Timer 555
Détecteur de noirceur / Dark Detector

Fig. 1 Dark Detector:

It will sound an alarm if it gets too dark all over sudden. For example, this circuit could be used to notify when a lamp (or bulb) burns out. The detector used is a regular cadmium-sulphide Light Dependent Resistor or LDR, for short, to sense the absense of light and to operate a small speaker. The LDR enables the alarm when light falls below a certain level.