Driver for minute clocks

This page describes a circuit for driving clocks which require a phase changing signal every minute. Clocks of this type were used in large offices, before battery driven crystal clocks were a commodity. An example of these clocks is shown below.

The hands of these clocks were driven by a simple single phase stepper motor. A mechanical construction garanteed that the rotor could turn in only one direction, clockwise. Each phase change moved the minute hand one minute further.

The 50 Hz is derived with an optocopler from the mains frequency. The circuit uses four dividers and a flip-flop to reducethe 50 Hz pulses to a symetric square wave of 0.833 mHz (milli-Herz), or a phase change each minute. On the board this is a 74HC390 as 100-devider, a 74HC390/74HC00 as 60-devider and a 74HC74 to make the signal symetric. A relay changes the phase on the clock coil.

This is the schema

Discussion

  • The adapter used must be AC, alternating current.
  • The optocoupler is optional. With some diodes and resistors a cheaper pulse generator can be made.
  • A short test period proved the circuit works, but seems to a bit fast compared with a stable clock like a atom-clock receiver (DCF77). This could be a problem in the circuit, or an effect of the short-term instability of the mains-frequency.

    fjkraan@xs4all.nl

    Last updated: 2005-11-27