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fun and frustrations with a 74HC74
Friday, November 18 2005
I spent nearly all of my time today pursuing the goal of creating the circuit described yesterday, this time using a 74HC74 dual-D flip flop canibalized from an old school full-length ISA modem card. 74HC74 can be used in conditions similar to 4XXX CMOS, but because of the negative-going logic of the reset and preset I could avoid using NOT gates. I managed to get one of the flip flops working exactly as I wanted it to, but then for some reason when I wanted deploy a second parallel system using the other flip flop (ultimately both the slab and hot water will have panel sufficiency determined by separate controls), the whole thing broke and could not be made to work no matter what I did. The problem was clearly the flip flops, because the op amp stuff was still behaving acceptably. I must have either exceeded some rating of the 74HC74 and burned it out, or it had been mortally wounded by the temperatures needed to desolder it from the modem board. Or perhaps there was something I didn't know about 74XX74s that the second half of my life hadn't taught me (I've been playing with 74LS74s on and off since 1984).
I had a housecall down in New Paltz this evening that ended up lasting an exhausting four hours. This housecall business feels much more like work than lots of things I've done for money in my life. Well I remember the good old days when I'd report to work for a day filled mostly with casual web surfing and personal projects for which I was rewarded regular paychecks, reliable healthcare, and even free bagel Fridays.
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