How connected devices work
Sensors, chips, radios, power and software, explained from the ground up. No brand is recommended here, and each fact links to its source. Six questions point you to the right kind of chip, from small microcontrollers to Linux-class processors.
One device, end to end
Follow a reading from a sensor to a decision and back. This is a local-first design: everything except the optional remote server runs inside the home. Many products instead send every reading and command through the maker's cloud.
The internet is off. In this local-first design, sensing, decisions, local control and the app on the home network keep working. Remote access, off-site backup, updates, phone notifications and voice assistants wait until it is back. A product that depends on its maker's cloud would lose control here.
Tap the diagram, or any block in it, to open it full size. Pinch to zoom, and tap a block for its details.
World Where it starts and ends
Physical world
Temperature, force, light, sound and motion. None of these arrive as numbers.
Everything starts as a physical quantity, and it comes with its own noise.
- → Sensorphysical quantity
Actuator
Turns a decision back into a physical effect: a relay, motor, valve, LED or speaker.
The return path: a command from the device, the gateway or the user becomes movement, light or sound.
Motors and relays draw far more current than the logic driving them. They need a driver stage, usually fed straight from the battery or supply rather than the logic rail.
The device On the circuit board
Sensor
Turns a physical quantity into an electrical signal.
Output is an analog voltage, current, resistance or capacitance change, a pulse train, or already a digital reading.
Digital sensors usually connect over I²C or SPI (sometimes I3C, UART, 1-Wire, or PDM/I²S for microphones); analog ones go through conditioning and an ADC.
Some sensors need continuous excitation; switching them off between readings saves energy.
A sensor can be fooled physically (heated, shaken, blinded) without touching any electronics.
- → Conditioningsmall analog signal
- → Microcontrollerdigital reading
- → Microcontrollerdigital sensor
Conditioning
Amplifies, filters and shifts the signal into the range the ADC measures best.
Gain boosts a millivolt signal to fill the ADC's input range. A low-pass filter weakens frequencies above half the sampling rate, which would otherwise show up as false signals; because real filters cut off gradually, designers sample well above twice the highest frequency they need.
Precision amplifiers draw continuous current; low-power designs switch them off between readings.
- → ADCscaled, filtered
ADC
Samples the analog voltage at fixed intervals and outputs numbers.
Resolution sets the step size; the sampling rate must exceed twice the highest frequency in the signal to capture it without aliasing.
Often built into the microcontroller; external ADCs connect over SPI or I²C.
- → Microcontrollersamples
Microcontroller
Runs the firmware: reads the samples, decides what they mean, and acts or reports.
Filters, averages or runs a small model, so only meaningful results leave the device.
Connects to other chips over I²C, SPI and UART, and drives simpler parts directly through GPIO and PWM pins.
Spends most of its life asleep; average current is set by its sleep current plus how often, and for how long, it wakes.
Firmware can be read out or replaced through an unlocked debug port, a serial bootloader left enabled, or unsigned updates.
- → Radiopackets
- → Actuatorcommand
- → Radioif separate chip
- → Secure elementkeys and signing
- → Actuatordrive (via a driver)
- ← Debug portif left unlocked
Radio
Sends and receives packets over the air: Bluetooth LE, Thread, Zigbee, Wi-Fi or LoRa. (Cellular goes through a mobile operator and the internet, so it bypasses the home hub.)
Battery devices send small packets, and not often. Every byte sent costs energy. Wi-Fi devices skip the hub's radio and join the home Wi-Fi directly.
Integrated in wireless MCUs; otherwise a separate chip or module on SPI or UART, or SDIO for faster Wi-Fi. Cellular modules are usually driven over UART with AT commands, or USB when the host runs Linux.
Usually the largest consumer among the electronics when active, especially Wi-Fi and cellular transmit; time spent listening costs energy too. Motors, relays and heated sensors can draw more.
Anything in range can listen or transmit; links need encryption and authenticated pairing.
- ↔ Home hubover the air
- → Home hubeavesdrop, spoof
Battery
Stores the energy; its chemistry sets the voltage range the rest of the device must tolerate.
Usable capacity depends on load current, temperature and the cut-off voltage of the circuit.
- → Regulatorraw voltage
Regulator
Converts the battery or supply voltage into the stable rails each chip needs.
Its own quiescent current is spent all the time, including while everything else sleeps.
