I work in a research lab with several freezers. One of our freezers was left open last night. Some of our samples are ruined. I am looking for a door alarm on a timer. If the door is left open for more than 5 minutes an alarm will sound. I haven’t been able to find an item to purchase that fits my needs. Anyone know of a circuit to build?

 

Just a start: Use a NO door switch to connect a battery to a 555 IC 5 min timer and a piezo buzzer. When door is opened ,timer starts, if door is not closed within 5 min, timer output will go low, activating buzzer. How loud does buzzer need to be?

 

Here's a couple pics of my mailbox door alerter circuit I made awhile back (10 yrs ago).
"Deep Space" is just a Co name I made up, etc..
I used half of a 556, you could just use a 555.
Anytime the mailbox door is opened, the piezo chime goes off every 60 sec till reset. Can turn circuit off if need be for whatever purpose...
Have changed the names to "freezer" for ya to better grasp the concept, etc...
Will need to put 5-min timer in front of it, etc.....