This circuit at some adjustment levels is sensitive enough to be triggered by ambient light if the photocell is not sufficiently covered with tape or other material.
Photocell PC1 should be a "photoresistive" device, meaning its resistance changes with changes in light. This is typical of Cadmium Sulfide (CDS) photocells. Many common CDS photocells vary from many thousands of ohms when exposed to very little light, to only several hundred ohms when exposed to significant levels of light. This circuit will work with a wide range of photocells.
Trimmer pot VR1 is used to adjust the sensitivity of the circuit to the light level on the photocell PC1 and can vary in value, possibly as high as 10,000 ohms (10K), however the sensitivity adjustment characteristics will probably change as the resistance of this device changes. The resistance values of the photcell and trimmer pot interact to some degree as they are changed. The trimmer pot will never be adjusted very close to counterclockwise (CCW). This could be changed by adding an additional resistor on the CCW end of the trimmer pot, but the value of the resistor would be dependent on the photocell and trimmer pot characteristics.
Transistor Q1 is used to amplify the small change in voltage/current created by the photocell PC1 and the trimmer pot VR1, and to insure that there is enough gain to turn on the LED at low light levels.
Resistor R1 is needed to limit the current through the indicator LED to its specified maximum current rating. If this rating is not available in the data sheet or other documentation with the LED, it can be assumed that a typical T 1 3/4 ("T one and three quarter") LED is around 20 milliamps (.02 amp). There may be wide variations in the amount of current needed to light a particular LED to full brightness.
Battery voltage can be reduced to as low as 3 volts but will cause the circuit to not work earlier in the discharge life of the battery compared to a 9 volt battery. Also, the snap type battery clip arrangement of most nine voltage batteries is generally more reliable than the pressure contact type of holders for AA and AAA sized 1.5 volt batteries.
SW1 is an On-Off switch, but the power consumption of this circuit is quite low when not triggered, so this switch may be eliminated in many cases.
If the battery voltage is changed the value of R1 should be changed accordingly, and can be calculated with this formula: R1 (in ohms) = battery voltage / .02 (amp). Then, the closest common resistor value is used.