- → Microcontrollerlogic rail
- → Sensorswitchable
- → Radiohigh peaks
Secure element
Optional separate chip that holds long-term private keys so they never sit in the main processor's memory. Some microcontrollers build a similar key store in.
Usually connected over I²C or SPI.
The device's identity key cannot be copied out even if the main firmware is hacked, though hacked firmware could still ask the chip to use it.
Read: Secure boot and the root of trustDebug port
JTAG or SWD pins used in development to program and inspect the chip.
JTAG or Arm SWD; a probe connects it to a debugger on a computer.
If left open on a shipped product, it gives full control of the chip. Lock it before shipping. On many chips, someone with the board in hand can still defeat the lock by glitching the power supply, so keep long-term secrets in a secure element.
- → Microcontrollerprogram, debug
- → Microcontrollerif left unlocked
Home network Inside the home
Home hub
A local, always-on device that connects low-power radios to the home network, and makes decisions without the internet.
Collects readings from every device, decides what to do and keeps the history, all without an internet connection.
Bluetooth LE, Thread, Zigbee or LoRa on one side; Ethernet or Wi-Fi to the home network on the other.
Mains-powered and always on, so it can listen for battery devices that wake only briefly.
It holds credentials for every device behind it, so if it is hacked, the whole site is exposed.
- ↔ Local brokerpublish
- ↔ Remote serveroptional
- ← Radioeavesdrop, spoof
- ← Remote servermalicious update, stolen account
Local broker
Passes messages between devices and apps. It runs on the hub, so nothing has to leave the building.
Devices and apps do not need to know about each other; each one just sends and receives messages.
MQTT over TCP, ideally with TLS even inside the home.
Needs per-device credentials and topic permissions, or any device can publish as any other.
- ↔ App or dashboardsubscribe
App or dashboard
Where a person sees results and sends commands, from a phone or screen on the home network.
Commands travel back the same way: app, broker, hub, radio, device, actuator.
Read: MQTT, CoAP and HTTP
Remote Optional
Remote server
Optional. Remote access from outside the home, an off-site backup, and delivery of software updates.
Only what needs the outside world goes here. In a local-first design, the home keeps working when it cannot be reached.
The update service, through the hub, can push code to every device. The firmware signing key is the most valuable secret in the system; keep it offline or in a hardware security module, not on the server. A stolen account or exposed remote access reaches the hub the same way.
- → Home hubmalicious update, stolen account
Read the diagram as a list
Physical world
Temperature, force, light, sound and motion. None of these arrive as numbers.
- Data Everything starts as a physical quantity, and it comes with its own noise.
Sensor
Turns a physical quantity into an electrical signal.
- Data Output is an analog voltage, current, resistance or capacitance change, a pulse train, or already a digital reading.
- Buses Digital sensors usually connect over I²C or SPI (sometimes I3C, UART, 1-Wire, or PDM/I²S for microphones); analog ones go through conditioning and an ADC.
- Power Some sensors need continuous excitation; switching them off between readings saves energy.
- Attack surface A sensor can be fooled physically (heated, shaken, blinded) without touching any electronics.
Conditioning
Amplifies, filters and shifts the signal into the range the ADC measures best.
- Data Gain boosts a millivolt signal to fill the ADC's input range. A low-pass filter weakens frequencies above half the sampling rate, which would otherwise show up as false signals; because real filters cut off gradually, designers sample well above twice the highest frequency they need.
- Power Precision amplifiers draw continuous current; low-power designs switch them off between readings.
ADC
Samples the analog voltage at fixed intervals and outputs numbers.
- Data Resolution sets the step size; the sampling rate must exceed twice the highest frequency in the signal to capture it without aliasing.
- Buses Often built into the microcontroller; external ADCs connect over SPI or I²C.
Microcontroller
Runs the firmware: reads the samples, decides what they mean, and acts or reports.
- Data Filters, averages or runs a small model, so only meaningful results leave the device.
- Buses Connects to other chips over I²C, SPI and UART, and drives simpler parts directly through GPIO and PWM pins.
- Power Spends most of its life asleep; average current is set by its sleep current plus how often, and for how long, it wakes.
- Attack surface Firmware can be read out or replaced through an unlocked debug port, a serial bootloader left enabled, or unsigned updates.
Secure element
Optional separate chip that holds long-term private keys so they never sit in the main processor's memory. Some microcontrollers build a similar key store in.
- Buses Usually connected over I²C or SPI.
- Attack surface The device's identity key cannot be copied out even if the main firmware is hacked, though hacked firmware could still ask the chip to use it.
