Timeclocks and photocells

What to look for

• What is supposed to decide. A photocell watches light level and switches at dusk and dawn. It does not know the time. A standard timeclock switches at the same clock times every day. An astronomical timeclock computes sunrise and sunset from the date and the site, then applies an offset such as twenty minutes after sunset.
• Indoor schedules. Offices, retail hours, and corridors that should follow the building's day want a standard timeclock or a schedule in the lighting controller. Sunset is the wrong trigger for a room that closes at 6.
• Outdoor, parking, and sign lighting. A fixed on-time drifts as the seasons change. An astronomical clock tracks that drift. A photocell is the simpler choice for a single dusk-to-dawn circuit with one threshold and no weekly schedule.
• Location settings on an astronomical clock. The clock needs the right time zone and site settings, or sunset is early or late every day. Confirm the offset after the first evening rather than assuming the factory default.
• Exceptions and override. Holidays, weekends, and service calls need a way to hold the lights off or on. A clock with no override leaves occupants stuck with the schedule.
• Contact rating. Many clocks only pilot a relay or a contactor. The contact has to be rated for the lighting load, including LED inrush, or the clock should drive a relay that is. A photocell has the same limit.

Common mistakes

• A standard timeclock on parking or sign lighting, then changing the on-time every month as sunset moves.
• A photocell aimed at the fixture it controls, a bright sign, or a window, so the light it switched holds itself on or off.
• Expecting a photocell to hold business hours. Clouds, headlights, and a dark storm can switch it. It only watches brightness.
• An astronomical clock left on the wrong time zone or site settings, so the whole year is offset.
• No manual override, so nobody can turn the circuit off for maintenance.
• Letting a pilot-duty clock or photocell contact switch a large group of LED drivers directly.

Shop timeclocks on eBay

General buying advice only. This guide does not speak for any manufacturer, and it names no brands or part numbers. The shop is separate.

Related lessons

• Occupancy vs Vacancy vs a Photocell
• Daylight Harvesting vs a Photocell
• Scene vs Schedule vs Occupancy
• Timeclock vs Astronomical Timeclock