In the video I completely mixed up the whole open vs. closed switch and referred to the reed switch as a Normally Open, it’s actually a Normally Closed reed switch as explained below. Also, I said that the buzzer was rated at 80dB, it’s actually rated at 100dB.
Reed switches are really cool. In a Normally Open reed switch two contacts are aligned very close together in a glass vial, so close that when a magnet is placed near the reed switch, the contacts touch, completing the circuit and allowing current to flow. However, I needed the reverse, I needed a reed switch that completed the circuit (i.e. turned it on, let current flow) when the magnet was taken away from the reed switch. This can be done any number of ways but the simplest is to use just use a Normally Closed reed switch.
The whole Normally Open, Normally Closed thing can be a bit confusing. In the case of reed switches they are in their normal state when a magnet is too far away to have an effect on them. So a Normally Open reed switch does NOT complete a circuit when the magnet is removed, where as a Normally Closed reed switch DOES complete a circuit when the magnet is removed.
In any case I used a Normally Closed reed switch as I needed the circuit to be completed (the timer to be turned on) when the magnet was removed (i.e. the freezer door was left open).
The buzzer I used can run off 3-12VDC and is rated at 100dB at 12VDC, I’m not sure what the dB level is at 9VDC but it’s plenty loud enough to be heard even on the other side of the house.
The alarm works great, the reed switch is sensitive enough (the magnet needs to be within ~4mm to open the reed switch and turn off the alarm) that the door has to be purposefully closed in order for the alarm not to sound. The buzzer is loud enough that we can here it from any room in the house and when the freezer door is closed (which is most of the time) the circuit does not draw any power, so the battery should last a good long while.
The freezer door alarm has been working great, except for one minor issue. When loading the freezer it was often open for more than the 27 seconds the timer was set for. So while loading the freezer (or staring blankly into it deciding what you wanted) you’d either have to close the freezer door to reset the timer, or suffer through the buzzing.
Not wanting to increase the length of the timer, I added a momentary reset button. Now if you want to have the freezer door open for longer than the timer allows, all you do is press the reset button which gives you another 27 seconds to do what you need to.
The reset button is a Normally Closed button that is wired between the battery and the timer; just like the reed switch is. When pressed it cuts power to the whole timer, just like the reed switch does. Since it’s a momentary button as soon as it’s not being pressed power to the timer is restored, turning timer back on. This allows you to reset the timer without the risk of leaving it in an off state.
I did not update the schematic shown below but to add the reset button just place it in-line with the reed switch. The pictures below show the reset button (which was salvaged from an old computer case) added to the alarm.