Rain Alarm Circuit using Transistors

A rain alarm sensor circuit is very beneficial device for alerting when rainfall. The following circuit diagram is Rain alarm circuit. The diagram shows a water detection circuit. It consists of two NPN transistors. When there is no  rain fall on sensor its resistance decrease and due to that small current flow from + to base.

When current pass through sensor to base T1 open and current flow from emitter to collector. Cotter of T1 connected with the base of T2 as base of T2 sink current from the emitter of T2 it open and buzzer ring.

water on sensor , transistor T1 is blocked and T2 also blocked because base is empty of both transistors. When the sensor resistance drops below 4MOhm, T1 conducts together with T2 which turns on the piezo buzzer. The alarm is active for about 15-20 seconds until capacitor C1 is discharged. After the alarm, wait about 5 minutes for the capacitor to be charged and ready to use again.
The sensor can be ordinary 2 wires on the surface where water may appear. A better option would be to solder the wires to a PCB board arranged as shown roughly on the diagram.

Component Used

1. Speaker pizo
2. Transistor BC547
3. Transistor BC547
4. Resistor 10k
5. Capacitor 0.01 mfd
6. 3v to 6v battery
7. Sensor  (Two built on a plastic board to detect water of rain)

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Rain Alarm Circuit 2

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