Radio
Sends and receives packets over the air: Bluetooth LE, Thread, Zigbee, Wi-Fi or LoRa. (Cellular goes through a mobile operator and the internet, so it bypasses the home hub.)
- Data Battery devices send small packets, and not often. Every byte sent costs energy. Wi-Fi devices skip the hub's radio and join the home Wi-Fi directly.
- Buses Integrated in wireless MCUs; otherwise a separate chip or module on SPI or UART, or SDIO for faster Wi-Fi. Cellular modules are usually driven over UART with AT commands, or USB when the host runs Linux.
- Power Usually the largest consumer among the electronics when active, especially Wi-Fi and cellular transmit; time spent listening costs energy too. Motors, relays and heated sensors can draw more.
- Attack surface Anything in range can listen or transmit; links need encryption and authenticated pairing.
Battery
Stores the energy; its chemistry sets the voltage range the rest of the device must tolerate.
- Power Usable capacity depends on load current, temperature and the cut-off voltage of the circuit.
Regulator
Converts the battery or supply voltage into the stable rails each chip needs.
- Power Its own quiescent current is spent all the time, including while everything else sleeps.
Debug port
JTAG or SWD pins used in development to program and inspect the chip.
- Buses JTAG or Arm SWD; a probe connects it to a debugger on a computer.
- Attack surface If left open on a shipped product, it gives full control of the chip. Lock it before shipping. On many chips, someone with the board in hand can still defeat the lock by glitching the power supply, so keep long-term secrets in a secure element.
Actuator
Turns a decision back into a physical effect: a relay, motor, valve, LED or speaker.
- Data The return path: a command from the device, the gateway or the user becomes movement, light or sound.
- Power Motors and relays draw far more current than the logic driving them. They need a driver stage, usually fed straight from the battery or supply rather than the logic rail.
Home hub
A local, always-on device that connects low-power radios to the home network, and makes decisions without the internet.
- Data Collects readings from every device, decides what to do and keeps the history, all without an internet connection.
- Buses Bluetooth LE, Thread, Zigbee or LoRa on one side; Ethernet or Wi-Fi to the home network on the other.
- Power Mains-powered and always on, so it can listen for battery devices that wake only briefly.
- Attack surface It holds credentials for every device behind it, so if it is hacked, the whole site is exposed.
Local broker
Passes messages between devices and apps. It runs on the hub, so nothing has to leave the building.
- Data Devices and apps do not need to know about each other; each one just sends and receives messages.
- Buses MQTT over TCP, ideally with TLS even inside the home.
- Attack surface Needs per-device credentials and topic permissions, or any device can publish as any other.
Remote server
Optional. Remote access from outside the home, an off-site backup, and delivery of software updates.
- Data Only what needs the outside world goes here. In a local-first design, the home keeps working when it cannot be reached.
- Attack surface The update service, through the hub, can push code to every device. The firmware signing key is the most valuable secret in the system; keep it offline or in a hardware security module, not on the server. A stolen account or exposed remote access reaches the hub the same way.
App or dashboard
Where a person sees results and sends commands, from a phone or screen on the home network.
- Data Commands travel back the same way: app, broker, hub, radio, device, actuator.
Seven tracks
Each track answers one question. Open a track to see its topics, from the basics to the deep dives.
