Wall Clock

Overview

A wall clock is a large analog timepiece driven by an inexpensive quartz module running from a single dry cell. It works on the same principle as a quartz wristwatch, a 32.768 kHz crystal divided down to a one-second pulse, but the mechanism is scaled up and simplified: moulded plastic gears, a single AA battery, and a snap-together housing. The result keeps reliable time for about a year on one cell while sweeping three hands across a wide printed dial.

A wall clock packages this timing module behind a protective face. The Quartz Movement Module generates and reduces the timing pulse, and it is mounted inside the Case / Housing, which holds the printed dial, applied numerals, and a protective lens within its frame. The Hand Set presses onto the movement's concentric shafts, power comes from the Battery Compartment, and the Shaft Mounting Nut & Washer clamps the movement shaft collar through the dial and case.

How it works

Inside the movement housing, a small oscillator circuit board carries the quartz crystal and a CMOS divider IC. The IC counts the crystal's 32,768 oscillations per second and emits one drive pulse every second to the stepper coil.

That pulse energises the drive coil of a single-coil Lavet stepper motor. The shaped soft-iron stator channels the coil's flux around a permanent-magnet rotor, which advances one half-turn per pulse. The rotor's motion enters the reduction gear train, where successive plastic wheel-and-pinion stages step the speed down to the correct hour, minute, and second outputs.

The slowest stages drive the concentric hand shaft, also called the cannon pinion, which carries the hour, minute, and second arbors out through the centre of the dial. A friction bushing on the seconds arbor sets hand drag so the second hand holds position between steps. Spring contacts feed the AA cell's voltage from the battery compartment to the board, completing the circuit.