INTRODUCTION:As automation is emerging technology nowadays, just imagine a door bell that rings automatically if someone tries to enter your home! This circuit is a sort of security device that continuously watches if someone is visiting your home. A shadow alarm is a circuit that activates an alarm whenever a shadow falls on it. This circuit can sense the motion by noting the changes in the shadow cast by the moving objects. By utilizing the light sensing property of a photo diode it determines whether a person is present or not. If any object causes shadow on LDR it activates a buzzer that will make the LED glow.
|12) Battery||9VOLTS dc|
WORKING: The pn junction of the photo diode gives light current when it is forward biased. The circuit is divided into two parts one is power supply and the second is logic circuit. The power supply circuit converts 9volts to 5 volts and the logic circuit operates the buzzer whenever shadow falls on it. Power supply circuit comprises of battery, capacitors, regulators and diode. The purpose of using diode in the circuit is to avoid negative voltages. The voltage regulator is used to regulate the output voltages & the capacitors are served to eliminate the ripples thus the constant produced voltage at the output of regulator is applied to the logic circuit. The logic circuit consists of LDR, Transistors, op-amplifiers and buzzers. The op-amplifier IC is designed as a voltage comparator it compares the two input voltages and produces an output voltage which is applied to the transistor. Its inverting inputs are biased by the voltage obtained from the junction of 100K and the collector of phototransistor. The non inverting input receives its biasing voltage from LDR and resistor. When LDR gets light its resistance will decrease thus providing a path for the conduction of current and gives a high voltage to the non inverting input of IC. This high voltage is given to the base of transistor through a resistor. Since the base of transistor has become high due to high output from IC the buzzer and led connected to its emitter will remain off. As the shadow of an object falls on it this reduces the resistance of LDR and decreases the voltage at the non inverting input of IC. This makes the output of IC low immediately activating the buzzer through conduction by transistor and indicating through led. The buzzer has two pins, one is connected to the led and the other is connected to the gate. Whenever the output becomes high the buzzer starts alarming and the led starts to blink.
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