As a decorative addition to my home, I developed a wall clock designed to illustrate the basic structure of a digital clock by making the individual components of a quartz clock as visible as possible. Originally, my mother gave me a picture frame intended to hold a photo collage. Since I had no photo collage to put in it, I repurposed the frame to build my clock.

Due to the rather unusual layout of the picture frame for a clock, it made sense to divide the circuit into five functional sections: Power Supply, Clock Generator, Clock Selector, Clockwork, and Display.

Power Supply

The internals of an original Sony PSP 5V power supply provide power to the circuit. Originally, I used a 12V power supply instead, with an LM7805 voltage regulator to reduce the voltage to 5V. However, this setup proved to be extremely energy-inefficient, particularly because of the linear voltage regulator. I therefore simply recycled the power supply from my old Sony PSP.

Since this clock was intended to illustrate how a digital clock mechanism works, I mounted the individual circuits directly onto the picture frame without any visible cover. An open installation is perfectly safe for the Clock Generator and the Clockwork due to their low voltage levels of 5V. The power supply, however, can have up to 230V present on exposed metal parts at certain points. For this component, installation inside a closed enclosure is therefore advisable.

The enclosure was originally intended solely to suppress the transformer's coil whine. A sawtooth-shaped interference signal with an amplitude of 20mV at a frequency of approximately 2.5kHz can be measured on the power supply line. I initially attempted to suppress this noise using an LC band-stop filter, but found that the filter only reduced the interference on the output side. At the input and at the transformer, however, the voltage spikes remained. Installing the power supply inside a sound-insulated enclosure proved to be extremely effective in combating the coil whine. The power supply is now acoustically imperceptible.

Clock Generator

A 32.768kHz quartz crystal and a 16-bit BCD counter are used to generate the one-second clock signal. Another 8-bit counter subsequently divides this down to generate the minute signal. The circuit can also generate a half-second and quarter-second clock signal, which can be used to conveniently set the time.

Clock Selector

A small control panel allows the selection of the clock signal used to drive the clockwork. This makes it possible to set the time. With the quarter-second clock selected, the day simply flies by; at half-second intervals, things move a little more slowly. Once I have set the hour correctly, I tap the T button repeatedly, with each press advancing the clockwork by exactly one minute, until the clock matches my reference time to the minute.

Clockwork

A little logic makes it possible to display the time in the usual 24-hour format. The clock signal from the clock selector is first fed into a 4-bit counter, which counts the units digit of the minutes — for example, the 2 in 01:52. It counts up to 10, at which point a logic gate resets the counter and simultaneously generates a new clock signal for the next digit — in this example, the 5 in 01:52. The next digit only counts up to 6 and then causes the next higher digit to advance. In this way, the individual digits of the time are incremented successively. As soon as the hour display reaches 24:00, the logic detects this state and resets all counters to 0.

Display

I was able to fully indulge my innovative side when designing the display. Since the clock's digits were meant to be as large as possible, I modeled the seven-segment displays myself and then printed them using my 3D printer — at the time, a Renkforce RF100. I designed the individual segments of the display so that as many of the digits from 0 to 9 as possible would have a rounded and harmonious appearance.