This is an Arduino mini based digital clock which displays time on a set of four 7-segment displays. It uses a I2C crystal oscillator (DS 3231 - https://www.maximintegrated.com/en/products/digital/real-time-clocks/DS3231.html ) to keep time accurate to 2 ppm, which is about 30 seconds in an year.
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Connections from D2 to D13 on the Arduino Pro Mini to the corresponding transistors are omitted in the diagram for clarity. Similarly marked points need to be connected together
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All Transistor, except T1, T2, T3, T4 are NPN type – Part No. BC 547
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T1, T2, T3, T4 are PNP type Transistors – Part No. BC 557
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All Resistors are 4.7 kΩ, unless specified in the diagram
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The 7-Segment LED Displays ( https://en.wikipedia.org/wiki/Seven-segment_display ) used here are “Common Anode (+ve)” type, and work with 9V and an 820Ω resistor for limiting current.
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The cathodes (-ve) of the segments of all 7-segment displays are connected together, i.e., cathode of segment ‘A’ of all displays are connected together, cathode of segment ‘B’ are connected together, and so on.
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Regulator IC 7805 is used to provide 5V supply to the Arduino, as the input voltage is 9V.
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The DS 3231 is a I2C Crystal Oscillator. It keeps time and provides the Arduino with the necessary clock signal.
An image of the PCB for this Clock is shown below:
