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The “Smart Coffee Machine with Arduino and Bluetooth” is a documented 2021 maker retrofit for an older drip coffee maker. It combines temperature sensing, a display, physical and Bluetooth controls, and relay-based heater switching. It is useful as an embedded-systems learning project, but it is not a certified appliance design: reproducing its features does not establish that a modified coffee maker is safe for unattended use.
The original project by Gyula Ősi was published on Hackster.io on July 7, 2021. Its controller is an Arduino Nano or Uno-class board paired with an HC-05 Bluetooth module. The original project and its code are described on Hackster.io and in a DFRobot project reproduction. Treat those pages as historical project references, not as evidence of current maintenance or appliance-safety certification.
What the coffee-machine retrofit does
This is more than a Bluetooth on/off switch. The controller reads a temperature sensor, displays a temperature graph, operates a relay assigned to the heater, and reports status through indicator LEDs and a buzzer. A user can start or stop operation from a physical button or a Bluetooth serial terminal. The firmware also includes optional temperature-drop-based automatic starting, Celsius/Fahrenheit selection, EEPROM-stored settings, and a test mode for using the sensor as a general temperature monitor.
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- Temperature display: Read the DS18B20 and plot readings on an ST7920 128×64 graphical display.
- Automatic start: Optionally infer that fresh water has been added from a temperature drop. This is a heuristic, not a water-level measurement.
- Status and diagnostics: LEDs and a buzzer indicate operating states; the firmware can report conditions such as sensor errors, insufficient heating, or a suspected missing-water condition.
- Test mode: Use the temperature graph without running the coffee-making sequence, with a configurable sampling interval.
The project is a handmade retrofit, not a commercial smart-appliance platform. The published material does not establish validated temperature accuracy, food-contact compliance, thermal protection, or suitability across coffee-maker models.
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- 12-CUP DURALIFE GLASS CARAFE: The sturdy 12-cup* carafe has measurement markings for accurate filling and an easy-grip handle for comfortable pouring. *Cup equals approximately 5 ounces (varies by brewing technique).
- SNEAK-A-CUP FEATURE TO PREVENT DRIPS: This feature temporarily stops the flow of coffee so you can pour your first cup before brewing ends without making a mess.
- EASY-VIEW WATER WINDOW: The front-facing window shows you the exact amount of water that will turn into coffee, and the brew basket requires a standard paper coffee filter for proper operation.
- DIGITAL CONTROLS WITH RUBBERIZED FEEL: Large, rubberized buttons give you full control of the coffeemaker, and the easy-read screen displays the clock, brew time, and programming options.
- QUICKTOUCH PROGRAMMING WITH AUTO SHUTOFF: Easily program the 24-hour auto brew feature so you can wake up to a fresh pot of coffee or whenever you desire. 2-Hour Auto Shutoff for added safety and peace of mind with every use.
How the system is arranged
In the original architecture, the phone sends text over a Bluetooth serial link to the HC-05. The Arduino processes commands, reads the DS18B20, updates the display and indicators, and controls a relay output. That relay is used to switch the coffee maker’s heater coil.
Phone
│ Bluetooth serial
▼
HC-05 ── Arduino Nano/Uno ── ST7920 display
│ ├── LEDs, buzzer and button
│ ├── DS18B20 temperature sensor
│ └── relay control ── heater circuit
The relay-to-heater connection is the high-risk boundary: the heater circuit may carry mains voltage. Keep low-voltage bench testing separate from any appliance integration; a relay board alone does not make mains switching safe.
