by
3lectrix
;
Published: 15th June 2011;
Last Updated: 4th September 2011;
Views: 7126;
Comments: 4
The circuit is based on National Semiconductors comparator LM311 (IC1) and an LDR. The non inverting input of IC1 is given with a reference voltage of 6V using resistors R3 and R4. The input to the inverting input will be the voltage across the LDR that is light dependent. At darkness the resistance of the LDR will be high and so do the voltage across it. In this condition the voltage at the inverting input will be higher than the reference at non inverting pin and the output of the comparator will be low. When the LDR is illuminated, its resistance drops and so does the voltage across it. Now the voltage at the inverting input will be lower than that at non inverting input and the out put of the comparator goes high. This makes transistor Q1 on and drives the relay. As a result we get a relay switching according to the intensity of the light falling on the LDR.
Schematic
Picture
Notes
Adjust POT R1 to set the desired light intensity for switching the relay.For this illuminate the LDR with the desire intensity light.The relay will be either on or off. Adjust POT R1 slowly so that the state of the relay changes.That’s it.Now the circuit is set for the given intensity of light.
Assemble the circuit on a good quality PCB or common board.
You can use either a 9 V battery or a well regulated & filtered 9V DC mains operated power supply.
The pin 5&6 (Balance & Balance/Strobe ) of IC LM311 are shorted to minimize the chance of oscillations.
The pin out of LM311 is also given together with the circuit diagram.
Caveats
Because there is no hysteresis in this circuit, they can be some chattering with the relay when the illumination is very near triggering point.
Hysteresis can be added by connecting a 100kΩ resistor from the pin 7 to pin 2. 100k may not be the optimum value, depending on the light source. It might also be helpful to connect a 10-100uF cap across the LDR, to filter out hum from artificial lights.
can you help me create a circuit that consist a motion sensor & led. The objective is the led will light up when the motion sensor detects movement..please?
I can not understand why I see so many circuits powered by 9V batteries which have low capacity & hold little charge.
Surely 4 x AA's or 6V lantern battery offer lower cost & convenience.
I am a bit disappointed the poster acknowledged relay chattering as a probable event but offered no solution, leaving it to Roff (thanks for the input) to suggest remedies.
Surely the poster might acknowledge & update his circuit?
Can you help me construct a circuit that can turn on a light when it is dark,and can turn it off when there is light in the surrronding.i am using a pic P16f84A.may God bless you!
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