The following circuit is used to test servos... exactly as the name suggests. It generates a 50 Hz square wave with a positive pulse width between 1 and 2 milliseconds, adjusted using a standard 10k linear taper potentiometer.
This circuit is currently in the "it works" stage of prototyping, having been both plugged together on a solderless breadboard and transferred to perfboard and aligned to the approximate correct values.
The circuit is pretty simple, it's two 555 timers (or a 556), with the first timer in an astable configuration to deliver a 50 Hz series of negative (low) pulses, which are used to trigger the next timer.
It's important that these pulses are shorter in duration than the lowest pulse period you plan to use in the final waveform... in this case, 1 ms.
The negative pulses trigger the second timer, which delivers a variable length pulse based on the setting of the 10k potentiometer.
The resistor value in series with the potentiometer can be tweaked slightly, as the values do not go all the way down to 1 ms or stop at 2 ms, although this accuracy isn't crucial. I managed to catch these screenshots at exactly the right time.
I've also decided to slap this together on a PCB, it's the first PCB I designed, and, NB: I'm still waiting on the product in the mail, so use caution before blindly copying it. Click here to download the Gerber files.