An alarm apparatus, for detecting radiation and/or pollutants including smoke and carbon monoxide, comprises an alarm circuit including a detector for radiation and/or pollutants and an audible alarm emitting device. A power supply circuit, connectable to an external AC power supply, supplies power to the alarm circuit. A controller operates the power supply circuit in a first mode of operation in which it supplies current to the alarm circuit at a first power level less than that required to energize the audible alarm emitting device and, in response to the detection of radiation and/or pollutants, operates the power supply in a second mode of operation in which it supplies current to the alarm circuit at a second power level sufficient to energize the audible alarm emitting device.