This circuit is very sensitive and can be placed in a room to detect the movement of a person up to 2 metres from the unit.
The circuit is basically a high-gain amplifier (made up of the first three transistors) that is turned on by the LDR or photo Darlington transistor. The third transistor charges the 100u via a diode and this delivers turn-on voltage for the oscillator. The LDR has equal sensitivity to the photo transistor in this circuit.



Source: http://www.talkingelectronics.com.au