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no flip flops necessary
Sunday, November 27 2005
I was back at work on the electronics of the solar heat sufficiency circuitry again, this time creating a system less responsive to glitches and voltage spikes. One of the main innovations of this second version was the inclusion of a pair of NAND logic gates to keep the flip flops from being reset by the basement heat insufficiency condition unless there was also heat insufficiency in the solar panel. In the end, it turned out that the logic I needed didn't even require a flip flop. It was as simple as this: if there is heat in either the solar panel or the line from the solar panel down in the basement, open the valves and run the pump. Otherwise turn it off. Sometimes it takes a long strange journey to figure out the simplest of things. I actually did leave the flip flops in the circuit and their presence makes the logic a little different from the simple formula just stated. Only heat in the panel can turn on the pump, but once the pump is on, a loss of heat in both the panel and in the basement is the condition that shuts it off. Just what constitutes "heat" is still selected by potentiometers, but I ended up wiring them as variable resistors so they'd be more like the thermistors and the two would be easier to compare in the LM339 comparator chip.
This evening our friends Dave and Penny came over with a bunch of Chinese take away, including one dish containing shrimp. Traif was fine with Gretchen so long as we ate it on paper plates. Amusingly, though Dave and Penny are both Jewish, I've overheard Gretchen giving Penny Judaism 101 lectures, stuff I've known since 1989.
As we sat in front of the fire drinking beers, Penny gave us a long account of her battle with bedbugs back in 1998. It all started with a day of pulling poison ivy for a friend (something she can do because she isn't allergic). The next day she woke up to find two mysterious bumps on her arm. At first she thought her days of immunity to poison ivy might have come to an end. But the bumps didn't go away, and were soon joined by others. Eventually there were so many that she decided to visit a dermatologist, who told her the bumps were insect bites, but from what? The bumps continued to accumulate, even spreading up her neck and onto her face. Her then-boyfriend began suggesting she wear a sweater in public. Doctors became concerned and expressed fear that perhaps the bumps were evidence of cancer. Biopsies were performed. Then one night Penny woke up and switched on a light, whereupon she became aware of thousands of bugs crawling on her bed. She still has that apartment in Manhattan, though she's been through three mattresses since the bedbugs tipped their hand.
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