Inside Portobello’s Clock Tower: Gears, Relays, and a PIC

At street level, the job looked like changing a few clock hands. Inside Portobello’s old police station, it became a small lesson in mechanical engineering, embedded electronics, and the value of tracing a system one signal at a time.

The building on Portobello High Street began as municipal offices and a town hall. Construction started in 1877 and finished in 1878; it later became a library and then a police station. On March 23, 2026, Action Porty completed its purchase for community use, and volunteers were already peeling back decades of interior alterations when the clock question arrived: how do you set a tower clock whose controls are hidden above the attic?

The answer involved a Victorian-looking machine, a small early-2000s control box, and one very loud test at four o’clock.

The clock is a transmission, not a single object

A tower clock looks like one object from the street, but its hands are the final part of a much longer chain. In the Portobello Police Station tower, an electric motor turned a series of gears. This interconnected set of gears is called a gear train. Its job was to reduce the motor’s speed and deliver controlled rotation to the clock hands.

The key part was a shaft, meaning a rotating rod that carries motion from one place to another. The shaft turned once per hour and drove the minute hand directly. Each clock face had its own branch from the shaft, while another gear arrangement moved the hour hand more slowly. In an ordinary twelve-hour clock, the hour hand makes one revolution for every twelve revolutions of the minute hand.

That arrangement explains why setting the clock was not a matter of pushing each hand into position. The motor had to be separated from the gear train first. The useful discovery was a pawl, a small locking lever that normally prevents a gear from moving in the wrong direction. Lift the pawl, and the shaft could be turned by hand.

There was one comic complication. Looking at a clock from inside its tower means looking at the back of the hands. A direction that appears clockwise from the street can look counterclockwise from behind the dial. The mechanism had not started running backwards; the observers had reversed their viewpoint.

The mystery box wasn’t keeping time

The more surprising discovery sat beside the mechanical clock: a control box containing relays, a power supply, a battery charger, and a PIC16F628-family microcontroller.

A microcontroller is a small programmable computer built into a single chip. The “8-bit” description means it processes data in groups of eight binary digits, but that modest capability is more than enough for reading switches, counting events, flashing an indicator, and controlling motors. The PIC family also includes timer and counter hardware, though the wiring observed in this installation appeared to rely mainly on signals from the clock mechanism itself.

The important distinction is that the control box did not appear to contain a complete clock. There was no obvious real-time clock module whose job was to track hours, minutes, and seconds. Instead, the system seemed to use the main mechanical clock as its timebase.

That is a useful design idea. The mechanical clock already turns once per hour, so the electronics do not need to measure every second. They only need to notice the moment when the clock reaches the hour and then perform the correct chime sequence.

The box also contained relays, which are electrically controlled switches. A low-voltage circuit can energise a relay, allowing it to switch a separate, higher-power circuit. Here, that meant a small controller could operate the motor that drove the striking mechanism. The presence of a lead-acid battery suggests the electronics may also have been designed to survive short power interruptions, although a backup battery does not create an accurate time source by itself.

A bell system built from events

How can a clock chime the right number of times without knowing the actual time? By counting physical events.

A switch on the clock shaft acted as the hourly trigger. When the shaft reached the correct position, the switch changed state and told the controller that an hour had arrived. The controller then powered the chime motor. As the striker moved, another switch reported each completed strike. Once the count reached the required number, the controller switched the motor off.

A simplified mental model looks like this:

on hour_trigger:
 target = hour_counter
 strikes = 0
 chime_motor = ON

 while strikes < target:
 wait until striker_switch changes
 strikes += 1

 chime_motor = OFF
 hour_counter = (hour_counter % 12) + 1

This is not recovered firmware. It is a compact way to describe the observed behavior. The important idea is feedback: the controller does not assume that the motor completed a strike. It waits for a switch to confirm that the striker moved.

That makes the system more tolerant of real mechanical problems. If the chain slips or the striker stalls, the controller should not blindly pretend that a chime happened. It has a physical signal to watch.

Why the Advance button seemed broken

The control box had a button marked Advance, but pressing it briefly appeared to do nothing. The clue was that the button needed to be held for several seconds. That behavior is consistent with firmware waiting for a deliberate long press rather than treating every accidental touch as a command.

Once the button was released, the clock chimed. Pressing and releasing it again produced the next hour’s sequence. In effect, Advance taught the controller where it was in its twelve-hour cycle. The operator could set the mechanical clock hands by hand, then step the electronic counter forward until both halves agreed.

This is a small example of a state machine: a program that moves between named situations according to events. The controller might be idle, waiting for the hour trigger, striking, waiting for the striker switch, or responding to the Advance button. It does not need to understand the whole world. It only needs to react correctly to the next signal.

The flashing Status LED was another clue. A diagnostic LED is an indicator used to show internal conditions, often through a repeated flash pattern. Without a schematic or the original program, a sequence such as long-long, short-short-short is evidence rather than an explanation. It might represent a mode, a fault, or a stored counter value.

The repair lesson

The Portobello clock is a hybrid system: old gears provide the motion, newer electronics supervise the chime, and simple switches connect the two. When a machine like this looks mysterious, the best approach is to map its boundaries.

Trace where power enters. Identify which parts move. Find the switches that report movement. Then separate the questions: what keeps the hands moving, what announces the hour, and what counts the strikes?

That method is useful far beyond clock towers. Factory machinery, automatic doors, lifts, and hobby robots often work through the same pattern: an actuator performs an action, a sensor confirms it, and a controller decides what should happen next.

There is also a serious practical boundary. The chime motor was connected to mains electricity, meaning the building’s high-voltage supply, and the tower involved steep ladders and an old mechanism. Understanding the system is valuable; working on it safely requires proper isolation, access control, and competent electrical and mechanical skills.

The best test was a real one

At four o’clock, the clock showed the correct time and the bell struck four times. The result was technically satisfying and acoustically overwhelming. The chime motor was disconnected afterward, which may have disappointed the mechanism but probably pleased anyone living nearby.

The success did not come from replacing the old clock with a modern black box. It came from understanding how the existing pieces divided the work. Victorian gears kept the town’s time, a small microcontroller counted the hours, and a few well-placed switches let the two generations cooperate.