Abstract: A sensing device which is suitable for use as a decelerometer comprises a magnetic field producing element and a Hall effect sensor element with one of the elements supported for movement relative to the other. The arrangement is such that when the device undergoes acceleration or deceleration the support allows relative movement between the elements so that the sensor element produces an output indicative of the change in field the sensor element detects from the field producing element due to the relative movement between the elements to thereby provide an indication of acceleration or deceleration of the device. The output of the sensor is connected to a differential amplifier, and the output of the amplifier is connected to a transistor in parallel with a capacitor. When the device undergoes deceleration, the output signal of the sensor increases, causing the output voltage of the differential amplifier to decrease, which in turn causes the voltage across the capacitor to discharge.