We describe a low-cost, low-power wireless sensor network we are developing for high time-resolution (ns-scale)
characterization of particle showers produced by ultra-high-energy (UHE) cosmic rays, to infer shower direction at sites
where hard-wired data connections may be inconvenient to install. The front-end particle detector is a scintillator block
monitored by a photomultiplier tube (PMT). We keep the sensor nodes synchronized to within 1 ns using periodic highintensity
optical pulses from a light-emitting-diode (LED) overdriven at very high current (~30 A) in short (4 ns) bursts.
With minimal optics, this signal is resolvable under free-space transmission in ambient light conditions at multi-meter
distances using a high-speed avalanche photodiode (APD) receiver at each node. PMT pulse waveforms are digitized
relative to this precise time reference on a Field Programmable Gate Array (FPGA) using a Time-over-Threshold
(ToT)/Time-to-Digital Converter (TDC) digitizer developed at BNL. A central server receives timestamped, digitized
PMT pulse waveforms from the sensor nodes via Wi-Fi and performs real-time data visualization & analysis. Total cost
per sensor node is a few thousand dollars, with total power consumption per sensor node under 1 Watt, suitable for, e.g.,
solar-powered installations at remote field locations.