I’ve designed a fridge alarm to prevent the door from being left open without any of us noticing. It is a working design and it meets the following design criteria.
Design criteria:
Works off 3V CR2032 cell
Can last for years on one cell
No (=0) current when off
Very low current when fridge is opened but alarm is not going off (it’s a couple of uA in practice, so this is negligible)
Alarm either goes off or it doesn’t (threshold)
Alarm goes off (virtually) instantly when door is closed
Time between open door and alarm can be set (using a pot)
No micro-controller; as simple as possible
Explanation: when fridge door is opened, the capacitor is slowly charged and the voltage climbs. Once this voltage reaches the reference voltage (which can be set using the pot), the piezo buzzer and red LED activate. When the door closes (regardless of whether the alarm went off, the voltage at the base of the PNP goes low and the cap quickly discharges.
General question: can anything be improved? Are there more elegant solutions? Am I making something unnecessarily complicated?
Specific questions:
Because my reference voltage only available when the switch is closed, once the switch opens again (the door is closed), my reference voltage drops below capacitor voltage and it fails to deactivate. The OpAmp power rail is only active when the switch is closed, so this is not a problem, but I was wondering whether it is bad practice or something? It is trivial to solve this ‘problem’ by having the voltage reference on at all times, but this would consume a bit of power. Granted, it would be very little, but nothing is better than very little.
It is my first ‘design’; I’ve made this completely from scratch for an actual, practical problem that I had.
I apologise in advance for the rather messy schematic.
Any kind of (constructive) feedback is more than welcome. Thanks!