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Darkness detector


Himanshu Choudhary

This circuit based project demonstrates the principle and operation behind the darkness detector. For example lamps which switch on automatically in the night. Various decoration lights are needed to be switched on automatically in dark.

The circuit described here is more of a sensor application with a speaker attached to it as output. However, depending upon the application, user can attach a night lamp or series of small LEDs to the circuit too.

Constructed around a 555 timer IC and LDR, this darkness detector circuit uses a 9V battery as a power source along with a couple of passive circuit components: resistance and capacitance.



In this circuit 555 works in astable mode producing a frequency of about 56 hertz. Pin 4 which is reset pin is connected to ground through a LDR. Reset pin which is active high if gets directly connected to ground and circuit does not functions, but in this circuit when there is dark, the resistance of LDR becomes very high and conduction does not takes place so high voltage is retained at pin4 and circuit functions; there by making it a dark activate circuit. In presence of light the resistance of LDR becomes low and reset pin takes ground and speaker does not sounds while circuit works in dark and speaker sounds. This project can have varied applications like by using this circuit for high frequency we can make dark activated decoration lamps or night lamps (they could be closed by switch too). In security systems where we need continuous light we can place this circuit along with speaker and this could intimate us if light fails.


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...

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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...
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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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