Many people ask how they can run DC (30 second) Slave Clocks without the Master Clock.  To do so you have to generate a pulse every 30 seconds, which is precision timed, with the ability to operate the slave clocks.

The precision clock is the difficult part as many timed devices use simple chips that tend to time slip.  What is needed is a time device with a quartz clock and a contact set that has a current rating high enough to operate a relay or a couple of clocks.

Please beware that all the devices do not have powered relay contacts and the clock power will have to be obtained from a separate source to the power feeding the clock control circuit.

Below are some ideas of which some are modern and some, Heath Robinson in their construction.

This web site does not endorse any of the methods/products below and they are presented to you at your own risk.

It is also advisable to place a fuse in the clock wiring and there must be a current limiting resistor in series with the clock/clocks.  The clocks generally have their current rating on the back and a web based Ohms law calculator can work out the resistor value.

One must also ensure that the relay on the clock controller is capable of the loads required and in many cases a slave relay will be required which has contacts capable of such loads.

The GPO used the following table to calculate the resistor value.  The clock resistances are assumed to be in the range 7.5 to 10 ohms. The resistors used were called a Resistor, Coil No. 9 and these were wire wound, on a ceramic bobbin and rated at 10 watts.

Values of resistances (Ohms)

No. of Clocks in circuit 24volts 40volts 50volts 60volts
1 70 135 175 205
2 65 125 165 195
3 55 115 155 185
4 50 110 150 180
5 40 105 145 175
6 35 95 135 165
7 25 85 125 155
8 20 80 115 150
9 10 70 105 140
10 5 65 100 135

The GPO also fitted a 1uf capacitor and a 200ohm resistor, in series, across the operating relay contacts.  This reduced sparking due to the inductive load.

Velleman VM206
These can be obtained from a company called Quasar Electronics.  This is a fully configurable timer.

Quasar advise of the following:-

  • The USB socket is used for programming the timer.

  • The board requires a 12Vdc power supply otherwise the relay will not operate.  Order codes 660.446UK (requires connector 777.001UK) or CFE103.

  • The board uses a microcontroller that has a quartz crystal oscillator for accurate time keeping.

It is also advisable to purchase a VDR as the clocks are inductive - Order code VDR300.

Follow this link for more information

A clock enthusiast purchased a VM206 and has been running two pulse clocks with it.  As there are only two clocks they were directly connected to the clock unit relay.  The VDR was also purchased and this smoothed out the pulse to the clocks.

Programming the clock unit is via a USB port and Quasar have an app to down load.  The app worked a treat, but they did, however, find that the clocks were running up to 15 seconds slow after 7 days, but other than that it ran fine.

EC4A Slave Clock Driver
This is an electronic unit that will work on 3 volts to 24 volts or 3 volts to 50 volts, depending on the model.

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Cheap and simple
This is a standard wall type kitchen quartz clock mechanism (with a second hand) that has the minute and hour hands removed.  The clock mechanism is fitted to a plate and two sensors are fitted 180 degrees apart, at the point coinciding with the end of the second hand.

There are two types of sensors that could be used:-

  1. The glass contact insert of reed relays could be used and a magnetic fixed to the end of the second hand.  The two inserts would be wired to a relay, via a battery, that operated the clocks as the reed relay contacts will not sustain high current.

  2. Speed sensors are used with the appropriate circuitry and contacts that can take the current required.


This is an archive of a previous article.  It refers to Maplins, which closed a number of years ago, and is not really relevant today.

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 x 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 32 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: October 15, 2023