Q:

Avoid flat battery from door left open

Is there a cheap device, or circuit I could make up, which alerts people to car doors being left open for more than X minutes, to avoid batteries being completely drained? 
I know of a device which goes between one of the battery terminals and the cable which usually connects to that terminal, and when the battery power gets below a certain level it breaks the circuit, but it's not cheap and in some ways it would be better to deal with it much earlier (e.g. after 5 mins), and get the door closed for security & weather reasons. 
Maybe just a buzzer which fits under the hood/bonnet, which detects power to the door light circuit (in some models), and beeps for a second every minute or so. 
I have a Toyota Corona 1993 and we left a door open which flattened the battery a few days ago.   Fortunately this model beeps when headlights, etc.  are left on, so that's not a problem for us. 
Thanks. 

A:

You can fit a buzzer between a permanent live and the interior light door switch - simple and works. 
Then you could refine it with a timer circuit : either brush off your electronic skills or you may consider re-purposing  a wiper delay circuit - which may give you an adjustment delay facility...
My car does the opposite - if you leave a door open, it turns off the interior lights after 25 minutes approximately...

