motor speed controller

This circuit (figure 1) will impose a maximum slew rate on a signal; positive and negative rates can be independently controlled. The circuit is useful in servo applications where the error signal needs to be limited to be within the power rails to ensure predictable operation.
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http://freespace.virgin.net/ljmayes.mal/pubs/slew.htm

I was in need to modify four expensive JR-DS8231 Ultra Digital servos to rotate 360° instead of the standard 130° or something. The mods were required so they could be used with a tethered blimp. The infra-red and night vision camera equipment is mounted underneath the blimp on a special frabicated aluminum cage and is [...]

Our next program will allow you to control the direction of the servo manually via a SPDT switch (with a center-off position) connected to ports B1 and B2. Without a center-off position, you will have to use two switches. A schematic is shown in figure 3 (below).
Read more original source:
http://www.imagesco.com/articles/picservo/03.html