In one embodiment, a method includes conducting a first signal to a first source electrode external to a touch sensor. The first source electrode is capacitively coupled to the touch sensor through a touch object. The method further includes measuring a mutual capacitance between the first source electrode and the first measuring electrode. The method further includes identifying, based at least in part on the measured mutual capacitance and using a controller of the touch sensor, the touch object touching the touch sensor at a detected touch position.