How part-night photocells work

At the heart of a Lucy Zodion part-night photocell is a microcontroller which measures the time elapsed between selected symmetrical light levels in the evening and morning. These light levels are DIFFERENT from the normal photocell switching levels, which are generally asymmetric and can introduce systematic errors into the timing.

Using this data, any part of the night can be calculated. This process is repeated each night and the photocell continually adjusts for changing light and night patterns throughout the year. The algorithm that controls the timing now requires one complete night before the GMT time is known (and hence part-night operation can be instigated). During the ‘GMT Time Learning’ period it will operate as a conventional photocell and activate the lamp on a dusk-dawn regime.

Part Night photocells Table

The use of daylight changes to determine the time of day is subject to an annual rhythm that naturally leadsto a +/-15minute variation in timing throughout the year.

As standard part-night photocells switch around the same time every night of the year, and do not change in spring and autumn when the ‘clocks-change’. Therefore, they have different switching times in GMT and BST. A unit that switches off at midnight and on again at 5:30 GMT will switch at 1am and 6:30am BST. The latest development for our part-night photocells includes automatic GMT/BST Daylight Saving correction.

The transition is achieved by observing changes in Night Length and automatically switching between GMT and BST. As the unit doesn’t know the exact calendar it has a period of uncertainty around the spring and autumn transition. The unit compromises the OFF period during the days around this transition, to ensure that the light is never OFF more than 15 minutes before the ‘OFF’ transition, and is always ON 15 minutes after the ‘ON’ transition (within about 10 days of each transition the light may be ON for an additional hour in either the evening, or morning). Apart from this period the unit accurately tracks GMT and BST. If DLS units are sited under heavily shaded tree canopies they will misjudge the season and will transition between GMT and BST late, and return to GMT early.

Programme Options

The photocell is programmable to allow for various part night options. The periods of control are factory set, but can be made in 30 minute steps and typical periods are shown in the table below. The unit is programmed when it is made and whilst it can be ‘re-programmed’ it is only possible to modify the regime in our factory. The programme and settings are non-volatile, so will not be lost during power-cuts however the part night timing can take up to 28hrs to re-acquire following a power cut.

Typical Burining hours

For comparison

Part-Night Burning Hours

On part-night to off units (SS9, SS19) the ‘dim’ period is the ‘off’ period

If you’d like to explore our range of part-night photocells, please head to our photocells page here. 


For more information about photocells and the way they work, please explore the related KnowHow articles below…