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SportsDesign complete (stages 1–6, rev A3 GroundCheck), not built, not validated

FieldFlag

Every WBGT reading taken for you, a lightning 30-minute clock everyone can see, a cricket ground-and-covers record, and a log the AD or league can check

A solar WBGT station on each practice field takes the heat reading 15 minutes before practice and every 30 minutes after. A StrikeClock pod starts the NFHS 30-minute lightning clock on the big sideline digits the moment anyone presses THUNDER or its sensor hears a nearby strike, restarts it on every new one, and logs it all. A RollCall tap pod at the gate counts each athlete's heat-acclimatization day. Measurement and record only: the designated person decides, and it is not a lightning-safety or compliance certification. For cricket clubs, the GroundCheck kit adds a rain gauge, tags on the covers and flush wet-spot sensors in the outfield, and sends the visiting captain, both umpires and the league one timestamped ground page before anyone travels; the umpires still walk the ground and decide.

Who buys it
Athletic director or head athletic trainer at a Texas high school or ISD (UIL mandatory WBGT standards from 1 Aug 2026), then Georgia (GHSA requires an on-site instrument), Florida and Mississippi. Second sport: club cricket (home club committee or groundsman, then leagues that fine clubs for unfit grounds or wrong cancellations).
Why it pays
For a school with 3 outdoor practice sites: about 90 coach/athletic-trainer hours a year no longer spent taking readings (about $3,150 at $35/h, est.) and 3 Kestrel 5400s avoided ($1,662), against $2,097 + $897/yr. The larger value is a timestamped record of every reading and acknowledgement.
Target price
$699 per field station + $299/yr; StrikeClock pod +$199; RollCall tap pod +$149; GroundCheck cricket kit +$349; sideline display +$449 (target)
First-minute wow
Someone hears thunder and presses the big THUNDER key: the horn sounds, the sideline digits turn red and count down from 30:00, and when the sensor hears another strike 12 minutes later the clock jumps back to 30:00 on its own.
See the CAD designConcept page
FieldFlag concept: the idea and where each part goes
The concept. The CAD design is below, under Printable CAD.

What the research found

And how this design answers it

  • Cricket ground fitness: the umpires alone decide (MCC Law 2.7, ECB guidance), but leagues fine a home club 24 points for cancelling without letting the visitors see the ground and make the home club cover the pitch and run-ups → GroundCheck logs rain, cover on/off and outfield wet spots and sends one shared, timestamped ground page to both captains, both umpires and the league; it never says "fit" or "unfit"
  • Lightning: NFHS says wait 30 minutes after the last thunder or lightning and restart the count on every new one, with detectors only an aid (NFHS SMAC, NATA 2013) → a StrikeClock pod with a THUNDER key and an AS3935 sensor runs that clock on the sideline digits, restarts it itself, and logs every reset
  • Acclimatization is per athlete: late joiners start at day 1 and missed days do not count (NATA, NJSIAA, UHSAA, MSHSAA) → a RollCall tap pod at the gate counts each athlete's day and shows gear and second-session limits at the tap
  • HeatFlag assumed no US rule uses WBGT → school sport does (UIL, GHSA, FHSAA, MHSAA), so the band comes only from the school's versioned WBGT rule-set, with Class 2/3 locked for the year
  • ±1 °C WBGT can skip a UIL band (the red band is 1.9 °F wide) → target MAE ≤ 0.5 °C against an ISO reference, a published Kestrel comparison, and a band-edge rule that never rounds down
  • The free UNC forecast is UIL-compliant → measured at the practice surface, no one walks out every 30 minutes, band-rise alerts mid-practice, and a per-practice PDF the AD can produce
  • Readings must never stop → on-device Liljegren drives the display without LTE, and the display is shed first at low battery
  • Fields are open after hours and balls fly → clamp screws hidden against the post, pin-hex bolts in nut traps, banded pole saddles, cable lock, polycarbonate diffuser, tilt and GNSS alerts

Use case

Saturday league match after a wet night

Home club groundsmanVisiting captainTwo appointed umpiresLeague fixtures secretaryFieldFlag station with GroundCheck

  1. 01Trigger: 14 mm of rain falls overnight; the rain gauge logs it and both cover tags show the covers stayed flat on the square with rain landing on them
  2. 02Detection: at 09:00 two of six wet-spot sensors (the long-on dip and the pavilion-end run-up) read well above this ground's "we played" baseline and one shows standing water
  3. 03Alert: at 09:30 the groundsman adds three phone photos and the app sends one ground page to the visiting captain, both umpires and the league, with the rain, covers and wet-spot history
  4. 04Action: the visitors still travel (their right under league rules); the umpires walk the whole field at 12:30, set the next inspection for 13:15 on the sideline digits, and start at 13:45 when both agree

Outcome: A 40-over match played that might have been cancelled by phone, no 24-point dispute, and a record the league can read if anyone complains (est.)

