There is a need to monitor and control the state of relays and circuit breakers within power distribution systems. The monitoring of a relay state, i.e. open or closed, is often performed by applying a monitoring signal to sensing contacts added to the relay. Manufactures of such systems have chosen many different voltages for their own monitoring systems making it difficult to interconnect dissimilar monitoring systems. A signal conditioning circuit is provided that can cope with a large input voltage range and can be configured to allow may items of equipment (which may be new item or legacy items) to be connected to a controller.