Electromagnetic Pendulum – making it tell the time

by Stephen Hobley on May 31, 2011


A while ago I posted a video showing a pendulum driven by a pulse motor:

It worked by using a single coil to detect the field from a magnet embedded in the pendulum base. An op-amp (wired as a comparator) detects when the pendulum reached the equilibrium point (vertical) a voltage (5-12v) is applied to the coil for around 20ms. This pushes the pendulum away. The accelerating force on the pendulum is therefore two-fold – the iron core of the coil attracting the pendulum on its approach, and then a small “kick” pushing it away.


Additionally I’ve found that the pendulum can keep still moving, at a reduced amplitude, if the kick is applied every second or third crossing of the equilibrium point. A uController like the Arduino [BlogCred + 10!!] makes it very easy to control this – along with other parameters like pulse width, and offset.

The interesting thing about pendulum mathematics, is that it’s only the force of gravity and the length of the pendulum that affect the period. Not the mass of the pendulum. So to get a 1 second, full cycle pendulum, the length needs to be around 24 cm long. – Just under the length of a standard ruler.

(Additionally, it appears the energy required to keep a pendulum swinging is not directly connected to the mass either – so in theory you can keep a 6-ton pendulum swinging with the same energy as a 6-ounce one. Of course, friction and air resistance will also play a part, and could be radically different for either pendulum).

Of course, this is for a light, inextensible, frictionless pendulum, operating in a vacuüm (gotta love O-level physics!)

In this new video, I added a simple ratchet mechanism, so that the oscillation of the pendulum could be converted to rotary motion, suitable for driving a clock:

In an attempt to control the amplitude of the pendulum I added a sliding weight, and damping springs. These allowed some degree of control, but both the amplitude and the frequency are affected.

There are some problems with this ratchet arrangement. Since the gear is able to spin freely, there is nothing keeping it engaged against the lock after each advancing “tick” of the pendulum. This may resolve itself once the rest of the clockworks are added.

It would also be nice if a mechanical arrangement could be devised to allow the use of a 60 tooth ratchet – eliminating the need for 15:60 conversion gears.

More to come…


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{ 13 comments… read them below or add one }

thedude June 2, 2011 at 4:11 am

things that would slow down a pendulum are frictional force and air resistance, which are related to mass and surface area, so the force needed to keep it moving would depend on mass.

siege June 6, 2011 at 3:15 pm

Interesting to see a pendulum be the driver via a ratchet rather than an inhibitor via an escapement.

Bob L June 6, 2011 at 10:50 pm

Take a look at Clayton Boyer’s Electra for a method to eliminate most of the gears by using cams:


Stephen Hobley June 6, 2011 at 10:57 pm

That’s fascinating and totally novel – although I’m still scratching my head to grasp how the escapement/drive works…

Never thought of spirals as a driving mechanism.

BigHank53 June 7, 2011 at 1:29 pm

There’s a book called “My Own Right Time”, by Philip Woodward, on the history and construction of ultra-precise mechanical clocks, which covers the Shortt pendulum clocks. They used a mechanical arm to impulse the pendulum, which was reset electromagnetically–that reset signal was used to drive slave dials. It’s an expensive book, but you should be able to request it through any decent library, and it’s a great read for anyone who like clocks.

Chris August 25, 2011 at 12:19 pm

Hi Stephen

Thanks for posting this information. I`m currently building Dr Woodwards gearless clock as “Bighank53″ mentioned above. The details are on my website – http://www.raynerd.co.uk, I just managed to get it running a few nights ago and I`ve posted a video. I found your website because my next project I intend to be a synchronome but now after reading this, I`m considering using a similar setup to above! Seems quite novel! I`m in the UK – Manchester, get in touch if you find time. I also second that you should read My Own Right Time by Dr Philip Woodward. An excellent book and certainly links to what you are doing! You could use a daisy motion to drive the minute hand to the hour hand and your clock would be gearless as well!


Stephen Hobley August 25, 2011 at 12:50 pm

Chris – this is awesome – the “gearless” clock design is *exactly* what I’ve been looking for to complete my Clock of the long weekend – I needed a compact 60:1 gear ratio, and was getting ever closer to the “nested ratchet” idea.


(My brother lives in Stalybridge)

TheKackler September 7, 2011 at 8:19 pm

Is using solar panels an option?

Stephen Hobley September 10, 2011 at 8:50 am

Yes – I think solarbotics sells a kit that does jjst this – not sure if it could drive a clock though.

Brent October 4, 2011 at 9:20 am

Amazing example! I thought the projects on those DIY tech sites where detailed, but this clock electro magnetic pendulum is taking DIY clocks to a whole new level.

Chris Kendal January 1, 2012 at 4:18 pm

Following this with interest, have built an electromagnetic pendulum clock with a simple transistor drive but the battery life is shockingly short! In my research for alternatives I have found this….
Do you have any circuit diagrams you would be willing to share to help me!
Also based in Manchester UK, well Stockport actually!!
Have just ordered the recommended book too.


Emma D'Arcy November 5, 2012 at 10:14 am

Hi there,
I am an art student and am currently constructing a 3.2m pendulum using an electro magnet as you have demonstrated. I was wondering whether you have an schematics that you could send me, and perhaps the arduino script you used!? Hope this is not too much to ask, am very new to the area but keen to learn quickly.
Many thanks,

SkipF April 19, 2013 at 12:20 pm

I’m considering a small fan frame as a pivot. I’ll mount a 3 mm pin (drill
shaft) in a 8mm fiberglass rod that goes in the fan’s bearings.
(I haven’t decided on sleeve or ballbearing, yet.) I’m also keeping the X coil assembly. There seems tobe twin windings, grounded by a latching hall switch. IF I mount 2 Neo-
magnets on either side of the pivot, I can use the coil assembly to push the pendulum.
Twin coils means I can put them in series or parallel, or only use one and use the
other as a sense. I’ll put my ‘scope on the coils and see what I get…

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