Original parts and pin assignments
The following pin map is specific to the published Nano/Uno-oriented firmware. It is not a general Arduino standard or a verified replacement-board pinout. The mirrored project description and code list these assignments at DFRobot.
| Function | Part or signal | Original Arduino assignment |
|---|---|---|
| Temperature data | DS18B20 digital sensor | D2 |
| Buzzer | Piezo/buzzer output | D3 |
| Counter clock | CD4017/CD4027-related circuit | D4 |
| Counter reset | CD4017/CD4027-related circuit | D5 |
| Bluetooth connection status | HC-05 status output | D6 |
| Graphical display | ST7920 128×64 | D9, D10, D11 |
| Heater control | Relay input | D12 |
| Pushbutton | Momentary switch | D13 |
| Red indicator | LED output | A0 |
| Green indicator | LED output | A1 |
The published display initialization is U8GLIB_ST7920_128X64 u8g(11, 10, 9, U8G_PIN_NONE);. The source also assigns the buzzer to D3, the counter clock and reset to D4 and D5, the button to D13, the relay to D12, and the DS18B20 data line to D2. Verify the downloaded source and the exact hardware revision before wiring: the project page embeds code, but does not establish a maintained, versioned release.
Rank #2
- BREW BY THE CUP OR CARAFE: Brews both K-Cup pods and coffee grounds.
- MULTISTREAM TECHNOLOGY: Saturates the grounds evenly to extract full flavor and aroma in every cup, hot or cold.
- BREW OVER ICE: Adjusts temperature for maximum flavor and less ice melt for single-cup iced coffees and teas.
- STRONG BREW & EXTRA HOT FUNCTIONALITY: Brews a stronger, more intense-flavored cup and the extra hot feature brews a hotter single cup.
- MULTIPLE BREW SIZES: Brew 6, 8, 10, or 12oz single cups or 6, 8, 10, or 12-cup carafes. 12-cup glass carafe specially designed to limit dripping.
A reproduction also needs an Arduino Nano R3 or Uno-class ATmega328P board, HC-05 serial Bluetooth module, DS18B20, ST7920 display, button, indicator LEDs, buzzer, counter logic components used by the build, and a relay/driver arrangement. The source does not provide a complete electrical-rating specification for a safe mains switching assembly. Keep any sensor, probe, wiring, or enclosure component out of the water path unless its materials and intended use are explicitly appropriate for that exposure.
Firmware, libraries and Bluetooth commands
The original firmware is an Arduino IDE project for the ATmega328P-style environment. It uses OneWire.h, DallasTemperature.h, U8glib.h, EEPROM storage, and a software-serial Bluetooth connection initialized at 9600 baud. The official Arduino IDE download page provides the development environment; the project’s Hackster page is the source for its downloadable code and project details.
The documented text commands are:
| Command | Action |
|---|---|
Auto |
Toggle automatic starting based on fresh-water temperature detection; setting is stored in EEPROM. |
Reboot |
Restart the controller. |
Start |
Begin the coffee-making sequence. |
Stop |
Interrupt brewing or leave test mode. |
Test |
Enter temperature-monitoring test mode. |
C or c |
Select Celsius. |
F or f |
Select Fahrenheit. |
1 through 999999 |
Set the test-mode temperature-measurement interval in seconds. |
Command capitalization varies in the project documentation, so follow the exact command handling in the firmware you compile. A serial terminal may also need a line ending configured before a command is recognized. The numeric interval applies to test mode, not to heater-control timing. The project identifies Kai Morich’s Serial Bluetooth Terminal as a preferred Android test tool; it is a developer interface rather than a dedicated coffee-machine app.
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- Fill the coffee maker with water, then power the controller and appliance only in an appropriately controlled setup.
- Pair the phone with the HC-05 and open a Bluetooth serial terminal at 9600 baud.
- Send
Startor use the physical button. The firmware records an initial temperature and commands the relay output. - During operation, the DS18B20 is read and the display and indicator outputs are updated.
- After roughly 60 seconds, the project’s firmware analyzes the temperature trajectory. The brew sequence then ends according to its temperature and timing logic, or when the user sends
Stop. - At completion, the controller switches its relay output off and returns to standby.
The automatic-start option looks for the temperature drop associated with adding fresh water. It does not measure water level or verify that a reservoir has been filled, so changing ambient conditions, sensor placement, or water volume can affect the inference.
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- ICED COFFEE: Bold iced coffee flavor that's never watered down.
- COLD BREW: Smooth, fresh cold brew in as little as 10 minutes.
- 8 BREW SIZES: Small cup, regular cup, XL cup, travel size, or XL travel size. Plus, you can choose from a quarter, half, or full carafe.
