How loud? Audible from the next room in a house or to be heard across a vast factory or as a public service siren? Simple would be something on the lines of tapping into one of the outputs of a divide by counter such as the CD4060 (using RC timing components instead of a crystal), driving a simple mosfet inverter used to control a monostable 555 whose 3 second output drives a buzzer (or drives e.g. a relay to power an AC siren if it's something powerful).
Yes audible from the next room i want to use it with water level indicator , by adding a relay with the low level wire so the siren will be active every 10 or 20 minutes to alert .
Hi, for that you can get away with a 5 - 12V DC "wallwart" as some people call them, and any old buzzer, no need for a relay, just use a mosfet between buzzer and 555, mosfet as pass device and buzzer to gnd or to V+, maybe a flyback diode around buzzer but mosfet should be enough.