Astronomic Timer vs. Photocell for Landscape Lighting
An astronomic timer calculates switching times from calendar and location settings. A photocell responds to the light it senses. A fixed timer follows the times you program. The best choice depends on whether you want seasonal adjustment, response to local darkness, or a simple fixed schedule.
Choose the behavior you need
- Astronomic: useful for a sunset-related schedule that changes through the year. Correct date and location settings matter.
- Photocell: responds to ambient light. Placement and nearby artificial light can affect operation.
- Fixed timer: suits a specific schedule, but may need seasonal changes.
Set an appropriate off time
Lights do not have to run until dawn merely because they turn on at dusk. Choose the operating period for actual use and any project requirements. A shorter schedule can reduce energy use when the lighting is unnecessary.
Check compatibility and features
Confirm the control's voltage, load rating, installation environment, and fit with the transformer. Astronomic scheduling does not automatically include Wi-Fi, independent zones, or dimming. Those require separately specified capabilities.
Commission and record
Set the date, time, location, and daylight-saving options as required. Test manual operation, return the device to its automatic mode, and check the next scheduled event. Save a photograph of the settings.
If lights operate at the wrong time, inspect settings and overrides before replacing the transformer. For a photocell, consider nearby lighting and obstruction. Follow the device's testing procedure instead of permanently bypassing it.
What is an astronomical timer?
An astronomical timer, also called an astronomic timer or astro time clock, uses its configured date and location to calculate sunrise and sunset. It does not need to measure the light at the sensor location to follow seasonal sunset changes. A photocell instead reacts to the ambient light reaching its sensor.
For example, you might want the landscape lights to switch on around sunset and off at 11 p.m. An appropriate astronomic timer can follow the changing sunset while keeping the off time fixed. Confirm that the specific model supports the events you need.
| Control | How it switches | What to check |
|---|---|---|
| Astronomic timer | Calculated sunrise/sunset or supported programmed times | Date, location, time zone, daylight-saving settings and event limits |
| Photocell | Ambient light at the sensor | Sensor exposure, nearby artificial light and the device’s switching delay |
| Fixed timer | Clock times you select | Clock accuracy, seasonal schedule changes and power-loss behavior |
Example: Intermatic DT200LT
The Intermatic DT200LT supports one daily on event and one off event, with dusk/dawn or fixed-time choices. Check the transformer compartment and the applicable load rating before selecting it. Read the manufacturer's specifications and setup instructions for the exact installation.
Timer and photocell questions
Will an astronomical timer react to a dark storm?
Its astronomical schedule follows calculated sun times, not the brightness of an individual storm. A photocell responds to the light at its sensor, subject to that device's threshold and delay. Choose the behavior your site needs.
Can I use both a timer and a photocell?
Some transformers support combined controls. Follow that transformer's specified arrangement: one control can prevent operation even when the other calls for light. Do not add or bypass controls without checking compatibility and the wiring instructions.
Why are the lights switching an hour early or late?
Check the clock, time zone and daylight-saving settings first. Then check the location setting, any sunset offset and whether a manual override is active. If the system also has a photocell, inspect its exposure. Test one setting at a time and record the working configuration.
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- Kevin Simon