Greetings,

I have a refrigerator that is poorly designed and very, very easy to leave ajar. I want to build a battery operated 555 timer based circuit that will sound an alarm after a set time has passed with the door open. 555 circuits are easy enough to find, but I don't know what kind of switch set up to use.

The circuit will use a 9v battery, and current draw when off is a priority.

I want the switch in the closed position when the door is closed, so some electricity will be flowing through it. I want this amount of energy to be as low as possible.

Basically, I want a "High" signal of extremely low current to stop the 555 from charging.

How should I do this?

 

why don't you just put a strong magnet on the door,

 

A thin enough magnet of sufficient strength is too costly. Plus, the doors are non-magnetic, and the magnetic door seals would probably rip off the fridge.

 

When the door is ajar, is the refrigerator light on? Can you detect that light as a trigger?

 

I second the 'detect the light' approach.

 

Careful, some fridge lights go out when the door is slightly ajar - unless you are detecting a small amount of room light instead of complete darkness inside the fridge.

 

Not my fridge, I'm a renter. Monitoring the wiring is not an option, and I don't want a wire poking through the door seal to run phototransistor.

Surely there is another option?

 

I don't want a wire poking through the door seal to run phototransistor.

You wouldn't need it. The whole caboodle (light sensor, beeper, battery, circuit) could be fitted into the space of a matchbox and live inside the fridge.

 

Imagine this. An assembly stuck (with double sided tape) to the top of the refrigerator near the front door. Assembly contains a battery, a timing circuit, an alarm, and a switch. The switch is normally closed and positioned such that when the refrigerator door is closed, the door keeps the switch operated (contacts open). When the door is ajar, the switch is released (contacts closed) and a timer starts. When the timer ends, the circuit sounds the alarm. Switch is something like this: http://www.jameco.com/webapp/wcs/st...roductId=187805&catalogId=10001&CID=PDF133CAT .

The only time power is drawn from the battery is when the door is open.

Can you build it?

 

Here you go. Standby current is just a couple of microamps. Beeper is assumed to be a self-oscillating piezo type. A CD40106 is used instead of a 555. Delay before the beeper sounds is adjustable up to ~2 mins.