This desk clock was made by me using the printed circuit board (PCB) from an old wrist LCD watch (figure 1), a LED display (figure 8) and a driver (figure 5) to adapt the PCB (figure 3) to the LED display (figure 7). I made this desk clock in 1989. The idea was to not depend on power grid frecuency, wich has an accuracy of ± 1.5% , that leads to errors for clock of ± 2 seconds per day. The clock made by me has a much better accuracy of ± 1 second per month.

The quartz crystal (figure 4) I used has the nominal frequency of 32.768 kHz. The frequency tolerance is ±20x10^-6 at 25°C. To obtain a even better accuracy for the final clock, I used an adjustable capacitor mounted in parallel with the quartz crystal (see figure 10).

Figures 5 to 9 contain the schematic diagram of the project. The watch had initially a LCD display (figure 9) multiplexed in two sections (roughly the upper and the lower part of the display). The schematic diagram shows a driver section (figure 5) that adapts the signals designated to LCD display to the new LED display. This LED display has a configuration capable to simulate the old LCD part. For that I used four modified modules of seven segment, one digit LED (see figure 8). The second digit has been used upside-down. In that way, the decimal points of digits 2 an 3 make together a blinking indicator for seconds. Before soldering to final circuit, digits 2, 3 and 4 must be disassembled, their PCB layout modified and then reassembled. The LED modules must be with common anode and must be components not sealed on the back. The modified layout is shown in figure 8. In essence, each one of the three modules will have two separated anodes after modifications.

Schematic drawing of the driver is shown in figure 5. Each pair of segments is controlled by a comparator. In this circuit section I used single supply, quad comparators type LM339. There are no resistors provided for limiting the current through LED display. The limitation is provided by 339 comparators themselves. The circuits for the two multiplexed lines (anodes) are made with transistors. Figure 6 is showing the power supply schematic.

There are some modifications I made to the original schematic. Figure 11 contains these changes:
-power supply for driver is variable and goes from 0 to 3.6 V (instead of 0 to 5 V)
-power supply circuit for watch PCB is fixed and was slightly modified, the functionality remains the same.
-I added capacitors in parallel with quartz crystal. There is also a multi-turn trimmer capacitor for an accurate and stable adjustment of the clock frequency.

Diodes EFD108 are germanium diodes. If are not available, can be replaced by Schottky diodes 1N5817. The diodes used in driver circuit at the inputs of 339 comparators can be replaced by 1.5 MOhm resistors as shown in figure 18 (a project made later). The later project has two major advantages: seven segment LED modules do not need to be modified and display is not multiplexed.

There are also other types of clocks I used in other similar projects (figure 12, 13) that have LCD layouts as in figures 14, 15 and 16. Datasheet for the clock IC (with 3 1/2 digit dispay) is shown in figure 17.

Beside the wrist watch PCB, the project contains three more PCBs manually manufactured (figures 18, 19 and 20): a base mainboard, a display board and a board for support connections to wrist watch PCB.

The enclosure has been manufactured using transparent plastic panels, glued together with a plastic glue. When ready, the enclosure has been covered with silver color paint. The rear panel is detachable to enable the pulling out of the circuit board. It was kept transparent and small holes has been drilled in it. This is for enabling the frequency adjustment and measuring the battery voltage from outside, without disassembling the clock.

Advantages:
-front panel potentiometer for brightness control
-the clock accuracy does not depends on power grid frequency. There is an internal quartz crystal driven oscillator
-possibility for accurate adjustment of the clock speed (by adjusting the oscillator frequency)
-powered from mains power supply with low power consumption: 1.42 VA
-the backup battery is in operation only when mains power supply is interrupted, therefore it last for long time (years)
-after about 35 years of continuous use it still works just fine

Disadvantages:
-making modifications to LED modules and connections to watch PCB without damaging the components is a hard job to do, require a good soldering iron, accuracy and attention in work

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Links:
[1] - RCL Semiconductors Limited - manufacturer of digital watches ICs
[2] - LM339 - quad comparator
[3] - C1937 - clock chip