Radio-Controlled Atomic Clock

Overview

A radio-controlled "atomic" clock keeps ordinary quartz time but corrects itself against a national time signal that is in turn locked to a real atomic clock. It does not contain an atomic standard; instead it receives a low-frequency radio broadcast, WWVB at 60 kHz from Fort Collins, Colorado, or DCF77 at 77.5 kHz from Mainflingen, Germany, that carries the current date and time as a slow digital code. Once a day, usually overnight when reception is best, the clock decodes that code and slews its hands and display to match, so it never drifts and handles daylight-saving changes on its own.

Several modules cooperate. The Quartz Movement Module is a quartz timebase driving a stepper that advances the analog hands. The Radio Receiver pulls the time code out of the air and hands it to a microcontroller, which corrects both the hands and the Display. Everything runs from a single Li-ion Cell, 18650 inside the Case, and the Button Set lets the user set the zone or force a reception attempt.

How it works

The everyday timebase is a quartz crystal vibrating at 32,768 Hz. The movement's circuit divides that down to one pulse per second; each pulse energises a tiny coil that flips the stepper rotor one step, and a reduction gear train turns those steps into the motion of the second, minute, and hour hands. Between radio syncs the clock simply free-runs on this quartz timebase.

Synchronisation rides on the longwave signal. The ferrite loopstick antenna, tuned by its capacitor to the carrier frequency, picks up the broadcast; the receiver IC amplifies and demodulates the amplitude-keyed code, whose pulse widths each second encode the bits of the full date and time. The microcontroller validates a complete frame, then commands the stepper to spin the hands rapidly to the corrected position and rewrites the LCD. A lock indicator on the display shows whether the last reception attempt succeeded.