20-11-2017, 11:40 AM
Working: water is a conductor of electricity. When the water is in contact with the probe, then there is a current flow to the base of the NPN transistor (BC548), which leads. With the NPN transistor driving, the electron reaches Q2, which is a PNP transistor. The PNP transistor (BC558) also conducts and the current flows through the loudspeaker. In a loudspeaker there is an inductive coil that causes movement in one direction and then produces induced current, which is in the opposite direction to the current flow, this current induced in the form of a pulse, flows through a capacitor, resistance and makes the first transistor BC548 off for an inter-well and after-that relaxes to the previous state. This process is repeated again and again until the probe is in contact with the water and an oscillation is created in the circuit. The diaphragm of the speaker vibrates and gives a tone. The frequency of the circuit depends on the value of the impedance of the coil, the capacitor and the resistance value.
![[Image: rain_alarm.gif?189db0]](https://circuiteasy.com/circuit_diagram/rain_alarm.gif?189db0)
Component used
1. Speaker
2. Transistor BC548, BC558
3. Resistance 330K, 10K
4. 0.01 mfd capacitor
5. 2-cell battery container
