A system for detecting the operation of the motor or engine of a motor vehicle equipped with at least one tire-pressure-monitoring-system transmitter including: a low-frequency stage including a resonant stage connected in series with an antenna, a processing stage, a first switch and a second switch, the first switch being connected to at least two capacitors so as to control the parallel connection of at least one of the capacitors, the second switch being connected to at least two resistors so as to control the parallel connection of at least one of the resistors, outputs of the low-frequency stage being arranged to receive input from the processing stage, in order to transfer the perceived signal to a processor of the signal, which is configured to compare the level of the perceived signal to a preset threshold level.