hey!
im using LDR in my project work...im using the circuit in the attachment...can you please tell me how to increase the sensitivity of the ldr?im using a laser pointer to activate the LDR.the LED doesnt glow to its maximum intensity.i want to make the ldr a switch

 

You need an amplifier. Do you know how to use a bipolar transistor to amplify the LDR's signal?

 

Use a comparator or transistor to read the LDR and then drive the LED. See towards the bottom of this page: http://www.kpsec.freeuk.com/trancirc.htm

 

This comes up every once in a while...

When I was learning electronics I knew them by the names of CDS cell (cadmium sulfide), electric eye, photoresistor, or photocell. I didn't learn LDR until after I joined AAC.

They vary between approximately 100Ω to several MΩ depending on the cell and light intensity. The variable resistor allows you to set the cross over point.

 

I have made the same circuit as you. I just used a double TTL inverter to have a transistor switched on at a stable voltage output. This means the transistor wont turn the led on before the LDR puts up a voltage higher than TTL high level (5 volts?).
Just use several resistors to experiment til you have the perfect voltage divider that works with your wanted light sensitivity. Your circuit will make the led fade on/off.

 

The 555 circuit shown will overcome fading. It will either be on or off, no in between, since the 555 configuration is that of a Schmitt Trigger. The hysteresis of the Schmitt Trigger guarantee's that it will not flicker (unless the lights are of course). A 555 can be powered from 4.5VDC up to 15VDC.

Moving the CDS cell top or bottom on show will dictate whether the light turns on in the dark or in the light, it is an equivalent function to an inverter.

 

See my bike light project for a similar 555 circuit which used a capacitor to add lag to the LDR, subsequently making it slower to change.

 

The 555 circuit shown will overcome fading. It will either be on or off, no in between, since the 555 configuration is that of a Schmitt Trigger. The hysteresis of the Schmitt Trigger guarantee's that it will not flicker (unless the lights are of course). A 555 can be powered from 4.5VDC up to 15VDC.

Moving the CDS cell top or bottom on show will dictate whether the light turns on in the dark or in the light, it is an equivalent function to an inverter.

the circuit(555)is perfect,could you please explain it briefly?when will the output be high(i.e. when the LDR is above or below)?

 

The configuration show will go high if there is light, so you need to reverse the LDR and R1/R2 to match the 1st circuit.