Rain Alarm : This circuit gives out an alarm when its sensor is wetted by water.A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water.The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz.The sensor is also shown in the circuit diagram.It has to placed making an angle of about 30 - 45 degrees to the ground. This makes the rain water to flow through it to the ground and prevents the alarm from going on due to the stored water on the sensor.The metal used to make the sensor has to be aluminium and not copper. This is because copper forms a blue oxide on its layer on prolonged exposure to moisture and has to be cleaned regularly.The aluminium foils may be secured to the wooden / plastic board via epoxy adhesive or small screws.The contact X and Y from the sensor may be obtained by small crocodile clips or you may use screws.
Theft preventer alarm:
This circuit utilising a 555 timer IC can be used as an alarm system to prevent the theft of your luggage, burglars breaking into your house etc. The alarms goes ON when a thin wire, usually as thin as a hair is broken.The circuit is straightforward. It uses a 555 IC wired as an astable multivibrator to produce a tone of frequency of about 1kHz which gives out a shrill noise to scare away the burglar.The wire used to set off the alarm can be made of a thin copper wire like SWG 36 or higher.You can even use single strands of copper form a power cable.The circuit operates on a wide range of voltages from 5V to 15V.The speaker and the circuit could be housed inside a tin can with holes drilled on the speaker side for the sound to come out.