DIY Temperature Control Systems with Arduino: Guide to Building Precision Temperature Controllers

Last updated: May 23, 2026

Building a DIY temperature control system with Arduino is one of the most useful electronics projects a homebrewer can take on, and it’s accessible to anyone comfortable with basic soldering and a willingness to follow documented wiring diagrams. My first Arduino temperature controller replaced a $60 commercial unit and added programmable step capabilities that the commercial unit didn’t have. The total component cost was $28 and the build took a Saturday afternoon. Here’s a practical guide to building a reliable Arduino-based temperature controller for fermentation or mash control.

Components for a single-zone PID controller

• Arduino Uno or Nano ($5–15): The Nano ($5 on AliExpress, $10–15 from US suppliers) is preferred for compact builds; the Uno is larger but easier to prototype on a breadboard. Either works identically for this application.
• DS18B20 waterproof temperature probe ($5–8): The OneWire digital temperature probe used in most brewing projects. ±0.5°C accuracy; rated to 125°C. Wiring requires a 4.7kΩ pull-up resistor between the data line and VCC.
• Solid State Relay (SSR), 25A DC control ($8–12): Controls the AC load (heating element or refrigerator compressor) from the Arduino’s 5V output signal. SSR provides silent switching and longer life than mechanical relays for inductive loads. Use a 40A SSR if driving compressor loads.
• 20×4 LCD display with I2C adapter ($5–8): Displays current temperature, setpoint, and mode. The I2C adapter reduces wiring to 4 pins (VCC, GND, SDA, SCL) rather than the 8–16 pins required for direct LCD connection.
• Momentary pushbuttons ($2–3 for a set): Up/down setpoint adjustment and mode selection. Three buttons is sufficient: UP, DOWN, MODE.
• Project enclosure ($8–12): A 150mm × 100mm × 70mm ABS project box from any electronics supplier houses all components neatly with space for the AC wiring.

Wiring overview

The DS18B20 probe connects to Arduino digital pin 2 with a 4.7kΩ pull-up resistor to 5V. The LCD I2C adapter connects to SDA (A4 on Uno/Nano) and SCL (A5). The pushbuttons connect to digital pins 4, 5, 6 with internal pull-up resistors enabled in code. The SSR control input connects between Arduino digital pin 8 (signal) and GND, the SSR triggers when this pin goes HIGH. The SSR load side (AC terminals) sits between the AC supply and the heating element or refrigerator power input. Keep AC wiring completely separate from DC Arduino wiring; use appropriate wire gauge for the current load (14 AWG minimum for 15A loads).

Software

Required Arduino libraries: OneWire, DallasTemperature (for DS18B20), LiquidCrystal_I2C (for I2C LCD), and PID (Brett Beauregard’s Arduino PID library). All are available through the Arduino IDE Library Manager. A complete single-zone fermentation controller sketch, reading temperature, running PID algorithm, controlling SSR, displaying on LCD, accepting button input, runs approximately 150 lines of code. Complete example sketches are available from the Arduino Homebrew community on GitHub and from BrewUnited’s brewing automation resources. The PID constants for fermentation control (Kp=2, Ki=5, Kd=1 as starting values) work for most chest freezer applications and can be tuned based on observed temperature response.

Safety requirements

AC mains wiring requires attention to safety: use properly rated wire gauge; mount the SSR to the enclosure wall or an aluminum heat sink (SSRs generate heat proportional to load current); ensure all AC connections are insulated with heat shrink; install a fuse rated for the load between the AC input and SSR. Do not use the Arduino 5V supply for anything except the DC circuit, powering the Arduino from a USB charger or a 7–12V wall adapter through the barrel jack is cleaner than deriving 5V from the AC supply. The completed controller should be tested with a multimeter confirming correct polarity and absence of AC voltage on DC components before first use.

Common Questions

How do I tune the PID constants for my specific brewing setup?

PID tuning for a fermentation chamber uses the Ziegler-Nichols method: set Ki and Kd to zero, increase Kp until the temperature oscillates around the setpoint at a consistent amplitude, this is the ultimate gain (Ku). Measure the oscillation period (Tu). Then: Kp = 0.6 × Ku, Ki = 2 × Kp / Tu, Kd = Kp × Tu / 8. For most chest freezer fermentation chambers, the response is slow enough that a simple on/off (bang-bang) controller with a 1°F hysteresis performs nearly as well as PID and requires no tuning. PID adds value when controlling faster-responding systems (RIMS heating elements, sous-vide baths) where overshoot with on/off control wastes time and energy. For fermentation chambers, start with simple hysteresis control and upgrade to PID only if you observe meaningful temperature overshoot affecting beer quality.