RAIN ALARM

This is a simple circuit that can be used to detect rain. It will give a loud beep when the first drop of rail falls on the sensor. So it useful to detect rain where sun drying is carried out.

 Op-Amp IC 741 is designed as a voltage comparator. Its inverting input pin2 receives half supply voltage (4.5V) through the potential divider R1 and R2. Its non-inverting input pin 3 is connected to touch pads and variable resistor VR1. Output of the comparator becomes high only when its pin3 gets a higher voltage than pin2. Normally pin 3 is low because it is not getting current from the positive supply. So output remains low. This inhibits transistor T1 and buzzer remains off. When the contacts of the touch pad gets water, current passes through the contacts to the pin 3 of the comparator. This higher voltage at pin 3 causes the output to go high. LED lights and transistor conducts. Buzzer then starts beeping to indicate rain.

Sensor

Sensor can be a short piece Strip board (line type). Solder two thin wires to the adjacent tracks of the touch pad and solder wires to points A and B.

Test

Connect battery. Buzzer remains silent. Place a drop of water in the touch pad. Buzzer starts beeping and LED lights. Adjust VR1 to regulate current to pin 3 of IC if needed.