- REMOVABLE RESERVOIR WITH AUTO-METERING: Easy-to-fill, removable water reservoir with auto-metering to sense water amount, so there’s no need to measure or constantly refill.
Likewise, the diagnostic labels are interpretations of temperature response rather than direct measurements. A rapid rise may be treated as evidence of missing water; a slower rise may be classified as “Coffee for Six,” while a faster one may be labeled “Coffee for Two.” Little or no rise may produce a “Heating Coil error” message, and abnormal readings may be treated as a sensor error. These are project-specific heuristics: they do not prove a particular cup count, brew quality, or failed component, and the available project material does not establish that they generalize to other coffee makers.
What test mode can and cannot tell you
Test mode repurposes the external DS18B20 as a general temperature probe and graphs readings. The project describes a firmware graph range of approximately −20°C to +128°C (−4°F to +262°F), with intervals from 1 to 999999 seconds and graph spans described from about 128 seconds to 11.57 days depending on the interval and display behavior.
Those are display and firmware limits, not a guarantee of sensor accuracy or of safe operating conditions for the probe, cable insulation, appliance, or anything that touches drinking water. A sensor attached to a heater assembly may not track water temperature: placement, thermal contact, insulation, and exposure to steam all affect what it reads.
Build and test in stages
Do not start by connecting an Arduino-controlled relay to a coffee maker. First prove the low-voltage logic independently of the appliance.
Rank #4
- 2 BREW STYLES: Classic or Rich, each brew is flavorful and never bitter with custom brew strengths.
- 12-CUP GLASS CARAFE: Brew small or large batches of coffee – perfect for day to day or entertaining.
- HOTTER BREWING TECHNOLOGY: Delivers even saturation and temperature control for ultra-flavorful coffee.
- SMALL BATCH FUNCTION: Ensure your coffee is never diluted when brewing a small batch (1-4 cups).
- WAKE UP TO HOT COFFEE: 24-hour programmable delay brew allows you to prepare your brew up to a day in advance.
1. Bench-test the controller
- Use an LED or other suitable low-voltage indicator in place of a heater load.
- Confirm that the board powers up, the display renders, the HC-05 link works at 9600 baud, and the terminal commands are received.
- Check that DS18B20 readings are plausible and that the physical button changes the simulated output as expected.
- Deliberately exercise error states and confirm that
Stopde-energizes the simulated output.
2. Validate the sensor behavior
- Check the DS18B20 wiring and pull-up arrangement against the sensor and library requirements.
- Compare a stable room-temperature reading with the sensor’s response in warm water, without using an unverified probe in drinking water.
- Disconnect the sensor and observe how the firmware handles invalid or missing data.
- Consider mechanical displacement, condensation, and poor thermal contact: a plausible reading can still fail to represent the water or heater temperature.
3. Exercise control and failure cases
On the low-voltage bench setup, test manual start and stop, auto-start both enabled and disabled, suspected missing-water and slow-heating behavior, sensor disconnection, power loss and restart, and Bluetooth disconnection during a simulated run. Verify the actual default output state after reset. A connection-status pin can indicate a link state, but it is not a safety interlock.
4. Treat appliance integration as advanced electrical work
The original project switches a heater coil through a relay. The available project description does not document a complete safety case, validated thermal-control procedure, or component ratings for a universally safe appliance retrofit. If you are not qualified to design and verify mains equipment, stop at the low-voltage demonstrator.
An engineered appliance modification would need, at minimum, a correctly rated switching device and driver, galvanic isolation, suitable overcurrent protection, enclosed and strain-relieved mains wiring, protective-earth continuity where applicable, separation of mains and low-voltage conductors, moisture protection, and an independent hardware thermal cutoff. It must fail with the heater off if the controller crashes, resets, loses communications, or encounters a sensor fault. A hobby relay board does not establish adequate contact rating, isolation, enclosure, or fault protection by itself.
