For the Dark Detector circuit above, when the level of light on the photo-cell decreases, the 555 is activated. Photo-cells (Photo-resistors) have a wide range of specifications. Some cells go down to 100R in full sunlight while others only go down to 1k. Some have a HIGH resistance of between 1M and others are 10M in total darkness. For the circuit above, the LOW resistance (the resistance in sunlight) is the critical value.
More accurately, the value for a particular level of illumination, is the critical. The sensitivity pot adjusts the level at which the circuit turns on and allows almost any type of photo-cell to be used.
More accurately, the value for a particular level of illumination, is the critical. The sensitivity pot adjusts the level at which the circuit turns on and allows almost any type of photo-cell to be used.