Photocell / Dusk to Dawn Sensor
A light-sensitive sensor that automatically turns lights on at dusk and off at dawn. Built into some fixtures or available as a standalone add-on for outdoor lighting.
A photocell is a semiconductor device that changes its electrical resistance based on ambient light levels. When it gets dark, resistance drops, current flows, and the light turns on. When daylight returns, resistance increases and the light turns off. No timers, no programming — just automatic response to natural light.
Fixtures advertised as "dusk to dawn" simply have a photocell built in. You can also add photocell capability to any outdoor fixture using twist-lock add-ons (that screw into a socket on the fixture housing) or inline modules wired between the power supply and the light. The advantage over timers is that a photocell adjusts automatically year-round — no seasonal reprogramming needed when sunset shifts from 5pm in December to 9pm in June.
One important compatibility note: not all photocells work correctly with LED fixtures. Older photocells designed for incandescent loads may not handle the low current draw of LEDs, causing flickering or the light staying on during the day. Look for photocells specifically rated for LED loads. Most dusk-to-dawn sensors also include an adjustable sensitivity dial to fine-tune the trigger point — useful if street lights or a neighbor's porch light are bright enough to prevent activation.
Related Terms
- IP Rating
Ingress Protection rating — a two-digit code indicating resistance to dust (first digit, 0-6) and water (second digit, 0-9). Higher numbers mean better protection.
Mentioned in
How To Adjust Dusk To Dawn Light Sensor?
Cover your dusk-to-dawn sensor with your hand and wait — that five-minute pause before the light responds isn't a fault, it's a built-in delay designed to prevent rapid cycling.
How To Set Landscape Lighting Timer?
A dusk-to-dawn photocell tracks sunset automatically year-round — so a timer that seems broken in December may just be reading a sky you accidentally blocked.
Do LED Lights Need a Special Photocell?
Ghost current leaking through a switched-off photocell is invisible to an incandescent bulb — but an LED driver needs only milliamps to produce a faint, all-day glow.