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Bluetooth loss or pairing trouble
HC-05 boards can differ in firmware, pairing credentials, labels, and voltage arrangements, while phone support for Bluetooth Classic varies. A phone can appear paired without a usable serial connection. Treat Bluetooth as a convenience interface, retain a physical stop control, and ensure that loss of communication cannot leave a heater on indefinitely.
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- ADJUSTABLE CARAFE TEMP: Our Cuisinart 14-Cup* Coffee Maker with adjustable carafe temp feature adjusts the warming plate temperature to Low, Medium, or High to keep freshly brewed coffee at the temperature you prefer.
- BREW STRENGTH CONTROL: This coffee maker customizes your brew to perfection with regular or bold settings, catering to your unique taste preferences every time.
- FULLY AUTOMATIC COFFEE MAKER: The coffee machine offers convenience with 24-hour programmability, 1-4 cup settings, auto-off (0-4 hours), and an optional ready alert tone.
- 14-CUP* CAPACITY: *Cup equals approx. 5 oz. (varies by brewing technique).
- FILL WITH EASE: Enjoy precise water filling with the easy-to-view water window, while the Cuisinart coffee maker Brew Pause feature lets you savor a cup before the brewing cycle finishes, enhancing your experience.
Sensor faults and misleading readings
An open or shorted sensor, damaged OneWire data, detached probe, poor sensor placement, steam, or condensation can produce invalid or misleading readings. The original firmware includes sensor-error handling, but that does not ensure it will detect a mechanically displaced sensor whose output remains plausible.
Dry-run inference and heater switching
The temperature-based missing-water logic can be confounded by unusual water volume, ambient temperature, an aged or scaled heater, changed probe position, or a different thermal design. Relay contacts can also weld closed, fail to energize, be wired with misunderstood active-low logic, or be under-rated for the load. Software cannot substitute for an independent thermal cutoff and a default-off power design.
Reset, power failure and moisture
EEPROM storage of settings does not guarantee safe recovery after a power interruption. On restart, a controller intended for any real heater load must establish the sensor state and output state before permitting another heating cycle. Keep unsealed hobby electronics out of wet areas; distinguish components merely near the appliance from probes exposed to water or steam, and do not assume a generic “waterproof” probe is suitable for drinking-water contact.
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Reproduce the original design or modernize it?
| Choice | Best suited to | Trade-offs |
|---|---|---|
| Nano/Uno with HC-05 | Studying or closely reproducing the documented build and its ATmega328P-oriented firmware. | Requires a separate Bluetooth module; module variants differ, and the published code should be treated as a project reference rather than a maintained library. |
| ESP32 development board | A new design needing integrated Bluetooth, possible Wi-Fi, or a richer interface. | Not a drop-in replacement: it uses 3.3 V logic, different GPIO constraints and Bluetooth APIs, and requires a new pin map and firmware adaptation. |
| Low-voltage demonstrator | Learning, classroom use, or projects where mains modification is outside the builder’s skill set. | Does not switch a real coffee-maker heater, but can demonstrate command handling, sensing, graphing, and control safely with an LED, suitable low-voltage load, or separate temperature vessel. |
For an ESP32 redesign, consult Espressif’s ESP32-DevKitC documentation. The change can eliminate the HC-05, but it does not remove the need to redesign firmware, signal levels, peripheral assignments, and any power-stage safety system. The official Arduino Nano hardware documentation is useful when matching a reproduction to the original board family.
If the goal is simply phone control, a low-voltage heater demonstration or a separate certified external control device designed for the appliance is a better choice than defeating the coffee maker’s built-in protections. Any external control method still needs to be legal, appropriately rated, and compatible with the appliance.
Verdict
The Nano/HC-05 project is a worthwhile embedded-systems reference for learning serial commands, temperature sensing, graphing, and basic control. Reproduce it first as a low-voltage bench prototype; choose an ESP32 only if you are prepared to port and redesign the system. Do not treat the temperature heuristics, Bluetooth link, or relay as certified protection, and do not operate a modified mains coffee maker unattended without a professionally engineered and independently verified safety system.
The original project is marked CC BY-NC on Hackster.io. Review the license and attribution terms before reusing its code, diagrams, or images: project and license information.
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