A data logger that avoids wasting power when data gathering is not needed (or is done by other means), but is activated automatically, without the need for human intervention, when it is needed. The data logger senses and stores data when a primary power source has failed. The data logger includes at least one back-up power source, and a detector switch connected to a primary power source and to one or more back-up power sources, the detector-switch detecting a condition indicative of the primary power source failure state, and connecting at least one back-up power source to provide power to the sampler, the storage medium and the sensor upon detection of the failure condition.