This electronic circuit is a very nice and simple alarm circuit that is based on the 555 timer integrated circuit . This alarm circuit can be used with many types of sensors like : light or temperature sensors .
As you can see below some types of sensors that can be connected at this type of alarm circuit ( darkness , light , cold and heat sensor ) . If the sensor detects that the temperature or light is outside of the desired range the alarm will be activated .By turning the potentiometer from the base of Q1 you will modify the sensibility of the sensor . When the alarm is activated the Q2 transistor acts as an audio amplifier and speaker driver for the 16 ohms speaker .
This alarm circuit require a 12 volts DC power supply circuit .