the observer and the observed
Tuesday, November 22 2005
Today I got the solar sufficiency circuitry working in the laboratory using the ULN2003 and then I applied it in the basement, where it also seemed to work as expected. I ran into problems, though, the moment I began trying to get the system to control the circulation of water. Since the control circuitry is a simple robot that is both an observer and doer, deploying it provided a vivid illustration of how observation can affect the observed. I quickly discovered that the thermistor probes could not effectively serve both the control circuitry and the digital cooking thermometers at the same time. Not only did my control circuitry throw off the displayed temperature on the digital readout by 100 degrees, but the digital thermometer circuitry introduced a strange pulsed waveform into my control circuitry. Evidently the digital thermometer polled the probe once each second, and every time it did so the resistance across it changed dramatically, fooling my control circuitry into thinking a momentary heat wave was upon us. But even without the the digital thermometer circuitry hooked up, the control circuitry wasn't able to do its job. The heat from the panel would tell it to turn on the pump, which it would, but then when the hot water had all passed through the basement thermistor and it registered cold water, which should have triggered a shutoff of the pump, the shutoff would immediately be answered by a mysterious "turn on" signal. I don't know if the relays were causing spikes that caused the flip flops or op amps to detect conditions that weren't there, but the result was lots and lots of clicking on the relays. There was so much confusion that one of the electric Grundfos water valves got stuck in the on position and I had to partially disassemble it in order to get it working normally again.
Obviously, I need to go back to the drawing board and develop a different sufficiency control prototype. In the meantime you can peruse a diagram of the circuitry I deployed today, as drafted using MultiSim (a fabulous and very expensive electronic circuitry designer available for free on most file sharing networks; even expensive information wants to be free).
Click to enlarge.
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