Abstract: A smoke detector working on the principal of interference of light between an L.E.D. source and a phototransistor receiver detects differences in smoke emission regardless of the color of the smoke. A pulsating power supply automatically turns the power to the unit on and off every thirty seconds so that the life of the unit is extended and energy is conserved. When a hazardous smoke condition exists, the unit produces rapidly interrupted alarm signals. A warning signal which is different than the alarm signal draws attention to an optoelectronic system malfunction or a power supply voltage drop below a preset level.