Attached is a pure analog circuit to generate the signal to control an ESC for a brushless motor. I use this circuit to control a brushless tail motor on an old FP helicopter. The brushless tail motor actually is too responsive and makes the heli more challenging to fly.
C3 can be made smaller to get faster response and the LM555 circuit can be changed to make the 20 mseconds between pulses shorter. As shown the circuit has almost no delay compared to the 20 msec between pulses.
This circuit has a small problem in that it requires at least 10 or 11 volts to operate. On my helicopter the 3S lipos would sag very fast to a lower voltage and the circuit would cut out. To fix the problem I added a small (150 mah) lipo in series with the main 3s battery to provide 16 volts instead of 12 volts. This battery could be even smaller like 80mah.
I suppose R8 could be a simple pair of resistors instead of a pot. I found the best setting is just about in the center so two 1K resistors would be about right.
The circuit is based on this circuit:
I replaced the poteniometer P1 with a voltage derived from the PWM signal.