A Quick and Easy Way of Running ex-GPO/PO
Slave Clocks

For those of you who aspire to own a working ex GPO/PO slave clock but do not have the money to afford or have the space for a master clock (e.g. type 36) and all its associated wiring and relays etc, here is a "quick and easy" method of achieving this.

This method was devised for those with limited knowledge of electronics and/or construction of printed circuit boards, and as such the accuracy of the timing functions cannot be guaranteed, but it is a good, cheap starting point. 

The whole thing is based upon a Velleman kit "adjustable interval timer" MK3 (Maplin cat no. VT27E).  This kit which comes complete with all components and a pre etched PCB and is all you need apart from the battery and some wire.

The kit requires basic soldering skills to complete and comes with graphic instructions on how to complete it ,when completed the kit can provide a pulsed output (via a relay) adjustable between 2.5 seconds and a minute, so set it for 30 seconds and hey presto 30 second pulses to operate a slave clock (the length of the pulse can also be set from 0.5 to 5 seconds).

The kit runs off 12v and if you use this same 12v to operate the coil in the clock the whole thing can be run from the same battery , there is no need to worry about limiting the current through contacts because there are none and the relay in the kit can handle 3A at 24V so the 12V battery will do it no harm, the output will easily operate two or three clocks in parallel as well.

The easiest way to supply the 12V is with 8 AA size batteries (8 x 1.5V) in a battery holder (Maplin cat no. RK44X @ 69p) and a PP3 battery clip (Maplin cat no. HF28F @ 29p) no switch is needed, to turn it off simply disconnect the battery.  This way both the circuit board and the battery easily put behind the clock, and can be secured with double sided tape or even Blu-Tak or similar.

Off-board wiring is simple, after connecting the battery clip to the terminals marked +12V/-12V take a second wire from the +12V terminal to the clock, link the -12V terminal to the NO (normally open) terminal and another wire from the COM (common) terminal to the clock (the polarity at the clock is not important) and that's that.

In practice and with careful adjustment of the variable resistor (part of the kit) I had mine running at 2 seconds i.e. pulses every 28 to 32 seconds, this sounds bad, but the early 28 second pulses tend to get cancelled out by the late 32 second ones.  And I did say quick and easy, not 100% accurate, but at around a fiver its certainly worth thinking about.

For those of you with a master clock already running or a number of slaves around the house etc, a unit adjusted to run at fast pulse rate (I got mine down to under 1 second) makes a wonderful way to adjust your clocks fast as putting a clock forward by an hour only takes 60 seconds, much better than standing holding the advance key.

Kevin Sanderson


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Last revised: January 03, 2022