Simplest yet durable light sensor.

oishi said:

is there any other way to use the circuit without depending to the LDR?

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The LDR is absolutely necessary if you want to sense light or dark. It changes resistance with the amount of light it receives, hence its name--Light Dependent Resistor

First glance, shouldn't the 10K and the 1K on the base be switched and don't you need 2 resistors for the feed back to work?
Jeff

what I mean is can the circuit work the same function if the LDR is replace with other component for experiment such as input AC voltage?

oishi said:

what I mean is can the circuit work the same function if the LDR is replace with other component for experiment such as input AC voltage?

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Yes, the circuit can easily be modified to work with a variety of sensors, for example a thermistor could replace the LDR for heat or cool functions. Yes, an AC signal could be applied resulting in an output of a square or rectangular waveform of constant amplitude. Since thjere is no mention of the amplitude, waveshape, or frequency that is about a generic answer as can be.

This link is a very good read on the use of comparators.

Ron