This report presents an averaged model and nonlinear observer for an on/off pneumatic actuator. The actuator is composed of two chambers and four on/off solenoid valves. The averaged model is elaborated which has the advantage of using only one continuous input instead of four binary inputs. Based on this new model, a sliding mode observer is designed using the piston's position and the pressure measurements in one of the chambers to estimate the piston velocity and the pressure in the other chamber. The finite time convergence of the observer is proven and its performance is verified in closed loop on an experimental benchmark.