An inexpensive and portable pulsed light emitting diode (LED) source with variable repetition rate up to 10 MHz suitable for time-resolved fluorescence sensing, in medical, environmental and industrial applications is described. The pocket-sized battery-driven source is pulsed by signals from a simple logic circuit, giving measured optical pulse widths of similar to1.9 ns at 525 nm as recorded using, time-correlated single-photon counting. Pulses are presented for three LEDs with peak emissions at 525, 560 and 590 nm. The measurement performance is illustrated by the fluorescence lifetime and anisotropy decay of rhodamine 6G in various solvents and a fluorescence resonance energy transfer (FRET) sensor for detecting metal ions in water which uses a new measurand, namely the site distribution function rho(r) of metal ion acceptors accumulating in an anionic porous polymer with respect to the donor fluorophores. The rho(r) dependence reveals a minimum fluorophore-metal ion separation of similar to7 Angstrom which explains the selectivity of such FRET sensors in discriminating against other quenching mechanisms.