This is a simple circuit using 555 timer IC which detects the dark and raise an alarm.

Working of this circuit is based on LDR (Light Dependent Resistor). When the darkness falls on the LDR, the resistance of the LDR increases drastically as explained in introduction, this increase of resistance in the second branch (one with LDR) of voltage divider will be enough to change the ratio of voltage sharing between the two branches of voltage divider section. Once this happen, the potential at the junction of voltage divider circuit rises from 0V to 2V (approximately). And similarly the voltage at the RESET pin rises. This rise of voltage will be enough to lift the 555 Timer ICĀ from reset mode. Once this reset mode is lifted, the timer generates square wave output. So it is concluded that once the darkness falls on the LDR the square wave output is generated by the timer. This output is fed into the speaker which starts the alarm sound.