In this paper a Lyapunov technique is presented for the synthesis of controllers characterised by control signals that, for construction simplicity and in order to attain a better efficiency, may only assume a finite number of values. In particular, a design technique of a control law with prescribed control levels for a class of continuous-time SISO uncertain nonlinear systems is provided, which guarantees the tracking of a given reference trajectory, with a prescribed maximum error norm, a prescribed rate of convergence and a low switching frequency.