A driver circuit is described for an ion measuring device associated with a cylinder of a diesel engine for the measurement of the conductivity of the combustion gas present in the cylinder, wherein a negative voltage can be applied by said driver circuit to a glow plug serving as a sensor and associated with the cylinder for a respective ion measurement. The indicating member is movable from its first position to its second position in response to the actuation of a brake system of the vehicle so as to indicate the extent of engagement of the brake system. The panel is pivotally attached to the base via the expandable walls and is movable between a closed position in overlying, face-to-face relation with the base and an oven position disposed substantially transverse to the base.