PIR circuit to switch off door alarm
June 21, 2011 5:36 AM   Subscribe

I am trying to build a circuit to turn off an alarm when a passive infrared sensor detects a human in a room. Yes this is a bit backward but it is for turning off a door alarm when our receptionist is at her desk rather than detecting intruders. I know enough about electronics to just get myself in trouble (which isn't much), so any advice on parts and construction would be excellent. Thanks!
posted by lake59 to Technology (8 answers total)
You might want to play with PIR detectors a little more before you get too excited about this idea. They generally detect movement rather than mere presence. I once set off the school alarms two hours after they'd been armed by getting up from the chair in the computer lab in which I had been sitting and writing maintenance scripts.

I think you'd find that a mat switch under the receptionist's chair would be a more reliable sensor.
posted by flabdablet at 5:52 AM on June 21, 2011

Flabdablet: You'll want a timer on the mat switch or the alarm will go off as soon as the receptionist gets up to let the visitor in.

Can you just rig a 'normally closed' relay to the PIR sensor? When the PIR sensor 'saw' a person it would have the alarm on and this would activate the relay. The relay would then switch off the alarm.
posted by Confess, Fletch at 6:08 AM on June 21, 2011

These guys do all kinds of circuit making for their 'hacks' and I'm sure if you sent them a quick email explaining your idea they would get back to you with at least some info/help/ideas.
posted by MansRiot at 6:50 AM on June 21, 2011

I've installed PIR's all over the house for my lighting needs. Folding towels in the laundry room results in darkness. In my experience, you have to actually be... moving to keep the sensor triggered. You'll have to look into some sort of other sensor, I suspect.
posted by mmdei at 10:32 AM on June 21, 2011

Given the motion requirements of the PIR sensors, I would think an ultrasonic sensor would be more appropriate. Parallax has them and STAMP microcontrollers. Their product page for one of their sensors is here. It has videos, spec sheets, code examples and more.
posted by buzzv at 11:02 AM on June 21, 2011

Jesus. Sorry, that link was totally wrong. Here is the Parallax link.
posted by buzzv at 11:04 AM on June 21, 2011

buzzv, thanks for the LOL.
posted by titanium_geek at 12:02 AM on June 22, 2011

Response by poster: Thanks for all your input! I still haven't figured out how to get email notifications when someone answers my questions. So, Sorry so long to respond!

I see how the timer would be needed since low activity like folding towels would let the sensor go off. I think I will explore the ultrasonic sensor and maybe contact the link MansRiot posted.

Do you know whether the ultra sonic sensor can be sensed by dogs? We do have a dog that visits often. I am assuming that the sensor pointing at the receptionist is what you are recommending. So, when she moves from her work station, the alarm will be active (unless someone puts a box on her desk)
posted by lake59 at 12:33 PM on June 30, 2011

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