This is a Phototransistor Detector circuit. In this circuit, when the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
The only critical part of this circuit is the value of resistor R1 which in most cases can be 470K ohms but may have to be increase if the room is dark or decreased if the room is well lit.
Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the phototransistor.
There are a number of phototransistors sizes and case styles. The smaller cases will be easier to hide but connecting wires may be more difficult.
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