The plan

Damp Diary: blueprint

Written 15 October 2026. It describes the business, the software and how the software and the hardware fit together.

Every part carries a status, so nothing is claimed beyond what exists:

Built: exists and passes its tests (the server: 27 tests on 15 October).
Prototype: written, but not yet run on real hardware (the firmware compiles).
Plan: not built yet.

1. In one paragraph

Damp Diary is a £29 wall sensor that a tenant buys and owns. It sits beside the corner that keeps going damp. It reads the room every ten minutes and pulls in the local weather. From these it works out when the coldest surface will run wet, and it warns the tenant the night before. After a week it answers the question every damp dispute turns on: is the moisture from how the tenant lives, or from a cold building? It answers with numbers. Every reading is signed into a log that cannot be quietly changed. If the cause is the building, it drafts the letter to the landlord with the readings and the law that applies.

2. The business

Customer and problem

Who pays: private tenants in England, starting in Manchester, who are in a damp dispute or heading into one.
The problem: one in ten privately rented homes in England has a damp problem, against 5% of all homes, about 1.4 million homes (English Housing Survey 2024-25).
The dispute: the landlord says it is how the tenant lives, and the tenant says it is the building. The tenant usually has only photos.
Why now:
- Awaab's Law has applied to social landlords since 27 October 2025.
- For private renting, the government consultation runs from 9 October to 18 December 2026.
- Tenants are about to have a reason to keep evidence.

What the customer gets

Part	Status
The sensor, with a green, amber and red light and one button	Prototype
The chat assistant (Telegram today): alerts, the night-before warning, the verdict, the letter, the log	Built
A signed, exportable log of every reading	Built

How it makes money

Line	Price	Status
The device, software included, no subscription	£29 one-off	Planned price
Bulk sales to groups that lend devices to members (tenant unions, advice services, student unions)	To be set per order	Plan, to test
Extra sensors for more rooms on the same account	To be set	Plan, to test

The second and third lines are options to test with buyers. Nothing is decided.

Unit costs

Item	Cost	Source
Core parts: SHT40 sensor £1.63, ESP32-S3 module £2.46, case £0.62, LED ring £0.40, battery £0.67	£5.78	Alibaba.com listed prices, 15 Oct 2026
Circuit board, assembly and test	To be quoted	Needs the board files and parts list
USB-C socket, charger chip, button, packaging	To be quoted
Shipping, duty and import VAT	To be quoted
Running costs per device: hosting, weather data on Open-Meteo's commercial plan, messaging	To be measured	Open-Meteo's free tier is for non-commercial use only

£29 includes 20% VAT, so the business receives about £24.17 per device before payment fees. The margin can only be stated once the board quote is in.

Channels to test

Direct: short videos on TikTok and Instagram, and a simple order page.
Groups to approach in Manchester: tenant unions such as ACORN and the Greater Manchester Tenants Union, student unions, and university accommodation offices. None of these is a partner yet.
Advice services: local Citizens Advice and housing charities, as lending pools. Not yet approached.

Competitors and the gap

Who	Who buys it	What it does	What it does not do
Aico HomeLINK, Switchee	The social landlord	Sensors in social homes, reporting to the landlord	Work for the tenant
Wi-Fi hygrometers on Alibaba.com (£4.19 and £6.78 to £7.54 listed)	Anyone	Show temperature and humidity	Forecast, judge the cause, keep a signed log or write the letter
Damp Diary	The tenant	Forecast, verdict with numbers, signed log, landlord letter

Validation plan, next 30 days (Plan)

Talk to 20 tenants who have reported damp. Ask what happened and what evidence they had.
Put prototypes on real walls for at least a week, with the tenants' consent.
Open a pre-order page and count real sign-ups. Quote only real numbers.

3. The system at a glance

Tenant's home

Sensor (SHT40)
ESP32-S3: reads every 5 s
posts every 10 min, or at once on a jump
Light: green/amber/red
Button: send now

HTTPSreading →← colour

Server (Node 24, TypeScript)

/v1/hello pairing window
/v1/reading store + verdict
physics: dew point, surface RH
assess: live level, pattern, tomorrow, living or building
store: signed log (HMAC chain)
letter: template from the log
weather: Open-Meteo, 7 days back and 3 days ahead

alerts →← commands

Tenant's phone

Chat (Telegram)
alerts
tomorrow
why, letter
log, delete

The device only measures and shows a colour. All the judgement happens on the server, so the model can improve without touching the hardware.

