A circuit for sensing the waveform of the current in a transistor, including a first transistor which is controlled on its base by a switching signal; a second transistor which is controlled on its base by a control signal which is inverted and delayed with respect to the switching signal, the collector of the first transistor being connected through a diode to the collector of the second transistor, the emitter of the first transistor and the emitter of the second transistor being connected to the ground; a third transistor connected with its base to the collector of the second transistor and with its emitter to the ground through a resistor, the third transistor being switched on when the second transistor is switched off and the first transistor is on.

The optical inspection apparatus consists of an explosion-proof light projector and a viewing periscope. In one embodiment, an external power source connects to an explosion-proof housing protectively enclosing a lamp. In another embodiment, the lamp, batteries, and a circuit are included within the explosion-proof housing, providing a fully portable, self-contained and explosion-proof light projector. A viewing periscope is provided for examination of meter tubes, tanks vessels and other enclosed machinery. An explosion-proof connector is also disclosed providing an explosion-proof and thermally resistant interface between a light source and a fiberoptic light guide. A timing circuit disclosed prolonging the operation time of the light projector on one charge of the battery.