How flip clocks work

Old fashioned digital flip clocks display digits on flip-down leaves. The mechanisms are described in this page.

Summary

An animation of how a split-flap display works

The earliest form of digital clock displayed the digits (and possibly days or the week and months), each split into top and bottom halves on adjacent leaves in a pack as in this animation. These are brought into Restart Parties from time to time as cherished legacy items, or treasured finds from a car boot sale or similar. They were electromechanical devices with ingenious mechanisms. An understanding of how they work helps greatly in any repair situation.

Safety

Old mains powered devices may have worn or perished insulation on the mains lead, creatng a shock or fire hazard. In the oldest devices, the design may be inherently unsafe, for example having exposed metal parts without an earth connection.

The Timepiece

Older clocks normally have a mains-driven synchronous motor, locked to the frequency of the AC mains supply. (The mains frequency is guaranteed to be stable over a 24 hour period, though it may vary slightly over the course of a day). In a very old example, if there are concerns over its electrical safety or if the coil appears to be open circuit, this website describes various options.

If the motor isn't turning even though the coil is definitely not open circuit, then the motor spindle or gears may need cleaning and lubrication. Excessive wear of the bearings may cause the gears to jam, which would be challenging to rectify.

Newer examples may have a standard quartz or radio-controlled timepiece. There are some branded Karlsson or Habitat (probably rebranded Karlsson) clocks where the flip display only displays the date. A conventional analogue time display is driven directly by the quattz movement.

In the case of a quartz or radio controlled movement, first check that the battery is good, and that it is a good quality alkaline type, and not a zinc or rechargeable type. Repair options are limited, and whilst replacements quartz clock movements are readily available online, it may be harder to find one containing the switch to activate the flip mechanism.

The Flip Mechanism

Purely Mechanical

Many flip-clocks have the flip wheels in-line, and like the digits of a mechanical odometer in a car, one complete rotation of each wheel triggers a flip of the next digit. This will take more force than a flip of a single digit, and the most force will be required at midnight when the minutes, hours and day are all changing together.

Such a mechanism is purely mechanical, and any repair will require you to understand what is hindering the correct operation, whether it be a need of cleaning and librication, or he result of wear.

Electromechanical

Karlsson, Habitat and possibly other similar flip clocks have an analogue time display, and separate electro-mechanical flip mechanisms for the day of week, day of month, month and year. These are all driven by separate electric motors and are linked electrically rather than mechanically.

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In the Karlsson and Habitat clocks, each flip mechanism triggers the next by means of a 2-way switch, the same circuit as is currently used to allow a landing light to be turned on or off equally by a switch at the top and another at the bottom of the stairs. This uses the following very simple circuit.

Note that a current can only flow and light the lamp if both switches are in the same position. Switching either switch will turn the light on if it's off, or off if it's on.

The sketch below shows two stages of an electromechanical clock. This assumes the quattz movement displays analogue time of day and the first two flip mechanisms display am or pm and day of month. A third (not shown) might display the month.

At 12:00 (am or pm), a 2-way switch in the quartz movement is flipped into its other position. This completes the circuit to the motor driving the first flip wheel via a reduction gear. The motor runs until the display has flipped, at which point a 2-way switch in this actuator is flipped into its other position, breaking the circuit and stopping the motor.

A further 2:1 reduction gear in this actuator operates a second 2-way switch connected to the next actuator. Thus, whenever the first flip display changes from pm to am (at midnight), the motor in the second actuator is activated, to flip the day of month display.

The reduction gear in this actuator would have a ratio of 30:1, causing the month to change after the 30th of the month. Manual adjustment is required formonths not having 30 days.