This simple circuit is called room security system circuit. The main purpose of this circuit is to secure your room in night. You need not any security guard. When any unwanted person wants to enter your room in night then this circuit raised alarm. On the other benefit, it is always activated on load shedding.
There are five separates parts in this circuit – regulated power supply, relay driver, door switching, alarm system and audio amplifier. First section is regulated power supply. The output of IC 7809 is regulated +9Volt which is converted from 9Volt unregulated power supply. Capacitors are used for removing ripple. A full wave 9 volt transformer is connected in the input of this power section.
The second part is relay driver. Q1 transistor is used for driving relay. When the Q1 transistor is forward bias then the relay is ON and the total circuit gets positive voltage from ac source. In the time of load shedding relay is deactivated and the total circuit gets power from 6Volt Battery.
The third part is door switching. When C1 capacitor’s two terminals are short, transistor Q2 is reversed biased. So Q2 transistor is OFF. When C1 capacitor’s two terminals are open, transistor Q2 is forward biased. So Q2 transistor is ON. For this reason IC UM66 is ON and it generates music which is feed another two transistors Q3 and Q4. A speaker is connected with collector of the transistor Q4 as circuit diagram.