How it works

Architecture

  1. 01

    Sense: HeatFlag head (SHT45, 150 mm black globe, ultrasonic wind) at 1.2–1.5 m, an AS3935 lightning sensor and THUNDER key in a strike pod, and a PN532 tap pod at the gate; for cricket, a tipping-bucket rain gauge, LoRa cover tags (LIS3DH + rain pins) and flush LoRa wet-spot pucks (stainless rods + water pins)

  2. 02

    Decide: nRF9151 computes WBGT every minute; the lightning clock restarts at 30:00 on any THUNDER press, app press or AS3935 strike within the school's radius, and never ends a stop by itself

  3. 03

    Send: LTE-M, MQTT over TLS with CBOR, a 7-day buffer; no school Wi-Fi

  4. 04

    Tell: red countdown on the sideline digits, the strike-pod horn, SMS/push to every coach and the AD, pod LEDs at the tap, per-practice PDF/CSV

Hardware

Real parts, buyable today

Bill of materials: about $597 per pilot station, $82 StrikeClock pod, $57 RollCall tap pod with 100 wristbands, $208 GroundCheck kit (2 cover tags, 4 wet spots, receiver, rain gauge), $38 pole kit and $333 sideline display (est.); volume targets ≤ $280 station, ≤ $140 display.

BlockPartWhy this part
Compute + radioNordic nRF9151 SiP on the HeatFlag board, beacon section not fittedPre-certified LTE-M + GNSS; runs WBGT on device
WBGT sensingSHT45, PT1000 + MAX31865 in a 150 mm Cu globe, Renke RS485 ultrasonicStandard globe and measured wind; the HeatFlag accuracy chain
Athlete tap-inElechouse PN532 NFC module, 3 LEDs and a buzzer in a sealed pod; NTAG215 wristbandsA quick tap counts each athlete's day; the tag holds only a random ID
Lightning clockSparkFun AS3935 board, XIAO RP2040, RS485, 12 V piezo horn and a 16 mm IP67 THUNDER key in a sealed podA person hearing thunder always counts first; the sensor adds strikes they miss and restarts the clock
Sideline display144 × WS2815 12 V IP65 pixels in 4 printed 200 mm digits behind opal polycarbonateBand colour plus number, readable at 30 m with margin (about 60 m by the 1 in per 25 ft rule)
Display driverRaspberry Pi Pico 2 + THVD1410 RS485 + 74AHCT1G125 + 12→5 V buckPIO drives the pixels; a Modbus slave on the station's bus
Power20 W (30 W with display) panel, BQ24650 MPPT, 12.8 V 12 Ah LiFePO44 Wh/day station (about 30 days in reserve); 58 Wh/day with display, which sheds at low battery
Cricket GroundCheckRAK3272S (STM32WLE5 LoRa) in 2 cover tags and 4–8 wet-spot pucks, a RAK3272S receiver box on the station RS485 bus, tipping-bucket rain gaugeBattery nodes reach the station across a 150 m outfield (est.) with no Wi-Fi; the receiver also counts rain tips as Modbus slave 4

Mechanical

Printable CAD

31 parts, generated from code and checked as closed solids. Pick a part to inspect it, or download the STL files to print.

See the CAD parts render
FieldFlag printable parts, rendered from the CAD
Render of the printable parts from the actual CAD. Not yet printed or photographed.

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Open risks we are working on

  • WBGT accuracy is unproven: it must reach MAE ≤ 0.5 °C against an ISO reference, and it will likely read about 0.5–0.7 °C below a Kestrel 5400
  • The UIL accepts a free internet forecast and only recommends records, so this is a labour-and-evidence sale, not a compliance sale
  • Perry Weather is entrenched in Texas districts and actively marketing the UIL rule
  • The AS3935 is a hobby-grade sensor with a coarse distance estimate and some false triggers; it is an aid only, and NFHS says never to rely on detectors over what people see and hear
  • RollCall makes it hold student data: districts may refuse a vendor cloud, and a wrong rule-set gives wrong advice, so each rule-set is checked against its source
  • Liability if a student suffers heat illness at a school using it; product-liability insurance cost is open
  • Georgia, Florida and Mississippi band tables and the GHSA 'scientifically approved' wording are not yet checked against the handbooks
  • GroundCheck: leagues or umpires may read the page as a fitness verdict, so it shows readings and photos only; wet-spot readings are relative to each ground and untested on clay and sand outfields; a puck flush in the outfield must survive mowers, rollers and spikes

Design stage only: nothing on this page is for sale. Specifications and prices are targets.