Abstract: A system for controlling the amount of torque applied by a hand-held power tool to fasteners during an assembly of parts. In the preferred embodiment, a torque control circuit is operated by air to deliver a selected volume of air under selected pressure to the tool and to stop the delivery of air when the desired torque has been applied to the fastener. The system can operate one or more torque control circuits. Visual color coded signals can be used to indicate the active circuit and the tools intended to be used with that circuit.