Volunteers Work Out How to Set the Portobello Police Station Clock

On 11 April a volunteer's friend asked: instead of Edinburgh, would they come to Portobello and try to fix the old police station clock? "We can't work out how to change the time on the clock tower," said Action Porty, the community group that recently bought the building.

The building and the tower

Construction began in 1877 for the Portobello burgh council, housing its meetings, administration and courts; later it was a library, then a police station. The Scottish Land Fund has since enabled its purchase for community ownership, and volunteers are stripping 1970s polystyrene ceiling tiles.

The mechanism sits high in the tower: the 1877 clockwork with three faces, now modified — electric motors power it, and a control box can switch the chime off overnight.

Setting the time

A small motor drives gears onto a shaft rotating once per hour; the shaft is split into three — one per face — and turns the minute hands directly, while a gear on each face derives the hour hand. Lifting a pawl on one of the gears freed the shaft; each hand carries a counterweight, so the time was set by imagining the hidden hand opposite it. They briefly feared the clock ran backwards — they were inside it, looking from the wrong side — but a check from the ground confirmed the time.

The mystery chime box

The chime was harder. The modified mechanism's switches and motor led to a box holding a circuit board not from 1877: a PIC 16F628 microcontroller with a suspected 2001 date code, relays and a charger for a lead-acid battery. The box is about a quarter of a century old but still works.

The box has almost no interface — just a button marked "advance". It switches a mains motor driving the chime striker, so the clock strikes while the motor runs; a switch on the striker counts the chimes and stops it, and a switch on the shaft triggers on the hour. Holding advance for a few seconds and releasing makes the clock chime, and each further press adds an hour — apparently how the time is set. The box has no clock, just an hour counter.

A successful mission

At 4pm the time was correct and the bell chimed four times — lucky, since 4pm was when the volunteers had to leave. Because some residents may not have welcomed the chime's return, the chime motor was disconnected. Over a beer, the group listed enhancements: finer control of chime times, thirteen chimes at Halloween and LED lighting. The community-owned building has far more potential, the volunteer wrote.