Hello! This is shobana here...am gona explain you a very simple and interesting project....THE RAIN ALARM CIRCUIT.Yes! This is very useful because it indicates us whenever the rain water gets in contact with the 2 probes which is connected to npn and pnp transistors.I have clearly explained my project with the images in each and every step.You can try and enjoy !

Step 1: Things Required

PNP (bc 558)
NPN(bc548)
10k ohm resitor,330k ohm resistor
0.1uf capacitor
A 9v battery
A piezoectric buzzer
Some Connecting wires
A breadboard

Step 2: Transistor Connection

Connect the npn and pnp transistors such that the emitter,base and collector is connected in correct position.I have connected my PNP transistor on the top and then the NPN transistor.U can connect as per ur wish..

Step 3:

Now its time to connect a 330k ohm resistor in the emitter of the pnp transistor and the other end of the resistor is connected to an open ended wire.

Step 4:

Connect a 0.1uf capacitor and a 10k resistor on the collector of the pnp transistor and the other end of the resistor is connected to the base of the npn transistor.we have used a resistor and capacitor in series so that it causes the 1st transistor to turn on and off followed by the second.

Step 5:

Now, in order to indicate the rain, we are connecting a piezoelectric buzzer.The positive side is connected to the collector of pnp and the negative end is connected to emitter of npn.Here collector of BC548 NPN transistor is cojnected to the base of BC558 PNP transistor

Step 6:

How does a buzzer works? Yes! Here we need a 3v supply...thus we are using a battery .connect the positive end to emitter of the pnp and the negative to emitter of npn..Dont forget to connect a probe(open ended wires) to the base of npn transistor
.

Step 7: Finally Done!

Its all over now..We all know that water is a conductor of electricity.Thus contact between 2 wires take place and current start flowing towards base of NPN .since collector of NPN is connected to base of PNP , when NPN start to conduct PNP also starts conducting.In this way the circuit works.