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My thermostat runs on batteries but it has an LCD, similar to (http://smile.amazon.com/Honeywell-RTH2300B1012-5-2-Programmable-Thermostat/dp/B007BHLUWM/ref=sr_1_1?ie=UTF8&qid=1425045609&sr=8-1&keywords=thermostat) but I thought any type of display (LCD) would suck up the battery. Am I missing something, is this a special type of display?

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    \$\begingroup\$ It's the backlight that sucks the power. If your thermostat has a backlight at all, you'll see that it is very low power and is only activated for a few seconds at a time. \$\endgroup\$
    – Dave Tweed
    Feb 27 '15 at 16:02
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LCDs (liquid crystal displays) require very little power to operate. The liquid crystal looks mostly like a small capacitor. To keep the liquid crystal in the dark state, a few volts of AC at a few kHz or so must be applied across this capacitor. That's not a lot of power.

This is exactly why you see LCD readouts and displays in low power battery operated devices to often. Think of a wristwatch. Those run for a year or more on a tiny battery.

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  • \$\begingroup\$ I have a wristwatch from the late 90s, it is still running... (on the same battery) \$\endgroup\$
    – PlasmaHH
    Feb 27 '15 at 14:19
  • \$\begingroup\$ TI's Chronos project is designed showcase extremely low power design driving an LCD. It uses their popular MSP430 processor, but the same sort of performance is possible using AVR or PIC chips. \$\endgroup\$
    – bigjosh
    Feb 27 '15 at 14:29
  • \$\begingroup\$ Also, whenever the thermostat is not demanding heat (or cooling), it can use the open-circuit voltage across the contacts to supplement the battery power. Since we're only talking about microamps, the leakage current is not enough to activate the HVAC. \$\endgroup\$
    – Dave Tweed
    Feb 27 '15 at 15:57
  • \$\begingroup\$ @Ryan: Huh, what? Nobody said anything about bad or good, what the parameters of OK versus not OK are, or even what that would apply to. Your question makes no sense whatsoever. \$\endgroup\$ Feb 27 '15 at 16:44

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