If you live in a tenement, you may well have been frustrated by power cuts affecting the time the stair lights come on and go off. 

The reason power cuts affect lighting timing is because the timing on each stair is controlled by an ‘Astro clock’, or timeswitch, which turns the lights on and off at the right time of day for the time of year. 

Astro clocks can be electronic or, in older stairs, electromagnetic. At least one firm of electricians advises stair-dwellers not to touch them: ‘Unless you are a qualified electrician you could be putting yourself at risk of injury and/or electrical shock. You may also find that any personal accident insurance you have might not cover you.

I ignored that advice and investigated the Astro clock in our stair. It lives in a box high up on the wall by the door.

Stair lighting timeswitch box

Our box isn’t locked. Open it up, and there’s the Astro clock timeswitch. (On the inside of the door there are stickers indicating that Edinburgh District Council serviced the timeswitch until the early 1990s.)

The time (GMT) is shown by the pointer at the very bottom of the dial. The date is shown in the calendar insert. An electric motor turns the dial round once a day, moving lugs on the outside of the dial past on and off switches (see below).

Stair lighting timeswitch

Without taking the cover off, it’s possible to turn the lights on or off using the manual override switch at the bottom left. To adjust the time and date, the cover needs to be opened using the clip at the top.

After opening the cover, you can pull the unit out with the handle seen under the plastic cover at the very bottom – it’s just like a plug, but this one has four pins (see below).

Back of stair lighting timeswitch

This is a Horstmann QMK timeswitch, with the date 5/78 (top right) – so presumably it has been humming away in our stair for about 40 years, without being serviced by the Council for 25 of them.

Horstmann QMK timeswitch

The brass mechanism (behind the dial) looks to be very solid. The nut in the middle seems to have a space for oil – but it’s well oiled, so no need to add any.  According to Edinburgh Stairlighting,  ‘the averge age of an Astro timer clock for stair lighting is 40–50 years old. Some clocks are 70 years old and some of the first generation digital clocks are 30 years old [and] are failing due to technology at the time of manufacture’.

To adjust the time, just twist round the dial using the two knobs on the main dial.

Adjusting the time on a timeswitch

To adjust the date, use a screwdriver to turn this screw (see below).

Adjusting the date on a timeswitch

Then there’s the reverse procedure to plug the clock back into place in its box in the stair.

I don’t think it’s necessary to take the clock out to adjust the time and the date – but you’d have to make the adjustments while standing at the top of a ladder.

There are no manuals for the Horstmann QMK online, but I found the following videos, produced by the timeswitch enthusiast community, very helpful in learning about how the timeswitch works:

David Sterratt

[Image, top-right, from Visualpharm.]