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Light detector


Himanshu Choudhary

Many a times we need a device which should activate itself according to outside light (like when we open door of drawer it should lit up). We sometimes feel the need of some light activated device, such a device could be made from circuit presented here. This light detector circuit project uses a light sensitive element which activates the circuit in light and switches it off in absence of light.

The light detector project presented here can be used in following applications. 

1. Such circuit can be used in refrigerators to automatically switch their light on or off corresponding to the status of door of refrigerator 
2. It could be used as morning alarm, which switches on with morning sunlight.
3. In drawers and wardrobes, on opening their door some light should automatically switch on. 





This circuit uses 555 in astable mode and produces a frequency of about 55.5 hertz. In this circuit, reset pin (pin4) which is active high is connected to VCC through a LDR (Light Dependence Resistor). LDR as stated light dependent resistor is a resistor whose resistance depends on light. Its resistance decreases to a few ohms in presence of light, and reaches to the order of mega ohms in dark. In this circuit as LDR is connected to reset pin (active high), so when light falls on it, its resistance decreases to large extent and reset pin takes high and circuit functions and produce pulse which could be used to drive a circuit (if it is a buzzer; frequency should be in audible range and if it is LED then frequency should be high to avoid flickering). In the absence of light, the resistance of LDR becomes very high and reset pin is at active low state and there is no output, thus forming light activated circuit. The resistance R4 connected between reset pin and ground is used as pull down resistor. Pull down resistor is the resistor which is used to pull down the voltage applied to a particular circuit not leaving it to floating state.



Circuit Diagram


Resistor Image

Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits.

Capacitor image

Capacitor is a passive component used to store charge. The charge (q) stored in a capacitor is the product of its capacitance (C) value and the voltage (V) applied to it. Capacitors offer infinite reactance to zero frequency so they are used for blocking DC components or bypassing the AC signals. The capacitor undergoes through a recursive cycle of charging and discharging in AC circuits where the voltage and curre...

LDR | Light Dependent Resistor

An LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained...

Speaker Image
A loudspeaker (or "speaker") is a transducer that converts electrical energy into acoustical energy. When an electric signal is applied to the input pin of speaker, it generates sound waves. The other pin is connected to ground. A speaker is commonly used to produce sound in response to a sensor output. For example, in an...
555 Timer | IC 555 | NE555
555 Timer IC

555 is a very commonly used IC for generating accurate timing pulses. It is an 8pin timer IC and has mainly two modes of operation: monostable and astable. In monostable mode time delay of the pulses can be precisely controlled by an external resistor and a capacitor whereas in astable mode the frequency & duty cycle are controlled by two external resistors and a...

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