DIY WiFi Programmable Thermostat

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basicchip
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Re: DIY WiFi Programmable Thermostat

Post by basicchip » Wed Apr 20, 2016 6:08 pm

Just checked on the TSOP version of the MCP73871, while on the datasheet, it doesn't seem to really exist. I was going to get a couple samples from MicroChip. They are usually a good source for a couple devices.

So I guess I am going to try my hand at soldering a small QFN with my hot air station. I know that may beyond most hobbyists, but hot air stations are really useful and not all that expensive on eBay.

Here is a really good howto

http://hackaday.com/2010/05/04/qfn-or-m ... der-paste/



basicchip
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Re: DIY WiFi Programmable Thermostat

Post by basicchip » Sun Apr 24, 2016 11:28 pm

Not much progress this weekend, as I took time out to attend a hackathon. I was under the impression these were to hack into websites/servers whatever. Actually this one was sponsored by Microsoft and NASA to get a bunch of people for a defined period of time (32 hours for us) to build a project mostly from scratch.
spaceapps.jpg
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Interesting, and I learned a few new programming tricks, met some local techie types. Getting dangerous again.

basicchip
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Joined: Fri Oct 19, 2012 2:39 am
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Re: DIY WiFi Programmable Thermostat

Post by basicchip » Wed May 04, 2016 11:42 pm

Finally back at it. I was originally thinking I could steal the power supply from the existing thermostats, but as the 2 examples I found so far ran at digital clock like power levels. So here is a design that will accept a single thermostat control pair and charge a battery from that, or take power from a heat/cool 3 wire setup. The bridge rectifiers charge a big cap and that is switched down to 5V using an OKI-78SR which is a quick way to handle that efficiently.

The optional MCP73871 charges the battery and supplies power when the batter is not place. That will be the first one I build. Without the battery charge circuit, I can run a spare 24VAC line for power and just jumper the power to the Vin of the Lua module, this assumes I use the Lua module, which are easier to solder.
thermo.pdf
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Now onto board layout, and some probing of the thermostats...

basicchip
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Re: DIY WiFi Programmable Thermostat

Post by basicchip » Sat May 14, 2016 11:40 pm

Back at the teardown of the existing thermostat. An interesting piece of 60s and 90s technology. The base board all thru hole, a half dozen transistors, a triac and triac driver for the heat demand switch. Looks like 2 different diode bridges, not sure why that is and an LED not visible from anywhere, must be used in testing. I won't bother tracing out too much of the base board, as all I want to save is the display board. The display board has a single chip micro mounted as chip on board, display, a pot and some bypass caps. The bypass caps are still through hole (really??). It runs off between 5 and 50 microAmps at 1.6V. And at least it keeps temperature, not sure I can fool it into indicating on or not, might just have to add an LED for that.

After reading a number of blogs I see this also has a bi-metal fail safe which shorts out when cold (the copper thing I bent away from the board). Got the old can of cold spray out and verified that. Not sure what it is set at probably around 40 degrees. Seems like newer one has no such thing, and the newer thermostats don't seem to have that anymore. I assume with fewer and all SMT parts reliability is improved by orders of magnitude.
parts.jpg
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Not sure what the need for the temp sensor on the battery is for (the small black thing hanging below the display board), I doubt they are trying to charge the silver type battery.

basicchip
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Re: DIY WiFi Programmable Thermostat

Post by basicchip » Sun May 15, 2016 12:52 am

actually that is the temp sensor for the whole thermostat hanging off for better thermal isolation. It seems to be checked every few seconds. In the upper left hand corner of the display board there are 4 jumpers, not sure what those select. Looking to figure out what some of the other 6 connections do.

basicchip
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Re: DIY WiFi Programmable Thermostat

Post by basicchip » Mon May 23, 2016 5:22 pm

Finishing up custom PCB, here are the connections to the micro/LCD board.
thermo_connect.jpg
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