PIR circuit with a timer - Page 1
Forum section: electronics / beginners

author: mrdrako
author_rank: Newbie
timestamp: October 02, 2013, 01:47:01 am
content: Hi, first post here ever, please be kind.I have a project in mind, and I'm not sure how to go about designing the circuit. I 'think' it should be fairly simple, but it is currently beyond my abilities. Here's a short description.I have a Halloween fog machine, from the manufacturer it requires you to step on a pedal to make the machine emit fog. I'd like to automate this process, with a PIR, so that when the kiddies come near my front door, it fogs up the porch as they approach. However, I don't want it triggering the whole time they are on the porch, so it really needs some kind of a timeout feature.I had thought to make a circuit similar to the one here:http://www.electroschematics.com/6039/security-light-switch-with-pir-sensor/?ModPagespeed=noscriptbut using a Parallax PIR (#555-28027-RT). The documentation page for the sensor is here:http://www.parallax.com/downloads/pir-sensor-rev-b-product-documentationThe original circuit as designed has a PIR that has a selectable delay built into it, but my Parallax one does not. I thought to use the relay in the circuit to splice into the pedal wiring and trigger the fog. A LED showing a triggered state is not necessary, but may be good to use during the testing phase. I could always remove it later.So, I have the following questions:1 - is this circuit still appropriate even, given the part differences?2 - Can anyone help me design the desired circuit? I am assuming that I need to potentially integrate a 555 timer into the circuit somehow, but I could be way off base here.3 - How 'programmable' can I make the timer delay setting? I assume I could probably do exactly what I'm searching for with an arduino, but I'd rather not dedicate a unit to a once a year purpose, I'd rather have a dedicated circuit board.Thanks for the help, in advance.

author: David_AVD
author_rank: Super Contributor
timestamp: October 02, 2013, 02:04:51 am
content: What you want is sometimes called a monostable or one-shot timer.Here's an example.http://www.dummies.com/how-to/content/electronics-components-555-timer-chip-in-monostabl.htmlYou can also buy little modules on eBay that do the same thing.

author: Stonent
author_rank: Super Contributor
timestamp: October 02, 2013, 03:38:17 am
content: I was thinking along the lines of those motion sensor lights that people put around their houses that turn on if someone gets near it. Though I don't recall what the warm-up time is for a fog machine. I have one in storage somewhere but haven't used it in ages.

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2013, 07:16:47 pm
content: PIR motion sensors to turn on lights, etc. that feature a timed-turn off (for instance for a hall light that only stays on for a min or so.) are easily available at your local hardware store. You should be able to easily hack it to fit your needs.

author: chibiace
author_rank: Regular Contributor
timestamp: October 02, 2013, 07:54:29 pm
content: the modules i got from the store for some jobs are mains voltage have 2 adjustment pots one for sensitivity and one for on time and are weather sealed, hook up to a relay and your set.