This low cost embedded system is dedicated to the detection of small variations in the magnetic flux density B. The measurement principle uses a giant magnetoresistance (GMR) Wheatstone bridge, whose conditioning is based on the ohmic balancing of the GMR by a virtual negative resistance connected in parallel. Associated with a capacitance, that provides a temporal base for an oscillator which delivers a frequency quasi proportional to B. A fast chronometry implemented on a microcontroller allows measurements in a dynamic range from 0.001 to 15 G without a flux concentrator and in a single range. To extract useful variations, we have developed an original digital filter that estimates the algebraic derivative of the measurement signal. The convolution product between the estimating polynomial and the signal is substituted by a recursive form. This leads to a simple implementation scheme that requires only five coefficients which makes it possible to adjust the analysis time window with great flexibility. Comparative real-time tests on the magnetic detection of moving vehicles are presented.