In one embodiment, a sensor for circuit testing has a first terminal and a second terminal. The first terminal is configured to be coupled to a first node of a first circuit via a first capacitor, and the second terminal is configured to be coupled to a second node of the first circuit. The sensor also has at least one transmitter and at least one receiver that measures a first transmission factor between the first terminal and the second terminal. The sensor determines that the first circuit is in a first state if the first transmission factor is above a first threshold, and determines that the first circuit is in a second state if the first transmission factor is below the first threshold.