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Rendered assembled Night Guard device

Engineering Concept · Rev E · Sibling project to Groove

Night Guard — Fridge Door Sensor

A 48 mm round, adhesive-mount sensor that lives inside the fridge, watches for the interior light switching on after a set hour, and fires a piezo siren to deter late-night raids. No app, no NFC, no audio playback — a sense-and-alarm circuit.

01Renders

Assembled view
Assembled — light window, LED, siren grille, one end button
Exploded view
Exploded — front shell / PCB stack / rear shell / adhesive pad
Quarter-section cutaway view
Cutaway — CR2032, piezo, standoffs, siren ports
Standalone front shell part
Front shell (standalone) — matches front_shell.stl
Standalone rear shell part
Rear shell (standalone) — matches rear_shell.stl

The last two are per-part renders of the actual STL files — a print/assembly reference: print both shells separately, seat the PCB on the rear shell's standoff posts, then press the front shell on until its pegs seat in the rear shell's sockets. No glue or fasteners in the base design; friction-fit pegs hold the two halves together.

02Key specs

Diameter
48 mm
Thickness
12 mm
Mount
Adhesive
Sensor
Phototransistor
Time source
MCU-internal RTC
Battery
CR2032
Battery life
5+ yr*
Alert
Piezo siren

* limited by the CR2032's own shelf life, not by circuit consumption — see the power budget below.

03Why adhesive, not magnet

Most fridge interiors are plastic liners, not steel — only the exterior door face is reliably magnetic. Since this device has to see the interior light (its only signal that the door opened), it has to sit inside the cabinet, where a magnet mount can't be assumed to work. An adhesive pad works on any interior surface — wall, shelf, or door bin — regardless of what's behind it.

04How it decides to fire

No phone app, no wireless pairing. Two side buttons set an "armed window" (e.g. 11pm–6am) directly into the MCU's own battery-backed real-time clock. From then on the circuit sleeps at microamp-level current until the phototransistor sees the fridge's interior light switch on. If that happens inside the armed window, the MCU drives the piezo; outside the window, nothing happens — a 2pm fridge raid doesn't trigger it.

MCU + internal RTC STM8L051, 1.3µA sleep alarm-gated logic Phototransistor sees fridge light turn on wakes MCU Hour / confirm buttons set the armed window sets RTC alarm CR2032 cell ~220 mAh powers Piezo siren 12.2mm, driven only if armed + triggered fires alarm

05Power budget

CR2032 battery life

Cell capacity220 mAh
Sleep draw (RTC + sensor bias)~2 µA
Siren draw, ~5s burst~5 mA
Assumed triggers~3/week
Estimated life~12 years*

What actually limits it

The math says over a decade, which is longer than a CR2032 lasts on the shelf before self-discharge takes over (typically 5–10 years). In practice, sleep current is so far below the siren's rare bursts that this circuit will never be the reason the battery dies — swap the cell on a calendar, not based on usage.

06Bill of materials (sourced from LCSC, ~500–1000 pc pricing)

ComponentReal partPriceStatus
MCU + internal RTCST STM8L051F3P6TR, TSSOP-20$0.58Verified
Ambient light sensorSenba SMD0805-20$0.057Verified
Piezo sirenTDK PS1240P02CT3, 12.2mm$0.164Verified†
Battery cellQIJEY generic CR2032$0.024Verified‡
Battery holderQIJEY CR2032-BS-6-1$0.109Verified
Hour + confirm buttons ×2ALPS SKSCLBE010$0.148Verified
Status LEDNationstar NCD0603R1$0.008Verified
PCBJLCPCB 2-layer, ⌀ <100mm baseline~$0.15Est.
Enclosure (2-piece shell)—~$0.45Est.
Adhesive mount pad—~$0.04Est.
Estimated BOM total$1.73

† Sound-pressure rating (dB) wasn't confirmed in this pull — verify against TDK's datasheet before treating "siren" as settled. ‡ Generic/unbranded cell price; a name-brand CR2032 (Panasonic, Murata) will cost more.

07Product break-even & margin (no marketing)

What the sensor itself needs to charge, independent of customer acquisition. See Groove's spec for the full formula derivation.

ChannelBreak-even (0%)Price @ 15%Price @ 20%Contribution @ 20%
Pre-order$5.23$6.15$6.54$1.31
Retail / wholesale$5.88$6.92$7.35$1.47

The product itself is cheap to justify — under $7 clears a 20% margin easily. Whether that leaves room for customer acquisition is a separate question, answered below.

08Marketing fund coverage (separate from product margin)

Same M values and sourcing as Groove's spec: $6/unit for pre-order/crowdfunding, $70/unit for standalone retail DTC.

ChannelContribution @ 20% priceCAC (M)GapFully-loaded price to still hit 20% after M
Pre-order$1.31$6.00−$4.69$14.04
Retail DTC$1.47$70.00−$68.53$94.85

Same pattern as Groove: at the $6.54 product-only price, neither channel's acquisition cost is covered by the contribution margin available. The $14.04 pre-order price is the one to actually plan around — this product's economics only work through a channel that supplies its own discovery (crowdfunding, marketplace search, word of mouth), not through funded advertising, where $94.85 would be required and isn't remotely competitive for a simple sensor.

How all-or-nothing crowdfunding actually charges backers: on Kickstarter (and Indiegogo's "Fixed Funding" option), backers' cards are never charged unless the campaign hits its goal by the deadline — a failed campaign collects $0, not money that then gets refunded. Indiegogo also offers "Flexible Funding," where backers are charged immediately on pledging and you keep whatever was raised even below goal — riskier for backers, since they can pay for a project that ends up underfunded.

09Manufacturing notes

Both shells demold with straight pulls except the two button bosses at opposite points on the rim, which are perpendicular to the main pull but simple single-direction side-actions. The light and LED windows are open apertures in this model — production would cover them with a clear IP-rated film to keep the sealed interior dust- and splash-resistant without blocking the sensor or LED. Four standoff posts spaced around the rim (not a full perimeter ring) support the PCB, keeping clearance from the outer wall and avoiding the thin-wall pinch that a tight-fitting ring risks at this scale. The round shell also drops the injection-mold complexity of the rounded-rectangle's four distinct corner radii to a single constant-radius core.

10Project files