Fixing the Portobello Police Station Clock

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The first step was to figure out how to set the time. This is probably easy to someone in the know, but we weren't.

We figured that the clock is driven from the small motor, through a series of gears onto a shaft which rotates once per hour. This shaft is split into three - one for each face, and drives the minute hand directly. A gear on each clock face derives the hour hand. If we could disconnect the motor maybe we could turn the shaft directly and set the time.

Sure enough - we found a pawl on one of the gears which could be lifted out of the way. Once this was done the shaft turned easily and we could set the time. Each hand had a counterweight on the clock face and we could set the time by imagining the hand we couldn't see opposite to each counterweight.

We had a slightly stupid moment when we thought we'd got it wrong, and the clock was running backwards - only to realise we were inside the clock and so were looking at it the wrong way. Doh. We had to head down to the ground to check that the time was correct - but - phew - time was OK and not running backwards.

Figuring out the chime was more difficult. The original mechanism had been modified, and switches and a motor had been added. These led to a box containing a circuit board which definitely was not from 1877. It contained a PIC 16F628 microcontroller which had a suspected date-code of 2001. There were a number of relays, a power supply and a battery charging circuit to charge the included lead-acid battery. I assume this box would be getting on for 1/4 century old, but it still seemed to work. No idea how good the battery was; maybe this should be tested another time.

We have idea who designed and built this box; there were no obvious description markings. Is it a standard thing, or just a one-off for this clock?

A mystery unmarked circuit board, ripe for deciphering.

This box didn't have much of a user interface, other than a non-obvious button marked "advance". The only problem was that pressing it did nothing. How did this thing work? Surely you had to set the time somehow.

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