A momentum-wheel installed to provide attitude-control torque actually produces undesirable force or torque disturbances owing to wheel imbalance and imperfection of the ball-bearings. To improve the pointing performance of observation satellites, a vibration isolator is used to isolate observation devices from these disturbances. This paper compares three types of semi-active isolators that consist of a piezoelectric material and a switch-controlled passive circuit. Since this isolation is implemented by controlling a circuit switch no external energy is supplied to the system, and so the system is stable even when a malfunction occurs in control. We propose a simple but effective isolation method that needs to know only one velocity value instead of the full state of the system. Numerical simulations with a simple model of an observation satellite demonstrated that the proposed isolator works well to isolate an observation device from disturbances caused by the momentum-wheel, without causing any degradation in the attitude control of satellites.