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Schematic for a Honeywell Thermostat — Parallax Forums

Schematic for a Honeywell Thermostat

Chris SavageChris Savage Parallax EngineeringPosts: 14,406
edited 2013-08-14 14:32 in General Discussion
Just checking to see if anyone might have a schematic for one of these Honeywell Thermostats.

2013-01-05_13-35-36_871.jpg
2013-01-05_13-35-36_871.jpg
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Comments

  • JonnyMacJonnyMac Posts: 9,108
    edited 2013-08-10 06:57
    Are you going to roll your own thermostat, Chris? I've long wanted to do that but am unsure about connecting to the HVAC side of the equation -- and don't want to break that part!
  • SapphireSapphire Posts: 496
    edited 2013-08-10 12:23
    Jon,

    Here's a recent discussion on that: http://forums.parallax.com/showthread.php/149531-How-to-override-HVAC-outputs-via-BS2
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2013-08-10 15:56
    JonnyMac wrote: »
    Are you going to roll your own thermostat, Chris? I've long wanted to do that but am unsure about connecting to the HVAC side of the equation -- and don't want to break that part!

    Jon,

    Yes, Sapphire posting something that got me back on track (he's done the research) and I am ready to move forward, however I wanted to see how this existing unit handled a few things before I started. Admittedly I didn't know enough about the signals before to continue.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2013-08-12 18:31
    Chris Savage wrote: »
    I wanted to see how this existing unit handled a few things before I started. Admittedly I didn't know enough about the signals before to continue.

    Ask away and I will try explain it to you best way I know how

    In a past live I use to be a HVAC tech

    I put a few thing in that post as well

    http://forums.parallax.com/showthrea...utputs-via-BS2
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2013-08-12 19:23
    Mainly I was trying to find out what components this unit is using...for example, I thought it had a relay in it, but it seems triacs are generally used in these devices. A schematic would help clarify that.
  • Don MDon M Posts: 1,652
    edited 2013-08-12 19:30
    Chris- It really doesn't matter. You could use either. You're only dealing with 24VAC controls. You would need a relay/triac (3) for fan, cooling and heating. Multi stage may be a bit trickier but most installations are single stage so 4 wires- common, fan, heat and cool.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2013-08-12 19:39
    Don M wrote: »
    Chris- It really doesn't matter. You could use either. You're only dealing with 24VAC controls. You would need a relay/triac (3) for fan, cooling and heating. Multi stage may be a bit trickier but most installations are single stage so 4 wires- common, fan, heat and cool.

    That all you need
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2013-08-12 19:50
    I completely understand and of course the triacs will use less power, however I am still curious about the internals of an existing unit and thought someone may have a schematic.
  • kwinnkwinn Posts: 8,697
    edited 2013-08-12 20:45
    Chris, most thermostats now consist a small microcontroller, a temperature sensor (lm34 or thermistor), and 1 to 3 triacs or relays. Even with a schematic there is not much to see.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2013-08-13 09:51
    Call it curiosity...I miss the old days (pre-90s) when you could get a schematic for a device from Sam's publishing. Sam's Photo Facts I think they were called. Fixed many TVs, Monitors and Stereos with that information.
  • Don MDon M Posts: 1,652
    edited 2013-08-13 11:46
    Chris Savage wrote: »
    Call it curiosity...I miss the old days (pre-90s) when you could get a schematic for a device from Sam's publishing. Sam's Photo Facts I think they were called. Fixed many TVs, Monitors and Stereos with that information.

    Whoa! Does that bring back some memories! I remember as a child growing up and hanging out in my dad's tv and appliance store, all the filing cabinets full of Sams. Such a memory... they are all gone now. Photofact of the month club...
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-08-13 12:32
    I too miss the good old days when everything had a published schematic, but these days it is all about reverse engineering... if you can. Having boards with 4 or more layers in them has made reverse engineering much more of a challenge.

