Motion Activated Exhaust Fan - Project

Discussion in 'The Projects Forum' started by khoegen, Jan 18, 2011.

  1. khoegen

    Thread Starter New Member

    Jan 18, 2011
    I have started working on a project installing an exhaust fan in a small room in my home where we keep the cat litter boxes. The reason for the exhaust fan should be obvious ;-). To-date I have simply used an ac plugin timer to turn the fan on and off at pre-set intervals (I've already installed a dedicated GFCI wall outlet and created both inlet and outlet holes on the exterior walls). The fan I WAS using was a standard bathroom exhaust fan which I installed at waist height. It worked OK, but was not very energy efficient. Bear in mind that the initial smell after a cat relives itself is what I am attempting to exhaust, since this is when particulates in the air are at the most concentrated. Afterwhich, the litter does the real work. The exhaust fan also assist with keeping the air balanced, as this room also has our furnace and water heater within it.

    What I would LIKE to do is to utilize a motion sensor as a means of switching the exhaust fan on and off (I would place the sensor near floor level to sense the motion of the cats as they enter and exit the room). My problem is that the motion sensors are not rated for the load of the fan, hence they do not work.

    A friend suggested I utilize a relay switch which the sensor would activate, and then the switch would actuate the fan. My basic question is, is this idea sound? How do I choose the correct relay switch?

    As an FYI I was planning on using a standard motion sensor typically found at any hardware store designed for use with lighting. These sensors have the additional feature of setting the length of time to remain ON, such as 5minutes, 10minutes etc.

    My apologies if I provided too much 'non-electrical' content, but my experience when dealing with projects such as this are, if others understand the intent of the project, the resulting advice is often much more helpful.

    TIA for any assistance you can provide. Feel free to hit me with other questions if necessary.

  2. hgmjr


    Jan 28, 2005
  3. khoegen

    Thread Starter New Member

    Jan 18, 2011
    Thanks hgmjr, I have actually viewed this video in the past. I would still like to look into using a relay switch since this will afford me the opportunity to use a larger fan and motor (the Motion sensor switch alone can only support 1/8 HP max motor). I could still utilize this switch, but connecting a relay to the circuit would allow me more options for exhaust fans. A bathroom exhaust fan really isn't designed to handle the excessive dust and just doesn't have the velocity I am seeking. I was theorizing that a higher CFU exhaust fan would push the dust out more efficiently, allowing less to accumulate within the ducting and on the motor assembly.
  4. wayneh


    Sep 9, 2010
    Hmmm... I wonder if you really need that much power. I've installed a motion sensor in my cats' feeding and potty area so that I don't need to leave the lights on for them. The fixture holds two bulbs, and without looking, I'd guess it would be OK with maybe 100 watts each, so 200W total being switched by the electronics. (I use much less, just a CFL bulb.) That's a lot of power - much more than, for instance, the fans in a computer use. But of course those fans have the enclosure to direct the air.

    To use a relay, you'll need a DC current. That's not hard, but it's the hurdle that stops you from easily just hooking things up. You need to get the signal from the detector and use it to control a DC circuit.

    You might also be able to use an SCR instead of a relay. There's probably one already in your motion detector, and the solution might be as simple as using a beefier SCR that can handle the extra load and heat dissipation.