This is probably the simplest doorbell or alarm circuit that is out there. The buzzer will not be constantly on since the buzzer gets switched on by a capacitor. The circuit draws hardly anything when off as you will see in the graph below. As the capacitor gets charged its current lowers down a lot. If you want to save money for parts and use a power supply use the one with the relay. The relay will make it work with a spst switch. I wish I knew that before ordering the parts
It uses a switching transistor to turn the buzzer on and off. It’s best to use a transistor since it can take very little current to turn on. It needs a SPDT magnet alarm switch since it has to switch the capacitor between +v and transistor base resistor. You cannot use a SPST switch. I didn’t need a transistor for the buzzer since it didn’t need alot of current.