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    This Circuit can compare the Light level in an area. It uses a PN Photodiode as the light sensor and IC CA3140 as voltage comparator. The circuit is ideal as the front end of burglar alarm circuits.

    The circuit uses an Op Amp as voltage comparator. In voltage comparator mode, the OpAmp compares the voltage levels between its inverting input (pin2) and the non- inverting input(pin3) and gives an appropriate high / low output. Generally in voltage comparator mode, voltage at one input is kept fixed using a Zener diode or a potential divider resistor chain. In the circuit, the voltage at pin 3 is set by VR1 so as to keep the output high in a particular light level. The high output keeps the LED off. The PN Photodiode act as the light sensor. When the light reduces, current through the photodiode decreases. This increases the voltage at pin 2 of comparator and the output swings to low state. Current then flows through R2 and LED into the comparator and LED lights. This indicates, low light level or darkness.

    Light Sensor Circuit

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    8 Responses to "Light Sensor Circuit"

    1. sir, how should we find the LED and photodiode for this circuit?

    2. what it means 2.2 M

    3. 2.2M means the resistor value is 2.2 Mega ohms or 2200000 ohms or 2.2X10E6 ohms which is colored red,red and blue.

    4. please assist me with a photodetector circuit diagram to detect a laser beam with a wavelength of 633 nm

    5. I want to make a circuit that when the light(invisible rays) cuts the alarm start ringing.plz help me.

    6. sir, if i want to connect series of LEDs and transformer instead of 9v battery.then will there any change require in circuit?

    7. What IC can I use as a substitute for CA3140 because I can’t find one in our are. thanks 😉

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