Abstract: A current limiting voltage sensing circuit especially suited as an annunciator input circuit is disclosed. It includes a radiation emitting device circuit, a current limiting circuit for maintaining the current supplied to the voltage sensing circuit at a substantially constant value while supplying the needed current to the radiation emitting device circuit, and a radiation sensitive device circuit coupled to the radiation emitting device and electrically isolated from it for converting from the radiation signal into an electrical signal. In one arrangement, the input to the voltage sensing circuit is a full-wave rectifier so that it can be used for detecting an ac voltage or a dc voltage in either direction.