HVAC zone controlled blower motor speed

Discussion in 'The Projects Forum' started by wogthefrog, Jan 10, 2011.

  1. wogthefrog

    Thread Starter New Member

    Jan 10, 2011
    3
    0
    Hi,

    I'm trying to build a circuit to work with my Central air zone controller board to modify blower motor speed based on # of open zones.

    I have very little electronics background- but I can get the zone's to trigger 5v logic states. I have 4 zones to monitor and basically need to set up digital gates to operate a relay (with a relay logic driver.)

    My goal is to take 4 hi/low logic states from the zones and set it up as follows:

    if total zones in use <= 1 then relay stays unactivated. if total # of zones > 1 then turn on relay.

    I have tried to draw this out as nand/xor gates and because I have never really done this before I'm unable to figure out how to make a circuit/truth table match what I'm trying to do-

    Is this accomplished with more a gate array than just normal and/or gates?

    Thanks for any info and sorry for the very basic question..
     
  2. #12

    Expert

    Nov 30, 2010
    16,262
    6,769
    Here's an analog method.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Does your air handler employ a multiple speed motor?
     
  4. #12

    Expert

    Nov 30, 2010
    16,262
    6,769
    It better, or this won't work!
     
  5. wogthefrog

    Thread Starter New Member

    Jan 10, 2011
    3
    0
    #12,

    Thanks for your post. I think I understand what's going on here- but like I mentioned, I have very little electronics experience. Could you tell me what your devices in the diagram are (part # for example) Are they gates or op amps?

    A correction to my earlier post- the zones work in reverse- power is applied to a zone motor to turn that zone off. So, the set up would need to be if # of zones powered on is =>3 then relay on if # of zones <3 then relay off (or vice versa)

    If I followed your diagram, then should I use inverters on the 5v from each zone?

    Thanks again for input.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Have you considered something like this? If the 24 VAC inputs can share a common you can eliminate three of the Optos and AND gate a single Opto with just 4 Diodes.

    BTW, if your relay coil draws more than the 4N33 can handle a relay driver transistor can be added.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,033
    So you need a relay to switch state from normally-closed ("on") to open (off), when at least one zone is OFF, right?

    CDRIVE's circuit, or proposed simplified version, will accomplish that. But you'll need a relay that is normally closed.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    The relay in my circuit is a SPDT.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    As I said previously, if it's permissible to common the 24VAC inputs you can simplify the circuit like this.
     
  10. wogthefrog

    Thread Starter New Member

    Jan 10, 2011
    3
    0
    Awesome,

    Thanks so much for the input. I'll see what I can find about making the zone's share a common- funny thing about Honeywell not publishing the schematic for their zone controller...

    Even if I had to employ 4 opto-isolators- this would be a minor issue.

    I'll breadboard this out and post back with how it goes. Thanks.
     
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