    I suspect that if you really want to see what's inside, you are just going to have to take one apart. Honeywell is just like most manufacturers these days... no reason for publishing a schematic as there are no service-able parts inside. A schematic would just inspire a competitor to clone the design.
  • ercoerco Posts: 20,256
    edited 2013-08-13 17:20
    Slightly OT, but I always thought the older mechanical thermostats with a mercury switch and a bimetallic coil were pure genius. The nearly-level mercury ampoule's CG would change as the mercury shifted right to left, which (combined with the coil's flexibility) mechanically added hysteresis to the system for snap-action switching to avoid a high-resistance connection at the transition temperature. Even in today's age of cheap and abundant microprocessors, I still appreciate an elegantly simple mechanical solution.
    http://www.youtube.com/watch?v=29tqil5Om-0&feature=player_detailpage&t=27

    And here's one that uses a magnet near the mechanical switch contacts to add hysteresis:
    http://www.youtube.com/watch?feature=player_detailpage&v=qzhgKWei2vY&t=50
  • kwinnkwinn Posts: 8,697
    edited 2013-08-13 17:59
    Chris Savage wrote: »
    Call it curiosity...I miss the old days (pre-90s) when you could get a schematic for a device from Sam's publishing. Sam's Photo Facts I think they were called. Fixed many TVs, Monitors and Stereos with that information.

    Same here. I can understand a simple item like a thermostat not having a schematic available, but I find it frustrating that schematics for more complex ones are almost impossible to come by as well.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-08-14 14:01
    Erco...
    Those old thermostats were really a profound piece of design.

    I am just wondering why we don't make clock-spring propelled robots that are merely controlled by micro-controllers ---- smaller batteries and more interesting design.

    It is just too bad that mercury switches have been banished as they still make great robotic sensors. I think I have 4 here that I hang on to for just the right project.
    erco wrote: »
    Slightly OT, but I always thought the older mechanical thermostats with a mercury switch and a bimetallic coil were pure genius. The nearly-level mercury ampoule's CG would change as the mercury shifted right to left, which (combined with the coil's flexibility) mechanically added hysteresis to the system for snap-action switching to avoid a high-resistance connection at the transition temperature. Even in today's age of cheap and abundant microprocessors, I still appreciate an elegantly simple mechanical solution.
    http://www.youtube.com/watch?v=29tqil5Om-0&feature=player_detailpage&t=27

    And here's one that uses a magnet near the mechanical switch contacts to add hysteresis:
    http://www.youtube.com/watch?feature=player_detailpage&v=qzhgKWei2vY&t=50
  • ercoerco Posts: 20,256
    edited 2013-08-14 14:14
    Now you're barking, Loopy! I'm a clock & clockspring fan. I've done lots of clock repair over the years. No doubt, my German clockmaker ancestry coming out. I could definitely go for a windup robot. BS2 and relay controlled, of course... :)

    A while back I bought some mercury switches from Ebay China just 'cuz someone here said they were illegal and couldn't be shipped into the country. They arrived fine and I need to find a use for them.

    Lithium batteries are banned from mailing in the UK, this from a forum across the pond:

    However, "officially" ALL lithium cells (not installed in a complete product) are now banned from our Royal Mail - yes even a single CR2032 for a RTC !

    http://www.picaxeforum.co.uk/showthread.php?24218-Reason-13-Why-I-Like-the-PicAxe&p=243762&viewfull=1#post243762
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2013-08-14 14:32
    erco wrote: »
    Slightly OT, but I always thought the older mechanical thermostats with a mercury switch and a bimetallic coil were pure genius. The nearly-level mercury ampoule's CG would change as the mercury shifted right to left, which (combined with the coil's flexibility) mechanically added hysteresis to the system for snap-action switching to avoid a high-resistance connection at the transition temperature. Even in today's age of cheap and abundant microprocessors, I still appreciate an elegantly simple mechanical solution.
    http://www.youtube.com/watch?v=29tqil5Om-0&feature=player_detailpage&t=27

    And here's one that uses a magnet near the mechanical switch contacts to add hysteresis:
    http://www.youtube.com/watch?feature=player_detailpage&v=qzhgKWei2vY&t=50

    That's what got me started on the original project was I had one of these, which had the mercury switch. Mine had a small slider to set the gap, but it didn't use a magnet.

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