Lightning Witness for Schools and Clinics — Design Blueprint: $91 Sferics and Electric-Field Warning Node and Three-Node District Arrangement

DESIGN BLUEPRINT — SIMULATED, NOT MEASURED — NOT BUILT. Build-ready design blueprint derived from 'A Lightning Witness for Schools and Clinics: A $91 Sferics and Electric-Field Warning Node, Its Computed Warning-Time Budget, and the Limits of the 30-30 Rule' (DOI: 10.5281/zenodo.24108716). Specifies a two-channel node — VLF sferics receiver (1-30 kHz, gated rate signal at a nominal 40 km gate) plus an electric-field plate (alarm at 2 kV/m sustained, crossed at 21.9 km) on an ESP32-S3, solar powered — at $91 in itemised parts, and a three-node district arrangement on a 12 km triangle at $318 total. Median warning 29.2 min single node (29.8 min district) before the first cloud-to-ground strike within 10 km; 8-minute evacuation need met in 82.4% (83.2% district) of dangerous storms; 45-minute default all-clear. Includes bill of materials with source-stated itemised costs, dimensions and siting rules, assembly, installation, calibration and commissioning, operating envelope, maintenance, failure modes and non-claims. Printed failure regimes at full size: zero warning in 12.7% of storms for a single node (5.2% district); the 30-30 rule warns before the first dangerous strike only 34.9% of the time with a median warning of 0 minutes; a 30-minute all-clear still carries 10.1% residual risk. All numbers are reproduced by the deposited compute.py.