Furnace Alarm Circuit

Discussion in 'The Projects Forum' started by chevylu, Dec 20, 2009.

  1. chevylu

    Thread Starter New Member

    Aug 15, 2008
    5
    0
    Our gas furnace intermittently fails to ignite, resulting in us waking up to a very cold house here in the frozen tundra.





    A Status LED mounted on the furnace control board indicates the status of the furnace as follows:
    • LED on continuously (steady glow): All systems are functioning normally
    • LED flashes on/off: A fault is preventing the furnace from starting
    [​IMG]


    When a fault is detected, the LED flashes a two digit number code. The first digit is determined by the number of short (about ½ second) flashes and the second digit by the number of long (about 1 second) flashes. For example, three short flashes followed by four long flashes is code #34.

    Unfortunately, unless one happens to be looking at the LED (an unlikely event), there is no alarm to indicate that a fault exists other than waking up (or coming home) to an alarmingly cold house.

    At this time, in order to be alerted to the flashing LED, I have aimed a webcam camera at the LED and set it up for motion detection. I wrote a Visual Basic program to monitor the webcam and when motion (LED flash) is detected, a computer: 1) sounds an alarm; 2) sends me an email message; 3) sends a text message to my cell phone.

    This system works OK; however, I would like to replace the webcam with a circuit somewhat similar the one discussed here: http://forum.allaboutcircuits.com/showthread.php?t=18501&highlight=%22furnace+led%22.

    Just as iONic’s picture shows (http://forum.allaboutcircuits.com/attachment.php?attachmentid=6843&d=1233100178 ), I want to fit an appropriate optical device (phototransistor?)in heat-shrink tubing over the control board LED and bring the leads out into a small circuit box.

    Also leading into the circuit box would be two wires from the computer’s DB9 serial port. One wire is connected to Pin7 of the serial port and the other wire is connected to Pin8.

    The computer program sets Pin7 high to provide approximately +12vdc to the furnace alarm circuit. The program then repeatedly polls Pin8. As long as the electronic switch remains open, Pin8 is low and all is well. If the switch closes, Pin8 goes high and an alarm event fires.

    Pin7 + >----
    .........../ electronic switch
    Pin8 <-----

    As I understand it, in this circuit when the R1 sensitivity control is adjusted appropriately and the light falling on Q1 flashes, Q2 will conduct and Pin8 will go high.

    [​IMG]

    I would appreciate it if someone would suggest how to modify (or replace entirely) this circuit so that it would work in the scenario I’ve described. Ideally the circuit would latch Pin8 high when flashes are detected.

    Obviously I’m a newbie, so ANY help will be greatly appreciated.

    Thanks.
     
    Last edited: Dec 20, 2009
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    It may be easier to simply place an opto-isolator in series with the existing LED. Use the output transistor of the opto to feed your serial port.
     
  3. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    I guess the first question should be; why does your furnace fail to ignite? If your circuit board is working and is calling for ignition, ignition should take place...unless the ignitor is bad or the HV tension lead has a gap, which may or may not allow a HV current path. The other problem is that the gas line pressure is too low to allow the gas valve to open, or the pilot to ignite.

    Regards, DPW [ Spent years making heaters out of op-amps.]
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    In addition to the post above, do you have a "proof of ignition" probe. Usually mounted near the burner that is the most distant from the ignition point so it will actually be in the flame. May simply be a steel wire probe with a single wire going back to the electronics. If so, try cleaning it with steel wool (don't use a pad with soap in it). This wire will have an AC voltage impressed upon it and depends upon "flame rectification" to prove a good fire.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    Bill, this sounds interesting. Could you please elaborate on "flame rectification"?
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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  7. chevylu

    Thread Starter New Member

    Aug 15, 2008
    5
    0
    At this point the furnace is working correctly. The gas valve was intermittently failing to open. Replacing it solved the problem. The intermittent nature of the fault was a bear to troubleshoot. However I would still like to see if I can get the alarm circuit working. A look at most any HVAC forum such as http://hvac-talk.com/vbb/ will attest to the need for an alarm, particularly when a homeowner is away.

    I intend on using this phototransistor as a starting point: http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=120221.

    With a peak sensitivity of 850 nm and a focused lens, it should respond well to being aimed directly at the red status LED on the control board.



    Here is a bit more on the project design constraints:
    • A computer’s serial port Pin7 is an output port; that is, it can be programmatically set True (high), producing about 10 vdc with respect to Pin5 (ground).
    • Pin8 is an input port which can be read for a True (high) or False (low).
    • If Pin7 is set high, touching Pin7 to Pin8 will cause Pin 8 to go high; disconnecting Pin7 from Pin8 will cause Pin8 to go low.
    • When the phototransistor receives a steady glow from the LED, Pin8 should be low; when the phototransistor sees the LED flash, Pin8 should go high.
    Will the circuit above as illustrated work this way? If not, what do you suggest?

    Time to break out the breadboard.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Probably the simplest way to couple that LED is with a Photoresistor taped or glued to the existing LED. If there's appreciable ambient light you'll want to tape the two of them light tight.

    Bill, thanks for the links on Flame Rectification, they were interesting. Sounds like pseudo vacuum tube theory. ;)
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    I made an opto out of an LED and a Photodiode by simply slipping a soda straw over them pointed head to head. Then applied electrical tape around it to eliminate stray light. Downside is that you can't see the LED. That's why I suggested using an OPTO-ISOLATOR in an earlier post. The opto will do the sensing and you still have the LED visible.

    As for the flame rectification, it depends upon ionized gas while a vacuum tube needs free floating electrons boiled from a cathode.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    The problem with that approach is that he'd be modifying the manufacturer's equipment. Doing so will surely make any warranties from the manufacturer, distributor and installer null and void, not to mention insurance liability and probable violation of many local and state codes.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    Very true........ Hadn't thought about that issue.....
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Bill, I forgot to thank you for correcting me on my erroneous analogy between vacuum tube theory and flame rectification. Not much in common there except heat!:rolleyes:
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    Some folks referred to the tubes such as the OA2 as vacuum tubes. The shell looks like one so it must be one, right???

    Good quiz of the day...... What is an OA2 and what is it used for???
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    That was a common gas filled voltage regulator. ;)
     
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    I'd bet that members under the age of 40 wouldn't have known that!!
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    Does anyone remember what the model number of the "Gas Eye Tube" was? They were used to replace expensive center zero meters when only a null indication was required. Heathkit used it in their model IT-11 C, R, L bridge.
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
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    6E5 and 6U5?
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    A gold star for you. ;):D
     
  19. BillB3857

    Senior Member

    Feb 28, 2009
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    Old age and treachery! Google is still my friend;)
     
  20. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    Since you are adding some type of circuit on the far side of the photodiode, why not add an LED to copy the OEMs LED? Or just pull off some of the tape when you need to see what the LED is doing?

    HTH,
    --Rich
     
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