31 March 2016 User ReviewCheck your delivery options:
Automatic Bell is a door bell, which can be placed under the carpet and when anyone stand up on the carpet, the existing calling bell inside the house will start ringing for alerting the householder about the presence of visitor. In the power supply section, LM 7805 voltage regulator will regulate the battery voltage to 5V. This 5V is given as supply voltage to the circuit. Capacitor 1000uf is connected across output of 7805 to stabilize the output voltage. POWER1 indicates the power led. In this circuit NE 555 timer works in monostable mode. Monostable multivibrator circuit is a retriggerable mono shot pulse generator. The duration of stable state or the pulse width is determined by the charging time constant of the R14C9 network.
Output of Monostable multivibrator remains in its stable state until it gets a trigger. We can transfer the multivibrator from stable state to quasi stable state by using push button trigger switch S1. The trigger causes the transition from stable state to quasi stable state. As the person stands on the carpet, switch S1 get pressed and timer gets triggered. In a monostable circuit, the output voltage becomes high for a set duration once a falling edge is detected on the trigger pin. A high output at 3 rd pin causes LED2 to glow. When the output of 555 timer is high, transistor T1 switches to ON state. As T6 is ON, it energizes the relay coil and relay switches from normally closed to normally open state. BELL connected to relay starts ringing.