Power flow sensing and auto cutoff

[John Gerrard]

Hi guys. Just wondered if anybody had any thoughts on this problem.
I have a coolroom which has a door which is opened and closed multiple times during any given day. When the door opens a fan automatically starts and will run while the door is open and will shut off as soon as the door is closed. Occasionally somebody will not shut the door correctly and the fan stays on. What I was thinking was that I'd like something to detect that power was flowing to the fan (ie. the door is open) and if it runs for, let's say more than 2 minutes, then we'll assume the door has been left open and I want this device to shut the power off to the fan and then perhaps initiate an alarm. I know I could do all this by just monitoring the door opening and closing, but I guess I'm just interested to know if there is anything more or less off the shelf, that I could just plug in between the fan plug and the power outlet that will monitor a circuit and detect when power is flowing and then, at a preselected time span (eg. 2 minutes) be able to do something like shut the power off, kick off an alarm or whatever ... any thoughts?

 

[awt]

monostable circuit will do the work for you. Let the opening of the door generates triggering pulse.

 

[Misterbenn]

OR
you could measure the current flowing to the fan....
OR
air speed sensor in front of the fan

There are lots of ways to do this, but monitoring the door is the quickest if you want to do an alarm as well. If your not bothered about an alarm then use the monostable

 

[KeepItSimpleStupid]

Search for door prop alarm.

Example: **broken link removed**

 

[KeepItSimpleStupid]

You can do current detect or even voltage detect, but you havn't told us what the voltage and/or current drawn by the fan is.