This simple rain detector will give you alert when a rain drop is detected. The buzzer is pulsed about once per second for a very short time, giving it a “dripping water” sound which seems appropriate when the circuit is triggered by rain water. The circuit is basically a handy flasher circuit using ordinary silicon transistors. It draws virtually no current when the sensor is dry and consume low current when the buzzer is activated. So a couple of AA battery will last a long time.
The rain sensor may be built any number of ways. You can use simply two conductors that are bridged by the rain water. Two conductors of bare copper wire are woven through the holes so that the conductors are near each other but do not come into contact. Or you can use a penny and nickel sandwiched together with a plastic insulator in the middle. Rain drops bridge the gap between the coins, triggering the circuit.