Limonchito working document, published as-is. Prices and lead times are estimates from September 2026 and change often.
Limonchito / Lemon: master plan (GMSR Studio)
Written 2026-09-17. Budget: two students, no patents. Detailed sources: SOURCING.md (parts, prices, lead times), COMPLIANCE.md (EU rules and test costs), ELECTRONICS_REVB.md, FIRMWARE_BACKEND.md, MECHANICAL_REVB.md (rev B design).
The honest picture
The full "500 certified lemons" plan costs about €90k: about €44k in parts and equipment plus about €46k in certification. Two students can't pay that up front, and they shouldn't. Most of that money buys certainty that nobody has yet: whether people still squeeze a lemon after a month.
So the plan is staged, and money only goes in once the previous stage has proven something:
| Stage | What it proves | Cash out | Each of you | Time |
|---|---|---|---|---|
| 0. Bench pair | The squeeze → buzz loop works over LTE-M, and the squeeze feels right | ≈ €300 | €150 | 4–6 weeks |
| 1. Pilot: 5 pairs, 1 month | Real people keep using it after the novelty wears off | ≈ €2,900 | €1,450 | ≈ 12 weeks |
| 2. Pre-orders | Strangers will pay for it | ≈ €100–300 (page, photos, video) | – | 4–6 weeks |
| 3. Certified batch | A business | paid from pre-orders, not savings | – | 4–6 months |
Patents: none. Nothing is kept secret any more, so the videos and renders can be shown, but reframe the kid-style advert first (see "Still true", below).
Stage 0: bench pair (≈ €300)
Two development kits act as the two lemons. Nothing is custom yet.
| Item | ≈ € | Note |
|---|---|---|
| 2 × nRF9151-DK | 175 | Onomondo and Wireless Logic SIMs are included. Later they become the production programmers, so this isn't wasted. |
| 2 × DRV2605L breakout + 2 LRA motors | 30 | ESTIMATE |
| Snap dome samples + 2 small LiPo cells | 25 | ESTIMATE |
| Small silicone trial kit (squeeze-feel test skins) | 60 | ESTIMATE |
| Relay server | 0 | Runs on the existing gerritsxd.com VPS |
| Total | ≈ €290 |
Done when: a squeeze on DK A makes DK B buzz in under 20 s, squeeze-back works, and you both like how the test skin feels.
Stage 1: pilot with 5 pairs (≈ €2,900)
Build 10 real rev B lemons and loan them to 5 pairs of people: grandparent and grandchild, a long-distance couple, a parent and a student. Squeezes per day are logged on the server.
| Item | ≈ € | Note |
|---|---|---|
| Round 1 parts (SOURCING.md §9 "Prototype R1") | 1,700 | 12 boards from JLCPCB, cells, casting materials, closure screws. Only 2 test pucks instead of the set of 4. |
| SIMs | 0–130 | DK/Onomondo trial SIMs first; 1NCE (€13 each) needs KvK |
| KvK registration | ≈ 85 | Needed anyway for 1NCE and for selling later |
| Minimum equipment | ≈ 940 | see below |
| Contingency 10 % | ≈ 270 | |
| Total | ≈ €2,900 | 3 pairs instead of 5 ≈ €2,300 |
Minimum equipment. Everything else in SOURCING.md §6 (hot air, microscope, pressure pot, fume extractor, J-Link, Form 4) waits until stage 3:
| ≈ € | |
|---|---|
| Pinecil v2 + USB-C PSU | 65 |
| Multimeter | 60 |
| Power Profiler Kit II (battery life) | 92 |
| NanoVNA-H4 (antenna tuning) | 70 |
| ESD mat + strap, solder consumables | 105 |
| Vacuum chamber + pump (degassing silicone) | 172 |
| 0.1 g scale | 40 |
| Respirator A2P3 (required for PU pouring) | 85 |
| Resin printer (Elegoo Mars-class), or €0 at a university or city fablab | 0–250 |
| Total | ≈ €690–940 |
Why the pilot doesn't need CE certification (our reading, confirm when asking labs for quotes): the lemons are not sold. They stay GMSR Studio's property and are loaned for testing. Products used only for testing and evaluation aren't "placed on the market" under the EU Blue Guide. Still use cells that come with UN38.3 and IEC 62133 reports, and don't charge pilot users anything.
Go / no-go at the end of week 4 of the pilot: - ✅ Go: at least 3 of 5 pairs still squeeze on most days in week 4, and at least 2 pairs ask to keep theirs. - 🔁 Fix: usage drops but people say why. Change firmware or feel, and run another 4 weeks (≈ €0–300). - ❌ Stop: usage is near zero by week 3. You've spent ≈ €3k and learned it cheaply.
Stage 2: pre-orders (≈ €100–300)
A pre-order page, either your own (Stripe, refundable) or Kickstarter/Bakers, with pilot photos, a reframed advert and quotes from pilot users. Certification only starts once the goal is reached.
