Here's a summary description of the code, have a look at the full guide for a more detailed explanation and a link to download the code - Mechanical 7 Segment Display Clock There is also a lot of repetition because there are four different 7 segment displays needing to be updated. The code looks complicated at first glance, but it's actually relatively simple thanks to the two libraries used. You only need to connect one servo driver to power and it will feed power to the second through the 6 wire ribbon cable connection. Power is supplied to the Arduino directly from the 12V power supply and to the servos using the 5V 5A BEC which is then connected to the two terminals on the top of the PWM driver. You'll need 5V and GND to each board as well as the I2C connections to your Arduino pins A4 and A5 (I2C on Arduino Uno), and the clock module pins CLK, DAT & RST to pins 6,7 & 8 on your Arduino respectively.

You'll also need to make up the wiring harness to connect the three boards (two servo boards and the clock module) to your Arduino. You'll also need to change the I2C address on the second board so that it is different to the first and uniquely identifiable. To chain the two together, you'll need to make up a 6 wire ribbon cable connector and solder a second header strip onto the other end of the first servo control board. So I have one board controlling the servos for the two hour digits and the second controlling the servos for the two minute digits.

I separated the servos into the hour and minute digits, with each pair of digits being driven by one board. You'll need to use two PCA9685 16 channel PWM drivers to drive your 28 clock servos.

Techutilities seven servos slow