SenseHow does a device measure the physical world? 5 topics
- What a sensor actually isA sensor responds to something physical, like heat or force, in a way a circuit can measure. Every reading depends on how well it does that.Foundation
- Analog-to-digital conversionHow a voltage becomes a number: resolution, reference voltage, sampling rate and the Nyquist limit.Foundation
- Accuracy, precision and resolutionAccuracy, precision and resolution mean different things, and datasheets often blur them. Mixing them up gives readings that look precise but are wrong.Foundation
- Signal conditioningRaw sensor signals are often tiny or noisy. Conditioning amplifies and filters them so the ADC can measure them well.Core
- The common sensor familiesHow common sensors for temperature, motion and sound work, and what limits each one.Core
ThinkWhat kind of chip does the computing, and why that kind? 7 topics
- Microcontroller or microprocessor?A microcontroller is a small computer on one chip. A microprocessor usually needs separate memory chips, but can run a full operating system like Linux.Foundation
- Instruction sets: Arm, RISC-V, x86 and othersThe instruction set is the language a processor understands. Who owns it, and who can build chips with it, shapes everything around it.Foundation
- Chip, module, SoM, dev board, SBCThe same processor is sold as a bare chip, a module or a ready-made board. Their prices are not comparable, and mixing them up is a common mistake.Foundation
- Anatomy of an application-processor systemWhat sits around the processor on a Linux board, and how it starts up, from power-on to a login prompt.Core
- Application cores and microcontroller cores on one chipMany processors that run Linux also include a small microcontroller core, which handles time-critical work while Linux does the rest.Core
- GPUs, DSPs, ISPs, NPUs and what TOPS meansAccelerators do one kind of work far more efficiently than a CPU. TOPS is the headline number, and it is hard to compare between chips.Core
- Choosing a microcontroller or processorAnswer six questions to find the kind of chip that fits your project, from small microcontrollers to Linux-class processors. You get a class, not a brand.Core
TalkHow do chips, devices and servers exchange data? 6 topics
- UART, I²C and SPIThe three buses that connect most sensors, displays and radio chips to a microcontroller, and how to choose between them.Foundation
- Bluetooth LE, Wi-Fi and 802.15.4Common radios in home and wearable devices, and how each one trades range, speed and battery life.Foundation
- CAN, RS-485 and Ethernet in the fieldBuses built for long cables and electrical noise, used in vehicles, factories and buildings.Core
- LoRa, LoRaWAN, NB-IoT and LTE-MLow-power wide-area networks trade data rate for range and battery life. They suit small, infrequent messages from devices that sleep most of the time.Core
- MQTT, CoAP and HTTPOnce a device is on a network, it needs a way to send its data. MQTT, CoAP and HTTP each suit different kinds of device.Core
- Matter, Thread and ZigbeeHow smart-home devices from different brands are meant to work together, and which part of the job each standard covers.Deep dive
PowerWhere does the energy go, and how long will it last? 5 topics
- Writing a power budgetBattery life comes down to arithmetic: how much current each state draws, for how long, and how often. Work it out before choosing a battery.Foundation
- Batteries for small devicesCoin cells, Li-ion and LiFePO4: their voltages, their capacity, and when the capacity on the label actually holds.Foundation
- LDOs, buck converters and PMICsHow a battery or adapter is turned into the steady voltages a chip needs, and why the choice of regulator matters for battery life.Core
- Sleep modes and waking upA battery device spends most of its time asleep. What stays on, what wakes it up, and what is lost while it sleeps.Core
- Designing for power that cuts outPower cuts and voltage dips are common in many places. A device should handle them safely and recover on its own.Deep dive
RunWhat software runs on the chip, and how does it start? 4 topics
- Bare metal: the super-loop and interruptsThe simplest firmware is a main loop and a few interrupt handlers, with no operating system. It handles more than you might expect.Foundation
- Real-time operating systemsAn RTOS adds tasks, priorities and predictable scheduling to a microcontroller, so deadlines can be met. Real-time means predictable timing, not raw speed.Core
- Embedded LinuxRunning Linux on a device usually means building or adapting your own small Linux system, and keeping it updated for as long as the product is in use.Core
- Debug probes and bench instrumentsTools for seeing inside a running chip and on the wires around it: JTAG, SWD, GDB, logic analysers and oscilloscopes.Core
SecureHow does a device stay trustworthy after it ships? 3 topics
- Secure boot and the root of trustAt start-up, each piece of software checks the signature of the next one before running it, starting from code that cannot be changed.Core
- Over-the-air updatesDevices need software updates after they ship, and a failed update must not break the device. A/B slots and rollback handle both.Core
- Security baselines and the lawSeveral countries now have security rules for connected products. The basic requirements are short and specific.Deep dive
InferHow does a small device run machine learning itself? 3 topics
- On the device or in the cloud?Where a model runs affects speed, privacy, cost, and what happens when the network is down.Foundation
- Machine learning on a microcontrollerSmall machine-learning models can run on microcontrollers, with no operating system and very little memory.Core
- Quantisation: making models fitStoring weights as 8-bit integers instead of 32-bit floats makes a model about four times smaller. Quantising the activations too lets integer-only NPUs run it.Deep dive
How this is checked
Every fact here was checked by hand against its source, such as a standard, a datasheet or official documentation. Example parts come from at least three vendors per class, and prices are dated listings from named sellers.
Data last checked 7 October 2026. No brand, part or tool is endorsed here: see the Terms for how examples, prices and links are used, and the Privacy Policy for what these pages do and do not collect.
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