To provide an undistorted square wave output signal under control of a switch subject to contact chatter so that the square wave will accurately reflect the operating instant of the switch, as commanded, regardless of contact chatter, a timing circuit is provided having a timing capacitor and a charge and discharge circuit therefor, connected to a threshold circuit, the threshold circuit response level and the charge circuit of the timing capacitor being dimensioned to cause the capacitor to charge to a level such that the threshold circuit responds only when the switch has changed state, typically closed after interrupting an inductive load, such as the primary of a spark plug, for a predetermined time period as determined by the switch contact chatter characteristics of the switch itself, for example about 0.6 to 1 millisecond in the case of automotive ignition breaker switches; a feedback connection is provided between the threshold circuit and the input stage to the circuit to disable transfer of a signal representative of change of the switch through the circuit unless the threshold level of the threshold circuit has been exceeded so that the threshold circuit could respond.