SS9-DLS

SS9-DLS ("Day Light Saving") photocell set at 55 Lux ON / 28 Lux OFF and part-night off from midnight until 6 AM. (2013)

Green-cased version set at 35 Lux ON / 18 Lux OFF and part-night from midnight until 5:30 AM (September 2014). The choice of colour for the casing allows part-night cells to be identified from ground level.

The clear component in the very centre of the circuit board on both units is the detector; immediately adjacent this is an LED that flashes in certain pulses depending on the cell's operation status:

Two flashes in quick succession every few seconds - the cell is operating in all-night mode (it has not yet established the midpoint of night). This operation occurs until the cell has operated dusk-dawn for one whole night.

One flash every second - the cell has calculated its night midpoint (and will therefore switch off its load at its programmed part-night time), but has not yet correctly established its programmed part-night early morning 'on' time. This operation occurs after the cell has operated dusk-dawn for one whole night.
The cell also operates in this mode at around the time of the changeover from Greenwich Mean Time to British Summer Time or vice-versa. The preparation for the transition automatically commences approximately one week prior to the "changing of the clocks" (assuming that the cell has a clear, uninterrupted view of the sky - i.e. it is not situated under tree foliage, etc.) and continues for approximately one week afterwards. After this period, the timings should be following the same clock settings being observed in Britain. (A slight "bug" that I have noticed with the transition period is that photocells with an early-morning ON time of 6 AM, as this example photocell has, accidentally skip an entire evening's operation at the start of the transition period, meaning that the load will not illuminate at all for one night. As far as I know, this only applies to photocells with the 6 AM calibration; all other early-morning ON times do not feature this error. After the first night of the transition period, the load will operate once again.)

One flash every ten seconds - the cell is fully calibrated for part-night operation, and will switch off its load at the programmed part-night 'off' time, and back on (if required) at the programmed part-night 'on' time. This operation occurs after the cell has been operational for approximately ten days without any interruptions to the power supply, and when the photocell has not entered its GMT / BST transition period. The ten-day period is reduced to four days when the date is close to the summer or winter solstice; presumably, the cell is quicker to calibrate its timing when the burning hours are above or below a set threshold.

A misconception is that the LED flashing equates to the photocell receiving the "MSF" Time Signal. This is not the case; the photocell does not have any means of receiving radio signals built into its circuitry. This is also proven by virtue of the fact that there is a variance of a few minutes in when the photocell deactivates its load at midnight - if the photocell did receive any sort of time signal, the timings would always be accurate, and there would not be the need for the calculation or transition periods.