I do not at all consider myself good at analog circuitry. It's one of those things that intrigues me conceptually, but has mostly eluded me in the few times I've found the time [and courage!] to try again over the last three decades.

Thus, seeing mention of the "Analog Challenge"  on The Blog gave me a bit of a forlorn sigh...

Until it finally came to me...

Somehow I'd forgotten this project, my proud analog moment of a few years back!

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This project is rather ridiculous, if you think about the number of transistors involved, and the crazy slew/arrangement of support components. And, I'd kinda prided myself on the fact it involed only one active component, heh! And a zener? Really? Can that be considered "passive?" 

But, at the time I didn't have my TTL collection with me, so I thought I'd see what could be done with what I had on-hand [What weird stage in my life did I go through when I'd have comparators with me but not TTL?!]

Progress was also motivated by the thought that maybe someday I could use the same inspiration for this circuit [a pen-clicker] to acheive similar with a single transistor. A fellow Hackadayer beat me to that (link in this project's description), which I actually found to be a bit of a relief, since this design relies heavily on the fast/full-swing switching of a comparator, which surely a single transistor wouldn't provide... 

...And, frankly, I'd kinda grown to love the ridiculousness of this circuit's excessive support-circuitry, when a few transistors in a digital 7474-like design could've surely done the job without any "spooky analog" weirdness like slightly-differing capacitor-charging delays or voltage-dividers outputting only slightly different values, or a friggin voltage-regulator, heh! And, frankly, how much more sophisticated, internally, a comparator is than a few NAND gates. 

And to think, it actually worked; this crazy analog design from a guy who had had little other than bad luck in decades of attempts designing analog!

...

Hmmm...

So, what about an Op-Amp?

I mean, they're just comparators with lower gain and larger delays, right? Albeit, internally even more complicated and precise... Further adding to this project's appealing ridiculosity...

Challenge Accepted.

Maybe I'll luck-out and it's a simple drop-in replacement[?]. But, as part of my own added challenge, I think I'll again attempt this with parts on-hand. Which basically amounts to LM-series from the 80's. We'll See!