The PIR sensor output goes low when a presence is detected.
The low signal is connected to the -Trigger of the Monostable and will commence timing if the -Reset input is high. The duration of this can be altered using VR1 and can time up to 110 seconds.
The output from the Monostable is connected to the Relay which activates when the Monstable output is high. The relay contacts can be used to switch a light or lamp.
The Light Sensor is connected upside down to create a Dark Sensor, whose output rises the darker it gets.
The output from the Dark Sensor is connected to a Comparator Switch which will send its output high when the voltage from the sensor reaches Vref, set by VR1. When the output goes high, (and it has reached a certain level of darkness), this enables the Monostable to function. When the light level increases, the voltage from the sensor reduces, the Comparator switches off sending the "-Reset" input low, thus disabling the Monostable.