Simple Servo Tester
Here is a circuit of a simple servo tester that I designed. It's easy to build and is useful for testing servos or anything else that can be plugged into a servo socket of your receiver - R/C switch, ESC, etc.
U1 is a 555 timer IC, U2 is a 74HC04 (or 74HCT04, 74HC14, 74HCT14), U3 is a 7805 5V regulator (a 78L05 would be OK). Resistors are all 1/4W, 5%. Capacitors C2 and C3 should be 10% tolerance. You MUST use a linear pot'!
If your servo (or whatever) doesn't like the maximum (or minimum) pulse width, it may be too long (or short). The simplest solution is to avoid turning the pot' right to the end of its travel! Alternatively, you could add a resistor in series with the pot'. About 1k added at the top end of the pot' (connected to U1 pin 2) will reduce the maximum pulse width, a resistor added at the bottom end (connected to R3) will increase the minimum pulse width.