Abstract: A safety device in a motor vehicle includes a sensor which provides an output relating to a physical quantity such as acceleration. The signal is passed to a subtracting circuit and the output of the subtracting circuit is fed to a further circuit arrangement which derives a signal value corresponding to an off-set component present in the signal, this signal value being subtracted from the original signal to provide an output which relates solely to the component of the original signal corresponding to the measured physical quantity. The output of the subtracting circuit is used to initiate the operation of the safety device such as an air-bag or seat belt pre-tensioner.