Please help me understand this schematic

Posted by Kromey at 3:35pm Sep 28 '13
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This is a modified version of the schematic of a "3D Xmas Tree", a mini soldering kit I bought from Radio Shack. This version is one that I prototyped out on a breadboard so I could tweak things and figure out what's going on and why this works.

Unfortunately, I'm still confused. :-(

[Note: For simplicity's sake, I will refer to *1 as being the group of components labeled R1, D1, D2, D3, and T1, and *2 as R2, D4, D5, D6, and T2.]
[IMPORTANT! Ignore the product numbers on the LEDs; I don't actually have product numbers for the ones I used/that came in the kit, but couldn't figure out how to make them go away. I can, however, tell you they are "standard" -- in the absence of the magic of this circuit, they do not blink.]
[Also, sorry for the fugliness of the wires going into the transistors' bases, the schematic editor wasn't all that intuitive to get wires to cross without joining and, while I didn't spend a whole lot of time on it, this was the only way I could get them to display correctly, i.e. not joined.]

When power is applied to this circuit, the LEDs in *1 will turn on for ~1 second, then turn off while those in *2 turn on for the ~1 sec; rinse and repeat forever. From my tinkering, the magic of the blinking comes from the capacitors -- remove them, and all the LEDs just stay lit.

Now, I'm assuming the differences in resistance between R3 and R4 are so that one side will start first (and, indeed, the side with R3, which has lower resistance, is always the first to blink on). But here's what I don't understand: The capacitors are wired in parallel to the other components, so how do they have this magical effect of turning the LEDs on and off??

In fact, I can pull the capacitors out of this circuit, and it still works -- albeit the LEDs stay lit rather than blink. Which proves a) they are the source of the blinking magic, and b) they are indeed in parallel with other components (because the LEDs wouldn't light if they were in series).

Basically, I've never properly understood capacitors. Or rather, I've never understood their interactions with other components in a circuit. I know they store up energy and then release it, but how that works in a circuit like this one has always confounded me.

Can anyone explain to me what is going on here, and why this circuit blinks the LEDs in sequence?

Bonus Round Questions!

1) Without showing me how, is it possible to extend this circuit so that I could have more sets of LEDs blinking in sequence? I'd love to build a line of LEDs that "sweep" a la Cylons; I know I could do it with an MCU, but if I could instead do it with discrete electrical components like this that'd be so much cooler (mostly because it would be new to me).

2) Anyone know of a good (preferably free) schematic editor? The one I whipped this one up on lets you use it for 60 minutes per week without paying, but won't let you save your schematics, nor export them as images, until you fork up some dough. (I took a screenshot and then used Paint to crop it and save it to show it here.) I found one one time that was obscenely difficult to use, couldn't even figure out how to do a simple battery-resistor-LED schematic. I don't really need simulation capabilities, just something where I can mock up some designs before I slap them into my breadboard....

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