Mine works around a bad design decision in my gate’s GSM intercom and stops endless missed deliveries.

The intercom does ring-around: a visitor presses the button, it calls the first number on its list (my mobile), and if nobody answers it tries the next (my wife's mobile). Except it counts voicemail as an answer. If the first phone goes to voicemail, the intercom decides the job is done and numbers two and three never ring. So the list is effectively one number long if my phone has no reception. I assume nobody tested it against a phone that was switched off or travelling on the London tube.

I gave up on the chain calling in a the first week of several failed delivery attempts. Now an ESP32 and a SIM7600 sit first on the list instead. The modem picks up after 3 seconds, sends a # DTMF tone (the intercom’s open command), hangs up, and texts me that someone came in.

Since this is a mass used car security gate it is a weak security control and more to hinder casual drivers parking where they are not supposed to - and now I get my packages too :P

Personally it made it very easy to build custom sensors/actuators for my Home Assistant setup. I have a folder with a bunch of esphome configs and just point Claude Code at it and tell it to add new entries for the stuff that I need, it's really good at that it can even write drivers for specific sensors autonomously. Granted you still need to do the wiring/soldering by hand (for now!)

Just around the home I have:

- a board that captures the audio output from my vinyl player and streams it on my LAN (so I can play my records on speakers across the house)

- one esp32 in each of my DIY speakers around the house driving the DAC+amplifier

- smart plant sensors that measure soil humidity, temperature, etc.

- a camera inside of a bird house, solar powered

- a 8x8 lidar ToF distance measurement board for detecting sleep phase

I am currently working on a Bluetooth Mesh home network allowing me to control lights, gather sensor data from various places, etc (think interfacing with a home alarm system for presence detection).

Also, a home air quality monitor, both with a display and reporting to my mesh network.

Or you could build a "plane radar" like this one :-) https://github.com/ironicbadger/ESP32-Plane-Radar

Cheap MCU dev kits in general, for me?

A somewhat-elaborate fan speed controller for a wonderful old pro audio power amp that also needed a good bit of other rehab

A window fan controller, for summer use, that only turns on the fan when it's cooler outside than it is inside -- so it only cools the indoors, seeks never to heat it

A sensor for automating the color temperature of my indoor lighting based on outdoor conditions

A macro pad that ultimately works kind of like a Streamdeck, but without the built-in display features (that was a download-and-print project)

Other random stuff, like an ESPHome widget that talked to a window aircon via infrared commands (mostly just a download-and-run project)

They're just cheap little programmable low[ish]-power computers on a board, with some flash storage and RAM, and a fair bit of low-level IO. Often with wireless network connectivity as a built-in. They do stuff. :)

If the code is tight-enough, they can do a remarkable amount of that stuff -- in terms of raw compute, they present something near-ish to the Pentium era of personal computing for a few dollars.

For me, they make sense for simple projects. When something like a disused laptop or a Raspberry Pi-like device [and the accoutrements, like a whole-ass operating system] seems like overkill, then: It's probably overkill.

With a dev board, I can breadboard a circuit and maybe even leave it that way forever, or get a custom PCB drawn up with KiCad and made inexpensively by JLC PCB or Osh Park or whatever, or just solder wires and parts to the dev board like a caveman.

All methods work, and the cost of screwing it up is usually very low.

After that, the thing gets powered up with one of the featureless USB wall warts that we all have accumulated too many of and it just sits there and does its job.

(And honestly, these Pi Pico and random ESP32 dev boards are also overkill for what I do with them. Most of what I've done could be accomplished with an MCU chip that costs pennies instead of dollars, but my quantities are low-enough that I just don't care much about a few dollars for occasional one-off projects.)

Similar class of chip, but I use the rp2350 to perform computer vision with LiDAR to detect when people leave dishes in a sink and beep angrily at them.

a microphone that streams to birdnet-go in my home assistant with that firmware https://github.com/Sukecz/esp32-birdnet-mic

Personally, I just finished up a project with one to poll outdoor and indoor weather sensors and notify the household over discord when it's warm enough outside to probably need air conditioning, when it's cooled off enough to probably not need AC, when the AC goes on, and when the AC goes off. It also parses and displays and notifies information from the National Weather Service when there are relevant alerts in my area.