What It Is and How It Works
A dusk-to-dawn sensor (technically called a photocell or photocontrol) contains a light-sensitive semiconductor - typically a photoresistor (LDR) or phototransistor - that changes its electrical resistance based on ambient light level. In daylight: resistance is low → the control circuit keeps the LED driver OFF. At dusk (when ambient light drops below ~10–50 lux threshold): resistance rises → control circuit triggers the LED driver ON. At dawn (light level rises again): resistance drops → control circuit turns the driver OFF. The entire process is automatic, continuous, and requires no power to maintain its state - it just responds to ambient light passively.
Types of Photocells Used in India
Twist-lock NEMA photocell: an international standard socket-mounted photocell that screws into a standard NEMA receptacle on the luminaire top. Easy to replace without opening the fixture. Built-in photocell (most common in Indian street lights): a small sensor window integrated into the fixture body. Cannot be replaced independently - requires driver replacement if faulty. Remote-mounted photocell: mounted separately on the pole away from the fixture - useful when the fixture orientation does not allow the sensor to see the sky directly. The photocell must face the sky - if it is accidentally shaded (by the fixture body, a tree branch, or a mounting bracket), it will think it is always night and keep the light on during the day, draining the battery (in solar) or running the light all day (in grid systems).
Common Photocell Failures
Light stays ON during the day: photocell is shaded, damaged, or wired in reverse. The sensor sees 'dark' even in daylight and keeps the light on. Fix: check for shading, clean the sensor window (dirty sensor = artificially low light reading), or replace the sensor. Light stays OFF at night: photocell has failed open (infinite resistance) - the circuit never triggers. Test by covering the photocell - if the light turns on when covered, the photocell has failed. Fix: replace the photocell. Light flickers at dusk: the ambient light level is right at the trigger threshold - the sensor switches back and forth as clouds pass. This is normal behaviour for 10–15 minutes at sunset and sunrise. If it continues throughout the night, the sensor's threshold has drifted and the photocell needs replacement.
