The composition of exhaled breath contains important information regarding the health of our body. Measurements of the level of exhaled carbon dioxide can help both diagnose respiratory diseases and determine metabolic rate. A low-cost NDIR sensor has been developed that offers the detection of CO2 from the ppm range up to 5% level in human breath. An innovative lock-in amplifier system allows a 10Hz drive signal to be recovered from the high frequency noise associated with a silicon thermopile infra-red detector. Laboratory experiments have demonstrated excellent stability (+/- 0.10% in 25% RH) and repeatability between dry and humid conditions (+/- 1.2% for 25% humidity increase). The response time is typically 2.4s, limited by the low drive frequency necessary for the MEMS-based wideband infra-red source. The current system has a resolution of ca. 10ppm of CO2. Further refinement in signal processing and a higher drive frequency should permit even lower concentrations of CO2 to be detected with an ultimate target of 1 ppm. Existing performance has been shown to be suitable for breath analysis using a side-stream analyser. (C) 2015 The Authors. Published by Elsevier Ltd.