4. Hardware

The prototype for the video and the final (Prototype)

Part	Price	Where
M5Stack AtomS3 Lite (ESP32-S3, RGB LED, button, USB-C, Grove port)	£9.00	The Pi Hut
M5Stack ENV III unit (SHT30 sensor, Grove cable included)	£6.80	The Pi Hut

The two plug together with no soldering. The firmware also detects an SHT40 on the same cable.

The production design (Plan)

Block	Part	Why
Brain and radio	ESP32-S3-WROOM-1-N8R8 module	Pre-certified Wi-Fi module, so radio testing is simpler. Enough memory to queue a week of readings
Sensor	SHT40	Datasheet accuracy about ±1.8% RH and ±0.2°C. Placed at the edge of the board, behind a slot, away from the chip's heat
Light	WS2812B ring, 12 LEDs	One colour at a glance from across the room
Input	One button	Send a reading now, start pairing
Power	USB-C, a TP4056 charger chip and a 602030 300 mAh cell with protection	Mains in normal use. The battery keeps logging through power cuts or a move
Case	36 x 36 x 20 mm wall box, vents on two sides	Air reaches the sensor and the case does not trap heat

Power, two modes (Plan; to be measured)

Plugged in: posts every ten minutes, and the light is live.
On battery:
- The device stores readings and sends them in a batch every few hours.
- It posts at once if the room crosses into amber or red.
- The server already accepts readings up to a week old, so no change is needed on the server side.
- Battery life is not estimated here: it has to be measured on the prototype.

Version 2: the corner probe (Plan)

A thin temperature probe on a lead, taped into the damp corner. It measures the coldest surface directly, so the model does not have to estimate it, and the verdict gets stronger.

5. Firmware (Prototype: compiles, not yet run on hardware)

Sensing: reads the sensor every 5 seconds. It finds an SHT30 or SHT40 by itself and checks every reading's checksum.
When it posts: a reading goes to the server every 10 minutes. It goes at once if the humidity jumps by 4 points (a shower, a kettle) or the button is pressed.
The light:

Light	Meaning
Green	Fine
Amber	Close to condensing
Red	The coldest corner is wet
Blue blink	Not paired yet; the device asks again every minute
Dim white blink	No Wi-Fi or no server. The colour then comes from the room's humidity alone (red at 85% and above, amber at 70% and above)
Slow red blink	No sensor found

Before sale (Plan):
- Set up Wi-Fi from the phone instead of compiling it in.
- Check the server's certificate instead of skipping the check, which the prototype does.
- Add over-the-air updates.

6. How the device and the server talk (Built)

Every request carries three headers:

x-device-id: the device's name.
x-device-key: its secret. The server keeps only a hash, and a wrong key is refused.
x-device-code: the short code printed on the label, used for pairing.

Call	Sends	Gets back
`POST /v1/hello`	Nothing	Whether the device is paired. If not, a ten-minute pairing window opens
`POST /v1/reading`	Temperature and humidity, plus a time if the reading is older	The level and colour to show

Pairing:

The tenant sends "pair" plus the label code in the chat.
The server links the device to that tenant.

Before consent: until the tenant agrees and the device is paired, nothing is stored. The colour then comes from the room's readings alone.

7. The server (Built)

Module	Job
app.ts	The routes, the chat commands, the evening nudge and alert timing
physics.ts	Dew point, absolute humidity, surface temperature and surface humidity
assess.ts	The live level, the room's hourly pattern, tomorrow's risk and the living-or-building verdict
weather.ts	Open-Meteo: 7 days of past weather and 3 days of forecast, plus place lookup
store.ts	People, devices and readings in one file, with the signed chain. 90 days kept
letter.ts	The landlord letter, built from the log, in two versions: social and private
telegram.ts	The chat
tools/	A pretend device and a simulated week, for testing and for the demo, kept apart from real data

Before sale (Plan):

Move from one file to a database.
Host it in the UK or EU.
Add WhatsApp or a web page next to Telegram.

