Simple and fast circuit needed...FAST!

Discussion in 'The Projects Forum' started by iONic, Jan 25, 2009.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    A quick circuit challenge

    My furnace has shut off twice overnight in the midst the middle of an artic freeze here in the North East US.

    I'm trying to build a very quick circuit that will detect the light from a red LED and sound an alarm. I am unable to get at the pins of the LED and thus need this Red LED light sensing alarm. I already have a cheap water alarm that when the water touches two contacts the alarm will go off. But the Burner LED flashes three times then repeats, thus I need a latching on circuit preferably.

    Any ideas.

    The current thru the LED is more than likely about 20mA and the voltage is probably 2V or less!

    Opto Sensors I have available...
    Last edited: Jan 25, 2009
  2. Audioguru

    New Member

    Dec 20, 2007
    My son's high tech high efficiency furnace stopped last winter and flashed a code on its diagnostic LEDs. It turned out that snow plugged its outside air intake. It couldn't breathe well enough to burn fuel so it shut down.
  3. pilko

    Active Member

    Dec 8, 2008
    You can buy cheap digital fhermometers with built-in set point alarm Set the temp below your normal operating minimum temp These things are portable,you can even take it to bed with you
  4. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    I will probable use a photo resistor and a 741 OpAmp to make the switch. In the end I'll want it to dial a phone number in case I am away from home. Never gave it a second thought with the oil burner. It never failed. But with this gas burner....trouble. It is rented and thus under maintenance contract so I can not alter it at all. I do, however, need to have them look at it soon.
  5. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
    I have a circuit working, sort of!!! See attached.
    It seems that when the thermostat first calls for heat there is a few second period when the LED indicator on the burner must light up as the alarm goes off then stops. It also has
    a shorter moment when the thermostat has reached it's temperature.

    Therefore, what I think I need is some sort of delay before the the relay is triggered.
    Any ideas for this tweak???
    Last edited: Jan 27, 2009