PURPOSE: Tracked power-tools are routinely used in computer-assisted intervention and surgical systems. In order to properly perform temporal and spatial monitoring of the tracked tool with the navigation system, it is important to know when the tool, such as an electrosurgical cauterizer, is being activated during surgery. We have developed a general purpose current sensor that can be augmented to tracked surgical devices in order to inform the surgeon and the navigation system when the tool is activated. METHODS: Two non-invasive AC current sensors, two peak detector circuits, one voltage comparator circuit, and a microcontroller were used to detect when an electrosurgical cauterizer is being powered on and differentiate between the cut and coagulation modes. The system was tested by cauterizing various substances at varied power ratings. RESULTS: By comparing the ratio of amplitudes as well as the frequencies of the signals, the current sensing system is able to differentiate between on/off, cut/coagulation, as well as when cauterizer tissue. DISCUSSION: The current sensing system is able to detect when the cauterizer is being powered on and can differentiate between monopolar cut and coagulation modes. CONCLUSION: This system shows promise for detecting when the cauterizer is being powered on and in the future could be integrated with a navigation system in order to easily temporally monitor the electrosurgical tool.