8. The model (Built)

Step	How
Dew point and moisture in the air	Magnus formula, from temperature and humidity
Coldest surface	Outdoor temperature plus 0.65 of the gap to the indoor temperature (the temperature factor used in building standards; 0.65 is a poorly insulated wall)
Surface humidity	The room's moisture against what the cold surface can hold
Live level	Red when the surface reaches 100% or the room 85%. Amber when the surface reaches 80% (the usual mould-risk threshold) or the room 70%
Tomorrow	The room's usual hourly pattern from the past week, combined with tomorrow's forecast, hour by hour
Living or building	Moisture added indoors (indoor minus outdoor, in grams per cubic metre) against the share of time below 16°C. Over 4 g/m³ added and not cold points to ventilation. Little added but cold more than 30% of the time points to the building. Both together is "mixed"

The verdict needs at least a day of readings and the outdoor weather. With less, it says so.

Weak point: the surface temperature is an estimate, and the corner probe in version 2 fixes it. Until then, the letter states the readings, and the estimate is named as an estimate.

9. The tenant's chat (Built)

Command	What it does
yes	Gives consent
pair, room, place	Set up the device, the room's name and the town
status	The current reading
tomorrow	The night-before warning: which hours, and what to do
why	The verdict, with its numbers
letter	The landlord letter, in a social or private version
log, events	The signed log as a file, and the list of alerts
delete my data	Deletes everything, after the tenant types DELETE to confirm

Alerts are spaced at least two hours apart, except that a change to red is always sent.

10. Evidence and privacy (Built, with Plan items)

Signed log (Built):
- Each reading is signed with HMAC-SHA256 together with the previous reading's signature.
- Changing or deleting any old reading breaks the chain, and the check shows exactly where.
- The chain still checks after old readings are trimmed.
Consent first (Built): nothing is stored before the tenant says yes.
Delete (Built): the tenant can delete everything at any time.
A device passed on (Built): if it is paired to someone new, the previous tenant gets their log first, then it is cleared.
Before sale (Plan):
- A privacy notice.
- Registration with the Information Commissioner's Office.
- Hosting in the UK or EU.
- A security review.

11. Rules to meet before the first sale (Plan)

Rule	What it means here
UKCA marking, Radio Equipment Regulations 2017	Radio, electrical safety and electromagnetic testing of the finished device
Product Security and Telecommunications Infrastructure regulations, in force since 29 April 2024	No default passwords, a published way to report security problems, and a stated period of security updates
Battery transport (UN38.3)	Certified cells, and sea freight or a UK supplier
WEEE	Producer duties for electrical equipment sold in the UK
Data protection	Privacy notice, ICO fee, and a lawful basis for each use of the readings
The letter is not legal advice	Every letter says so and points to Shelter or Citizens Advice

12. Build path to the final

Date	Step	Status
15 Oct	Software built and tested, firmware written, suppliers selected	Done
16 Oct	Application submitted, video link added if ready
23 Oct	Finalists announced
By 27 Oct	Order Alibaba.com samples (about £80 to £95) through Trade Assurance, after messaging each supplier	Plan
By 7 Nov	Prototype on a real wall, a week of real readings	Plan
19 to 20 Nov	Final at ExCeL London, with the working prototype

13. Risks

Risk	Answer
The surface estimate is wrong for a given wall	State the readings and call the estimate an estimate. Add the corner probe in version 2
Landlords dismiss the evidence	The log is signed and dated, and the verdict shows its arithmetic
Tenants do not pay £29	Test with a pre-order page and bulk orders from groups before making a batch
The device is moved or unplugged	The battery keeps logging, and the log shows every gap
Privacy	Consent first, delete on request, a minimum of data, and UK or EU hosting
A health claim slips in	Say "condensation", "damp" and "mould risk" only. Never "prevents mould"

14. Not built yet

A device on a real wall: no real readings exist yet.
The production board and case.
Wi-Fi setup from the phone, certificate checks and over-the-air updates.
Database hosting.
An order page.
Any sales or users.