Assessed as Utility
Letterbox Post Sensor
Assessed From Published Builds
A battery sensor inside the letterbox or mailbox that notifies you when post has actually been delivered, rather than only that the flap moved. A reed switch wakes the board; a distance sensor then decides whether anything was left behind.
Before you order anything
- Hardest part
- Soldering. About twenty minutes: a reed switch, a battery holder, a charge board and a breakout onto a bare ESP32-C3 module. Through-hole throughout, no surface mount.
- Point of no return
- Drilling a metal letterbox for the aerial. Everything before that is reversible, and a plastic or wooden box never reaches this step at all.
- VL53L0X time-of-flight distance sensor breakout
- The published build costs this part at $5, which is marketplace pricing for an unbranded clone. At a named retailer it is three times that and is the single largest line in the build.
- Reliability bar
- It has to survive months on a battery in an unheated metal box, and it has to not cry wolf. A sensor that fires on every flap movement is a notification you learn to ignore, which is the same as no sensor.
What it costs
| Item | US, dollars | UK, pounds |
|---|---|---|
| Seeed XIAO ESP32-C3 | $4.99 | £5.80 |
| VL53L0X time-of-flight distance sensor breakout | $14.95 | £16.20 |
| Build | $19.94 | £22 |
| Aqara Door and Window Sensor MCCGQ11LM | Not checked | Not checked |
- If it does not work
- The parts and a weekend, unless the letterbox is metal and gets drilled, in which case a rented front door has been modified and the deposit is in scope.
- Expected cost
- Not yet available. It is the build cost plus the failure rate times the cost of failure, and the failure rate comes from builder reports. Nobody has reported on this one.
What it asks of you
SolderingNeeded
- How much
- About twenty minutes: a reed switch, a battery holder, a charge board and a breakout onto a bare ESP32-C3 module. Through-hole throughout, no surface mount.
- If you get it wrong
- The most common reported failure is not a ruined board but an intermittent one: loose wiring on the distance sensor shows up as initialisation failures weeks later, in a box you have screwed shut.
Escape hatch
A pre-soldered ESP32-C3 with headers and a breakout with a plug-in connector removes most of it, at the cost of a larger assembly that may not fit a slim letterbox.
Firmware and Command LineNeeded
- How much
- An ESPHome deep-sleep configuration, which is meaningfully harder than a mains-powered one: the device is asleep when you want to update it.
- If you get it wrong
- One source reports over-the-air updates timing out because the sleep cycle is too aggressive to catch. Recoverable, but only by opening the box and connecting a cable.
Escape hatch
None worth the name. Deep sleep is the whole reason the battery lasts.
Mechanical FittingNeeded
- How much
- Mounting a weatherproof box and a magnet inside the letterbox, aligned closely enough that the reed switch triggers on the flap and not on the wind.
- If you get it wrong
- A misaligned magnet gives either constant false triggers or silence, and both look identical from indoors.
Escape hatch
None for a metal box: one source notes that a metal mailbox attenuates WiFi badly enough that the aerial has to go outside it, which usually means drilling.
Tools
Not owning a machine does not make this impossible. It changes what it costs.
- soldering iron and multimeter
- Required. Most makerspaces have one, and many public libraries now do too.
- basic power tools
- Substitutable. Only needed for a metal letterbox, where the aerial has to sit outside the box. A plastic or wooden box needs no drilling at all. A drill is the most commonly borrowed tool there is.
If it goes wrong
- Reversibility
- Free to Retry
- What is at risk
- The Thing Being Modified
- Do this first
- Prove the whole thing on the bench with the magnet held by hand, and confirm WiFi reaches inside the closed box with the aerial still inside it, before drilling anything. If it reaches, the drilling step disappears.
Is it worth doing?
- Verdict
- Costs More Than Buying
- On what ground
- Costs more than buying
- Still right for
- Anyone who wants to know post ARRIVED rather than that the flap moved. Whether that distinction is worth roughly double is the open question below.
The build does
- Confirms that something was actually left in the box, rather than only that the flap moved.
- Distinguishes a delivery from a collection, and from the flap being blown by wind.
The product does
- Runs about two years on a battery against four to eight months.
- Needs no build, no enclosure and no alignment.
