistor filters on each LED.  Is this to keep the schematic simpler since the focus of this data sheet is the giant 555 rather than the fading LEDs?


  • Yes, exactly.  It looks better (to us, at least) with the low pass, but this does keep it simpler. :D
  • Now that I've built your newer (2313) version and your older analog one (555+4017), I'd love to better understand why the transistors are helpful.  The output from the 4017 pin is passed through a 47k resistor to the base of a 3904, which is also connected to a 47uF capacitor, together forming a low-pass filter, right?  Assuming a 9V power supply, the resistor drops the current from the 4017 to about .2 milliamps, which the 3904 then boosts it back up by 100X to about 20milliamps for the LED, right?  But doesn't a low-pass filter work without the transistor too?  That is, could the 47k resistor and 47uF capacitor be replaced with a 470 ohm resistor and 470nF capacitor and get the same effect?  Or am I missing something?

    For what it's worth, I tried this on a breadboard plus oscilloscope, and it seemed to partially work.  The LEDs faded off but snapped on, and the fading out may have been shorter and thus not as nice.  But I also may have not had the right component values or had it hooked up quite right.  (I find working with breadboards very frustrating because I don't want to cut the leads off the diodes, resistors, or capacitors, but then its hard to keep them from touching and shorting out without realizing that I'm inadvertently bypassing a diode or other component.)

    Anyway, none of this is important, I was just curious why you made the original low-pass filters with transistors rather than just resistors and capacitors.  Thanks as always.
  • Had we done this more recently, we might have approached it differently. In a sense, we did, with the digital version.  

    I don't exactly recall the original reasoning that we used for this construction, but one thing to note is that the CD4017 only outputs up to about 8 mA per pin. With the transistors, you can get it quite a bit brighter. It's not particularly uniform in the intensity profile, but it was (to us at least) an improvement over not using the low-pass circuit.
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