preset level set by the 20K variable resistor. The second circuit is a light sensor that will activate the output LED when the light reaches to a preset level set by the 20K variable resistor.
The sensitivity of the circuit can be further increased by experimenting with the values of 10K resistor and variable resistor. The circuit can be operated with any voltage from 3V to 30V. The LED current limiting resistor must be increased if operating voltage is higher than 12 volt is used.