Revive a railway clock

Since few summers ago I have kept my eye on a popular bidding site. I was attracted by a few German railway clocks (as a less-famous brother to their Swiss counterpart). I have since brought several, some even have second hands (not synchronised though), and rescued them from the brutal conversions by the greedy vintage dealers. I then also buy the British ones, which are mechanically much more fun. These dealers normally buy these clocks at around £30 and try to sell them over £100 after converting the slave movements to quartz — and people buy them!? I would not stand such behaviour, so I took the problem to my own hands, aiming to save the whole clock with a cost under £20. Inspired by a similar project from veeb.ch, I decided to make a more robust and reliable code.

Different types of clock drives

As far as it’s know to me, there are two main types of slve clock drive mechanism. A latch system, and a motor system. I will give some technical details on both of them and compare them. Please note that this is not been looked into in a great detail, so please take it as personal opinion. The latch system, all remaining examples accessible on the auction sites, are half-minute pulse driven. The motor system all seems to run under one-minute alternating pulses.

The mechanism of these two systems are different, and in modern engineering’s understanding, the motor system should be more reliable than the latch system. Let me explain why:

The motor system resembles an AC motor, where an alternative magnetive field generated by the stator — an extended ferrous horse-shoe shaped stuff winded by a electromagnet — and a rotor connected directly to the minute hand — a two-pole magnet. There are some magnetic design to avoide system spinning anti-clockwise (it’s a clock so it mustn’t do it!), we might go in to that in future. The electromagnet is excited by a pulse with alternating direction direction each minute so the clock spins. This system was initially desined to run with mechanical master clocks and mechanical rotating junction to change polarity of the pulse, but modern application shouldn’t touch those.

The latch system is far more simpler electrically, it only needs a pulse of any polarity. A electomagnet is positioned near a spring-loaded lever, with a steel plate on one end and a latch on the other end. Every minute, the lever ticks once. This drives the latch down one tooth on the main gear, which turns the clock as the lever resprings. There’s a gravity-driven anti-anti-clockwise mechanism, running by a latch stationed above the main gear. Everything else is very much the same, a 60:1 ratio gearing between minute hand hour hands — that sort of stuff.

Materials

• Clock electrically and mechanically working
• Raspberry Pi Pico W
• Cables
• Power supplies
• Converters
• (additionally if you wish) a local NTP server

Clocks

There are two type of clocks mechanisms drive systems widely available in the UK: minute alternative pulse, and half-minute pulse.

Minute alternative pulse is a simplified AC motor, extremely similar to a modern quartz movement, but runs on a much lower frequency at 1/120 Hz. Pulses at +V and -V (usually 12, 24, 60 etc.) with duration less than a second at one-minute interval are sent to the movements, and the clock turns. Clock from European manufactures (such as Mobatime, T&N etc.) majorly adopt this mechanism.

Half-minute pulse works with a latch system, used by many British manufacturer. The clock at Baker Street tube station also works with this mechanism unmodified (I presume?). A pulse at same voltage and current is sent to the clocks every 30 s to trigger the latch, and advance the minute hand for 1/120th of a turn. This is simpler in electrically as it doesn’t require alternative pulse, but it is mechanically less durable as mechanical contacts are requires. I might go about how to CNC mill a replacement latch but that’s a story later.

Code

The code is available at Github, if you can’t understand them, then please stop here and visit the vintage dealers.

Circuits

I will provide more detail to this later, as I am currently working on a continuing project with a half-minute pulse clock running on a lower voltage.

Future work?

It worked, it works, and it will work. Not much future work is required right now for the drive, apart from the occasional network glitch that ruins the timekeeping. However, I need to fix the coil on a movement and rubber gear on another so they can be connected up to the current driving system.