Automatic WBGT readings, a lightning 30-minute clock on the sideline, a cricket ground-and-covers record, and a log for every field
Software: $299 per station per year
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How it fits together
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01
The whole FieldFlag
A fence-post station finds the heat band every minute, drives a sideline display and lightning clock, and texts coaches, with a log for the AD.
02Brain
nRF9151 station board
The brain: it runs the fieldflag-station program
03
Air sensor in shield
Reads air temperature and humidity in the shade
nRF9151 station board · I²C (address 0x44) · Temperature, humidity
04
Black globe probe
Reads radiant heat from the sun at the globe's centre
nRF9151 station board · SPI (MAX31865 chip) · Globe temperature
05
Ultrasonic wind sensor
Measures wind speed with no moving parts
nRF9151 station board · RS485 Modbus (address 1) · Wind speed
06
RollCall tap pod
Athletes tap NFC wristbands in and out at the gate
nRF9151 station board · 4-core cable, UART · Wristband tap IDs
07
StrikeClock pod
Hears lightning and starts the 30-minute clock
XIAO RP2040 · RS485 Modbus (address 3) · Strikes, THUNDER key presses
08
Sideline display
Shows the heat number and colour or the lightning countdown
Raspberry Pi Pico 2 · RS485 Modbus (address 2) · Digits, colour, brightness
09
GroundCheck tags
Cover tags and wet-spot pucks report rain and soil wetness
RAK3272S receiver · LoRa 868 MHz radio · Wetness and cover state
10
All connected
8 key parts, one program, and your phone
What it is
A solar heat-stress (WBGT) station for a practice field, with an optional sideline display.
What it does
Who it's for
Athletic directors and head athletic trainers at high schools, and cricket club groundsmen and committees.
How the parts connect
Every part, what links it to the brain, and how the data reaches your phone. A solid line is a cable; a dashed line is a radio link.
The program
for (;;) {
sample = sensors_read(); // SHT45 I2C, MAX31865 SPI, wind RS485
wbgt = liljegren_wbgt(&sample); // on-device, authoritative
band = band_for(wbgt, ruleset); // G, Y, O, R, black
display_write(wbgt, band); // Modbus holding registers, addr 2
if (band != last_band || uplink_due())
mqtt_publish_cbor("ff/dev/tele", &sample, wbgt, band);
buffer_push(&sample, wbgt, band); // 7 days offline
if (band > last_band)
evt_band(last_band, band); // starts acknowledgement chain
last_band = band;
k_sleep(K_SECONDS(60));
}To your phone
Coach and AD phone or laptop browser
One screen for each job FieldFlag does.
Heat readingFifteen minutes before practice the station reads the heat on the field. When it crosses into a hotter band, every coach is told at once.
Lightning stopThe trainer presses THUNDER. The horn sounds and the sideline digits count down 30 minutes; a new strike nearby restarts the clock.
Heat acclimatizationPlayers tap in at the gate during preseason. The app counts each band's day under the state rule-set and flags a second session that comes too soon.
Cricket ground checkAfter a wet night the cover tags and outfield pucks log the rain and wet spots. One timestamped page goes to the visitors and umpires, who decide.
App screens with sample data. The interface is in development.
| Heat reading | WBGT 87.4 °F, band Orange | Every 30 min in practice |
|---|---|---|
| Band change alert | Yellow to Orange | On change |
| Lightning clock | 28:15 left | During a stop |
| Athlete day | Band 17, day 3 | Each tap |
| Station battery | 82% | Every 5 min |
Logs store band numbers, not names. Names appear only in exports by the athletic trainer or AD.
CAD design · Design complete (stages 1–6, rev A3 GroundCheck), not built, not validated
| AI processor | Nordic nRF9151 SiP (LTE-M + GNSS); Seeed XIAO RP2040 in the StrikeClock pod; RAK3272S LoRa (STM32WLE5) in the GroundCheck tags, pucks and receiver; Raspberry Pi Pico 2 in the optional display |
|---|---|
| Sensors | Sensirion SHT45 in an 8-plate radiation shield · PT1000 + MAX31865 in a 150 mm copper black globe · Renke RS485 ultrasonic wind sensor · ams AS3935 lightning sensor (SparkFun board) with a 16 mm IP67 THUNDER key in the StrikeClock pod · Tilt and GNSS for tamper alerts · PN532 NFC reader in the RollCall tap pod, with NTAG215 wristbands · GroundCheck: tipping-bucket rain gauge, LIS3DH motion and rain pins in each cover tag, stainless soil rods and standing-water pins in each flush wet-spot puck |
| Connectivity | LTE-M, MQTT over TLS with a 7-day buffer (no school Wi-Fi needed) · SMS and push alerts · RS485 Modbus to the sideline display and the StrikeClock pod · LoRa from the GroundCheck tags and pucks to a receiver box on the station bus |
| Power | 20 W solar panel (30 W with display), BQ24650 MPPT and 12.8 V 12 Ah LiFePO4; about 4 Wh a day for the station (about 30 days in reserve), 58 Wh a day with the display, which is shed first at low battery |
| Enclosure | 3D-printed white ASA station with hidden-screw fence clamps (41–76 mm), band-strap pole saddle (Ø100–420 mm) and cable lock; a 76 × 104 × 32 mm sealed RollCall tap pod on its own fence clamp, a 90 × 150 × 42 mm sealed StrikeClock pod with a horn port and guarded key; optional display of four black ASA 200 mm digits behind opal polycarbonate |
