This application report describes how to implement an ultra-low-power photo-diode-based smoke detector using the MSP430F2012 and an external operational amplifier that is powered from two AAA batteries. The complete schematic is shown, and the solution is discussed. The complete code is downloadable from In addition, a solution for a piezzo-electric buzzer driver is shown that produces >85-dB sound pressure level at a 10-foot distance.