An apparatus in an electronic control system allows two or three wire operation. A power supply can supply power to the enclosed circuitry in both two and three wire installations. The system may have a high efficiency switching type power supply in parallel with a low frequency controllably conductive device. A control circuit may control the controllably conductive device. The control circuit may switch from the two wire mode of operation to the three wire mode of operation upon the detection of a zero cross detection signal. A detector circuit may be used to detect the presence of an output signal from a zero cross detector. Two separate zero cross detectors may be used such that timing information can be collected in both two and three wire installations. Both zero cross detectors may be monitored and used to automatically configure the electronic control.