switch on delay to stop nuisance alarm

Discussion in 'The Projects Forum' started by standingwave, Jul 14, 2007.

  1. standingwave

    Thread Starter New Member

    Jul 14, 2007
    I saw some discussion, but it seemed intended for more complicated projects.

    I've set up a very simple alarm circuit to monitor an 80mm 25 CFM fan (110VAC) using a Stego air flow monitor switch (500mA 10W), 6V 20mA piezo buzzer and 6V 200mA AC adapter. The Stego switch is normal closed with a threshold air flow 8.2 feet/sec, which this fan just barely exceeds. I'm getting some nuisance alarms due to occasional backdrafting.

    I want to introduce delay between switch closing and buzzer buzzing. Fan failure is not an emergency, so adjustment from at least 15 seconds to a few minutes (hours?) would be nice.

    Seems to be a job for a variable resistor controlling a capacitor which switches a transistor that shows 6V to the buzzer. Or some kind of 555 timer circuit.

    That's the limit of my understanding of circuit design. I can, however, breadboard from a diagram or put together a kit.

    BTW, higher CFM fan would exhaust more air than necessary, a waste of heat in winter. And maybe I can learn something along the way here.
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    Can you show wiring. I've done that with a normally open switch but can't think how to do it with a normally closed switch.
  3. standingwave

    Thread Starter New Member

    Jul 14, 2007
    The Stego switch acts as an air prover, ie the flow of air holds a small flap off its rest position, opening internal switch. Fan stops, flap returns to rest position, closes internal switch. Buzzer in series, just like the basic lightbulb, dry cell and knife switch in my elementary school science class.

    Had a similar problem on larger scale, ventilating an interior propane furnace closet to make code official happy. Wind drafts kept setting off the air flow switch in the exhaust stack. Only there, instead of a dinky 75 dB 6V squeeker, the contractor installed a 100,000 dB 110V KLAXON, probably audible 35 miles away on top of a 14,000 foot mountain peak. Users simply had to disconnect the monster. So this draft delay issue has greater applicability than just my little project!
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    Depending on what you consider a nuisance alarm, you could use the air flow switch to keep the Vc of an integrator at zero until the loss of airflow. Then the time constant of the integrator will wait less than 1 time constant to trigger the alarm condition.

    I would build another circuit that requires a push button reset, so you can know if there was a loss of airflow and no alarm happened.

    You can further interface that type of an alarm with an autodialer to call you if the alarm is remote.

    I also used a time delay relay [drop out] to do some remote monitoring ... if there was an alarm condition for five minutes and no one had reset it ... the autodialer called three people. Surprised the hell out of the controllers when my techs were calling them inquiring before they had a chance to call the techs.