What the goal has to be. Certification is a fixed cost: it's the same whether you make 50 pairs or 500. That sets the minimum batch:
| Batch | Parts ≈ €/lemon | Parts total | Certification (NL only, no cradle) | Cost | Revenue at €199/pair (−10 % fees) | Result |
|---|---|---|---|---|---|---|
| 100 pairs | 60 | €12k | €15–30k | €27–42k | €17.9k | ❌ loses €9–24k |
| 250 pairs | 55 | €27.5k | €15–30k | €42–57k | €44.8k | ≈ break-even to −€12k |
| 400 pairs | 52 | €41.6k | €15–30k | €57–72k | €71.6k | ✅ +€0–15k |
(Parts per lemon without the cradle: electronics + SIM + cell + casting + closure + box. ESTIMATE from SOURCING.md; certification is COMPLIANCE.md's low to mid case minus the cradle tests.)
Conclusion: with LTE-M, a certified batch needs roughly 250–400 pre-ordered pairs at about €199. That is a real Kickstarter, not a side project. Two things make it much easier; decide after the pilot, not now:
- Leave out the cradle. The box says "charges on any Qi pad". That saves ≈ €12 per lemon, about €4–6.5k in certification and a whole mould. The leaf cradle becomes a €19 add-on later. Recommended.
- WiFi instead of LTE-M (ESP32-C3 class module that is already certified). That saves ≈ €25 per lemon (no nRF9151 at €14, no SIM at €13), avoids the 16-week nRF lead time, and certification drops to roughly €8–15k (ESTIMATE; 2.4 GHz only, no cellular test caveats). The cost: you lose the best selling point, "works with no phone and no WiFi". Setting up WiFi is hard for exactly the grandparents it's meant for. At 100 pairs × €149 it roughly breaks even. Only if the pilot shows buyers care more about price than about zero setup.
Other ways to pay for certification without your own savings: your university's student-startup programme or incubator, and RVO early-stage financing (check eligibility). Both are much easier to get with pilot data and a pre-order count.
Stage 3: certified batch (from pre-order money)
Follow SOURCING.md §8 (ordering timeline) and COMPLIANCE.md §5 (test plan), sized to the pre-order count, and: - Test rev B only, at one lab that is also a Notified Body. Sell in NL (and BE) only. - Do the EN 18031 self-assessment yourselves; the lab only reviews it. - Order nRF9151 16 weeks ahead and production cells before Chinese New Year (order by week 50 for a Feb 2027 CNY). - Battery deadline: units placed on the market from 18 Feb 2027 need a user-replaceable battery. Rev B already has one, so this is covered.
Still true (from earlier decisions)
- Sell as a 14+ gift, not a toy. Toy rules add ≈ €6k of testing, so reframe the kid-style advert
(
demo/lemon_advert.mp4) before any public use. - User-replaceable battery (2 screws, JST connector) is the default design.
- EN 18031 security (locked debug, secure boot, signed updates, per-device keys) is needed before selling, but not for the pilot. Build it in anyway, since it's in the firmware already.
- Energy harvesting from squeezes: rejected (≈ 0.2 mJ per press vs 30–40 J per day needed).
Things only prototypes can answer
Antenna tuning in the silicone skin · Qi coupling through the skin and base · squeeze feel and dome travel · cast tolerances and skin tear strength · real battery life with the LED and accelerometer · whether a Qi2 puck charges the 5 W receiver.
Rev B cross-check (2026-09-17)
All four rev B work streams are finished: electronics, firmware and server, mechanical and cradle, sourcing. Checked against each other:
| Check | Result |
|---|---|
| LED P0.15 (PWM), accel INT1 P0.16, LIS2DW12 @ 0x19, CHG_STAT from BQ51013B | ✅ netlist = firmware/lemon.overlay |
| Battery connector J2 JST GH BM02B-GHS-TBT at (−1.0, 10.3) | ✅ BOM = sourcing = mechanical (fits, 0.22 mm to cell: measure real cells) |
| LED light rod at (10, 11) under 0.8 mm skin window | ✅ electronics = mechanical |
| Qi coil Würth 760308101214, caps 47 nF + 47 nF / 1 nF | ✅ BOM = electronics; coil now 2.0 mm from rest face |
| Cradle puck Samsung EP-P2900 | ✅ sourcing = mechanical (diameter is a parameter) |
| 32.768 kHz crystal | ✅ removed everywhere (SOURCING.md updated) |
| Rev B envelope | 63.9 × 44 × 27 mm, 32 g, fit check 27/27 OK |
Still open (only prototypes answer these):
- nRF9151 pad numbers: copied from a third-party board, so check them against Nordic's footprint before ordering boards.
- Accelerometer INT1 trace: factory accel should prove it with a real interrupt test.
- SIM: it runs at 1.8 V only, so confirm the 1NCE card supports that.
- Charging: the small Qi coil on an unmodified puck.
- Squeeze force: 12–20 N through skin, film and plunger.
- Casting: silicone cure on printed masters, and bubbles in the dimples.
- Brass inserts: they may crack the printed core.
- Firmware: nothing has been compiled yet (devicetree/Kconfig names marked HW-VERIFY), and there's no factory reset or re-pairing path for resale.