| Dimensions | Display 710 × 250 mm on two 1 in rails; station mounted at 1.2–1.5 m |
| Ingress rating | Display: 144 × WS2815 IP65 pixels in sealed digit boxes (hose test planned) |
| WBGT accuracy | MAE ≤ 0.5 °C vs an ISO 7243 reference (test target) |
|---|---|
| Calculation | Every minute, 5-minute means |
| Official readings | T-15, every 30 min, and on band rise |
| Band agreement | ≥95% vs reference (test target) |
| Rule | 30 min after the last thunder or strike; every new one restarts it |
|---|---|
| Inputs | THUNDER key, app button, AS3935 strikes within the set radius |
| Sensor range | About 40 km, coarse 1–40 km distance steps (AS3935) |
| Outputs | Red countdown on the digits, horn, text to every coach and the AD |
| Ends a stop | Never by itself; a person gives the all-clear |
| Coach alert | Within 60 s of a band change |
|---|---|
| Escalation | AT at 10 min, AD at 20 min |
| Log | Hash-chained, PDF/CSV per practice |
| Athlete tap | ≤ 0.5 s read, ≤ 3 s per athlete at the gate (test target) |
| Student data | Wristband numbers in the log; names only in AT/AD exports |
| Digits | Four 200 mm RGB digits + colour bar |
|---|---|
| Readability | Number at 30 m, colour at 60 m (to test) |
| Power | ≤14 W at full brightness |
Targets for production units; they may change during development.
Hardware: $699–$1,496 per field station
Software: $299 per station per year
Target pricing: $699 per field station, StrikeClock pod +$199, RollCall tap pod +$149 with 100 wristbands, GroundCheck cricket kit +$349, sideline display +$449, pole kit included; district bundle of 10 stations $5,990 + $2,490 a year. The subscription covers alerts, logs, the lightning clock, acclimatization tracking, the cricket ground page, the district dashboard, LTE and a recalibration swap. Estimated pilot BOM about $597 per station, $82 per StrikeClock pod, $57 per tap pod, $208 per GroundCheck kit and $333 per display.
Indicative launch pricing in USD, excluding taxes and shipping. Final pricing is confirmed before any balance is charged.
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From the product lab · ideation phase
No. FieldFlag is a concept in the in-house design and testing stage. The design is complete on paper and in CAD, but it has not been built or validated, and all specifications are targets.
No. The UIL allows an on-site instrument or an internet forecast and only recommends records. FieldFlag saves the labour of taking readings, measures at your field, sends alerts and keeps evidence; it is not a compliance certification. Georgia's GHSA does require an on-site instrument.
No. FieldFlag measures and records. The coach and athletic trainer make the decisions.
No. Under MCC Law 2.7 the umpires alone decide whether conditions are dangerous or unreasonable, and ECB guidance says they must walk the whole field themselves. Without umpires, the captains decide under the league's rules. GroundCheck shows rain, covers, wet-spot readings and photos with times, and never shows "fit" or "unfit".
Only if the league's rules allow it. Many leagues (for example Shropshire and Birmingham) deduct 24 points from a home club that cancels without letting the visitors view the ground. The page helps both captains agree early; it does not replace the visitors' right to travel and look.
They sit in a standard 108 mm hole-cutter hole with the cap flush with the soil, like an irrigation box, placed by the groundsman in known wet spots and at the end of the run-ups, never on the pitch. Survival under mowers, rollers and spikes is still to be tested.
When anyone presses the THUNDER key or the app button, or the AS3935 sensor hears a strike inside your school's radius, the sideline digits count down 30 minutes in red and every coach and the AD get a text. Any new thunder press or strike restarts it at 30:00, as the NFHS guidelines say. When it reaches zero it only tells your designated person; they check the sky and give the all-clear.
No. The AS3935 is a low-cost sensor with a coarse distance estimate and some false alarms, and NFHS says never to rely on detectors over what people see and hear. That is why the THUNDER key comes first and FieldFlag never ends a stop by itself. It does not replace your lightning policy or safe shelter.
A published comparison found a Kestrel read about 0.7 °C warmer than an ISO-compliant meter at high-school sites. If FieldFlag is accurate it should read about 0.5–0.7 °C below a Kestrel. We will publish our accuracy test and never add an offset to match.
Each athlete taps a numbered wristband on the RollCall pod at the gate on the way in and out. A day counts only after both taps inside a scheduled practice. FieldFlag follows the rule-set your school picks: late joiners start at day 1, missed days do not count, rest days are added, and a second session is flagged if it comes too soon. The coach and athletic trainer still decide, and can correct a day by hand.
Only if you use RollCall. The wristband holds a random ID, the log stores band numbers, and names appear only in exports by the athletic trainer or AD. Data is not sold or shared and is deleted at the end of the season unless your district keeps it. Without RollCall, teams are labels and the only contacts are staff.
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Concept · target $699