A switch circuit, particularly suitable for dimmer switches, detects zero-crossing or similar points in the supply and uses those to make predictions of future zero-crossing occurrences. The predicted occurrences may be used to time the operation of the switch itself to chop the supply which results in less variation in the power supplied (flicker if the switch circuit is operating a lamp) since the positions of the measured zero-crossings can be subject to noise but flicker is reduced if the switch is operated at times with respect to the true supply waveform. The predicted times may be obtained from a local oscillator having a period set by, for example, low pass filtering the period of the measured zero-crossings. The phase of the oscillator may be adjusted such that an error between the predicted and measured zero-crossings is reduced.