Please click the link (below the photo) to my blog for more details.
Thanks to my maker friends for their assistance on this project. Especially Tony!
Giant four digit mechanical 7-segment display uses 28 servos. Arduino Uno and four transistors. No shift registers, no servo drivers.
Wonderful, but you could have also controlled the servos using shift registers, one output per segment. You can use SPI transfers to 4 74HC595s in series. Then at each 20ms:
1) shift FF to all shift registers then pulse RCK to make them all go to 1
2) after 1ms you update the shift registers with the correspondent state of the segment. 1 is to remain lit, 0 to unlit. Then pulse RCK to make them all change
3) after one more ms you shift zero to all shift registers, then pulse RCK
4) wait 18ms until the next update. In this meanwhile you can do keyboard scan, etc....
Notice that this can be easily done because you have only two possible states for each servo, which means pulses with 1ms or 2ms, nothing else.