interfacing with the Light Dependent Resistor

[aliyesami]

i want to use the LDR to detect if the room is lighted or not , how can i interface it to generate variouis voltage
levels for dark or lighted situations ? i will be using it in a 3v env.
The LDR specs are :
dark resistance 500Kohm
illuminated resistance 16-33 Kohm

thanks

 

[Audioguru]

You need to know the actual LDR resistance with the room light, not with sunlight nor moonlight.
When the room is not lighted then is there any other light that would reduce the resistance of the LDR? Measure the LDR.
Then you will be able to design a simple circuit.

 

[aliyesami]

should i use a simple transistor or there are better ways to interface ?

 

[Audioguru]

should i use a simple transistor or there are better ways to interface ?

Simple arithmetic shows that a transistor with a 3V supply being turned on by a 33k LDR on its base has a base current of only 2.4V/33k= 73uA and the transistor can saturate with a collector current of only 730uA which is almost nothing. The currents will be less if your light is dimmer than whatever the LDR is rated with.

What will this transistor be driving?

 

[tariq.sahi]

should i use a simple transistor or there are better ways to interface ?

a 555 based circuit will be much better and you will have the luxury of varying the amount of light where you want to trigger the circuit with the help of simple variable resistor.

 

[colin55]

A standard TTL 555 does not work on 3v.
A transistor in a high-impedance configuration will work perfectly.