Letterbox Post Sensor
A reed switch on the flap wakes an ESP32 from deep sleep; a distance sensor then looks down into the box and decides whether anything was left behind. You get told that post arrived, not that the flap moved.
Before you order anything
The published build lists its parts at about $15.50 in total. Two of those parts, priced at named retailers on 11 August 2026, come to $19.94 on their own, and the build needs six more items after them. The gap is not an error in the source: it is marketplace pricing against retailer pricing, and it is worth understanding before ordering, because it moves this build from cheaper than the commercial option to roughly double it.
The second thing to settle first is whether your letterbox is metal. If it is, the aerial has to sit outside the box, and that means drilling a door.
What it costs
An Aqara door sensor costs around $10, runs for about two years rather than four to eight months, and needs no build at all. On every measure a price comparison can see, buying wins, and the verdict above says so.
The open question
The commercial sensor cannot do this particular job. A door sensor reports that the door opened; it has no way to know whether that was the postman leaving something or you taking it, and no way to ignore the wind. The distance reading is the entire point of the build and has no commercial equivalent at any price.
Whether that is worth roughly double and a weekend depends entirely on whether you want the distinction, and this page has no way to know that. So it asks:
Once it is running, do you actually act differently on “post arrived” than you would on “the flap moved”?
That is answerable, and only by people who built one. If builders report that they stopped caring about the distinction within a month, the extra cost and complexity are not earning their place, and the verdict gets looked at again.
Where this one dies
The reward is end-loaded, which is the pattern that kills projects during the wait for a part. Get the reed switch notifying your phone on the first evening. It is useful on its own, it is fifteen minutes of work, and it is the difference between a finished build and a box of components in a drawer three weeks later.
Who already has this, and how they differ
| Source | Weight | Licence | What is different about it |
|---|---|---|---|
| Botmonster: ESP32 Mailbox Sensor with Reed Switch and VL53L0X | Complete Published Build, Unknown Provenance | Unstated | The only source that solves the real problem rather than the easy one: a reed switch alone reports that the flap moved, and pairing it with a distance reading is what distinguishes post delivered from post collected. Also the only one to publish a battery arithmetic and then contradict its own figure honestly, estimating 205 days and telling the reader to expect four to eight months. Its parts list is priced at marketplace rates and does not survive named-retailer pricing. |
| Home Assistant Community: Mailbox sensor running on batteries | Owner With Sustained Use | Unstated | Where the build guides give calculated battery life, this thread gives measured figures from people living with it, and the two do not agree. Also carries the constraint no build guide leads with: deep sleep is only available on ESP32 parts in ESPHome, which silently rules out the cheaper ESP8266 boards most beginners already own. |
| Ingenious Makers: A very low consumption ESPHome mailbox sensor | Independent Build, Bought Hardware | Unstated | Optimises for consumption above everything and accordingly drops the distance sensor, so it reports flap movement only. Useful precisely as the counter-example: it is what the build becomes when you chase battery life, and at that point the commercial door sensor does the same job for less. |
Where they disagree · Time in Service
The build guide calculates 205 days of battery from deep-sleep current; the community thread reports shorter real figures. Neither is wrong. Calculated life assumes a fixed wake count and no WiFi retries, and a letterbox in winter delivers cold cells and a marginal signal, both of which cost more than the arithmetic allows. The guide's own four-to-eight-month caveat is closer to the thread than its headline number is.
What builders reported
No reports yet. This assessment is a theory, built by comparing what has already been published. It stays one until people who built the thing say otherwise.
Open question · features
Once it is running, do you actually act differently on 'post arrived' than you would on 'the flap moved'?
Builder reports, and the six-month still-in-use question. If people who built the distance-sensing version report that they stopped caring about the distinction, the extra cost and complexity are not earning their place.
Built this? What actually happened?
No account, no email required.
Safety
No Special Hazard
Battery powered, low voltage, no mains. The only physical risk is drilling a door or letterbox, which is a property question rather than a safety one. Placeholder only: the class has not been set by a person.
Region note
A UK letterbox is a slot in the door; a US mailbox is a box on a post. The build is the same and the mounting is not, and a US mailbox is far more likely to be metal, which is the case that needs the aerial outside.