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The principles of two circuits which may be capable of detecting fluorescence lifetimes down to less than 1 s are presented. These are intended for sensing applications involving fluorescence quenching, which can be used to measure such process variables as dissolved oxygen and pH. The principles of the circuits are explained, and the theoretical relationship between the output of the sensors and the fluorescence lifetime is derived. Thus far, prototype circuits using these principles have been unsuccessful, being unable to detect changes in fluorescence lifetime of a Ruthenium compound in the presence of oxygen. These failures are attributed excessive noise and/or large system transients which may be overcome in the future. If successful, the new circuits may represent the potential to make stable and robust new sensors for oxygen which have lower cost and greater simplicity than others available. This would greatly improve the ability to collect data from increasingly small, distributed, and multiplexed biological systems.