8051-Based Temperature Controller for a Laboratory Oven with Data Logging and Alarm

8051-Based Temperature Controller for a Laboratory Oven with Data Logging and Alarm is a classic instrumentation-and-control project done properly: a real closed loop on a real oven, not a bulb-on-a-breadboard demo. A wooden bench oven is heated by low-voltage halogen lamps driven through a MOSFET; a K-type thermocouple with a MAX6675 cold-junction-compensated converter feeds temperature to the AT89S52 with 0.25 °C resolution.

The controller runs two control modes so their behaviour can be compared honestly: plain on-off control with a 2 °C hysteresis band, and proportional (time-proportional) control that adjusts the heater duty cycle over a two-second period. The LCD shows setpoint, process value and mode; three buttons change the setpoint and switch modes; a buzzer raises an overshoot alarm if the oven exceeds the setpoint by more than 15 °C, and the converter's open-thermocouple flag handles a broken sensor safely by switching the heater off.

Every run is recorded: the firmware logs samples into a 24C02 I2C EEPROM and dumps them over the 8051's UART as CSV to a PC terminal, so the step response and control ripple of each mode can be plotted in the report. The driver circuit is simulated in Multisim / TINA before build, and the sensor chain is verified against a lab thermometer in a water bath — calibration discipline that the Instrumentation & Control diploma is expected to demonstrate.

College and small-industry ovens still depend on mechanical thermostats that hold temperature only within a wide, drifting band, show the setting instead of the actual temperature, and keep no record of behaviour over time. A failed sensor on such an oven is invisible: the controller simply heats forever, which wastes material and can damage the oven or its contents. There is no data to tune the loop or to prove a process was held at temperature. What is needed is a low-cost electronic temperature controller that measures with a proper thermocouple, holds the setpoint with a selectable control law, alarms on overshoot, shuts down safely when the sensor fails, and logs every run so the control behaviour can be analysed and improved — at a price a college lab or small workshop can justify.

Abstract

Temperature is the most controlled variable in industry — dryers, reactors, curing ovens, plastic moulds — and the workhorse of that control is still the simple electronic temperature loop. This project builds one at bench scale: an 8051-based controller that holds a small laboratory oven at a setpoint, offers on-off and proportional modes, logs every run, and protects the process with an overshoot alarm and sensor-fault shutdown. Everything is measured and plotted, so the report reads like a small control study.

Introduction

Every instrumentation diploma student meets control theory on paper: on-off control, proportional band, dead time, overshoot. Very few build the loop and watch the oscillation happen. Our project closes that gap with a safe, low-voltage bench oven and a controller built from syllabus components — the AT89S52, an SPI temperature converter, an I2C EEPROM and the serial port. Because the oven is small — minutes of thermal lag instead of an hour — the effect of every control decision is visible within a single lab session.

Existing System

• College ovens use mechanical thermostat or bimetallic control: a wide, drifting band, no real temperature indication, no record.
• Dial settings are approximate; the heater runs full-on or full-off, so temperature cycles far around the setpoint and contacts wear quickly.
• There is no alarm for a failed sensor — a broken thermocouple reads ambient, so a naive controller would drive the heater forever.
• Limitations: no setpoint precision, no proportional action, no data for tuning or audit, no sensor-fault protection.

Proposed System

• Sensing: K-type thermocouple with MAX6675 converter giving 12-bit readings (0.25 °C resolution) with built-in cold-junction compensation and an open-thermocouple flag.
• Two control modes, switchable at the touch of a button: on-off with 2 °C hysteresis, and proportional control that maps the error to heater duty over a 2-second cycle, so overshoot and ripple can be compared.
• Setpoint control and display: three buttons adjust the setpoint in 1 °C steps; the LCD shows setpoint, process value and mode; a buzzer fires on overshoot beyond setpoint + 15 °C.
• Data logging: each run's samples go into a 24C02 EEPROM and dump as CSV over the UART to a PC terminal for graphing.
• Safe heater stage: 12 V, 50 W halogen lamps as the heat source, driven through a MOSFET — no mains wiring in the student rig.
• Sensor-fault protection: an open thermocouple immediately latches the heater off and shows an error code.

Feasibility Study

• Technical: the MAX6675's SPI read, the EEPROM's I2C bus and the UART are all standard 8051 interfacing topics from the 34052 and 34066 courses; the driver stage is simulated first.
• Economic: the bill of materials is roughly ₹1,600–₹2,000, with the thermocouple and modules the only special purchases.
• Operational: the rig heats to 100 °C in a few minutes, so a full run fits inside a lab period, and everything runs from a 12 V supply with no mains hazard.

In scope

• A bench oven: wooden enclosure, two 12 V / 50 W halogen lamps as heaters, a 12 V fan for deliberate disturbance tests, and a K-type probe fixed at the shelf level.
• Two control modes (on-off, proportional) with measured step-response curves for each, setpoint range 40–120 °C, overshoot alarm and sensor-fault shutdown.
• EEPROM run logging with UART/CSV dump to a PC, plus calibration and test records.

Out of scope (v1)

• No autotuning or PID with integral and derivative action — proportional-only is the honest first step, and tuning is left as future work.
• No mains-voltage heater; the rig deliberately stays on 12 V. A 230 V version with a properly rated SSR is a supervised extension.
• No multi-zone control, no PC-graphical software (a plain terminal receives the CSV), and no real process fluids.

The project follows a phased build-and-test plan, with the two reviews timed to the M-scheme schedule: Review I after design and simulation, Review II after the controller holds temperature on the breadboard.

Testing approach: sensing is validated against references before control is trusted; then each control mode gets a recorded run — temperature versus time from cold start to five minutes past setpoint — repeated three times for repeatability. Disturbance rejection is tested by switching the fan on for a minute at steady state. Alarm thresholds, fault latching and the logging dump are tested explicitly, and every case is written up as expected versus observed.

The system is a closed control loop around the AT89S52: sense, decide, act, and record everything.

flowchart LR
  T["K-type thermocouple"] --> MX["MAX6675 converter (SPI)"]
  MX -->|"bit-banged SPI read"| U["AT89S52 (8051)"]
  PS["12 V supply + 7805 rail"] --> U
  B1["Setpoint / mode buttons"] --> U
  U --> L["16x2 LCD"]
  U --> Z["Overshoot buzzer"]
  U -->|"PWM duty pin"| DR["MOSFET driver"] --> H["2 x 50 W halogen heaters"]
  F["12 V fan (disturbance)"] -.->|"cools oven"| O(("Oven"))
  H -->|"heats"| O
  O -->|"air temperature"| T
  U -->|"I2C"| E["24C02 EEPROM (log)"]
  U -->|"UART 9600 baud"| PC["PC terminal (CSV dump)"]

Sensing. The MAX6675 does the hard part of thermocouple measurement — amplifying the microvolt-level signal and compensating the cold junction at its own terminals — and hands over 12 bits through SPI. The 8051 has no hardware SPI, so the firmware clocks the 16-bit frame out with software, checks the open-thermocouple flag bit, and converts the remaining 12 bits to 0.25 °C steps.

Control. The control task runs on a timer tick. In on-off mode the heater is fully on below setpoint − 1 °C and fully off above setpoint + 1 °C. In proportional mode the error maps linearly to a duty cycle (full on beyond 10 °C error, off at zero error, clamped at 95 % maximum), applied by chopping the heater over a 2-second period. The two laws produce visibly different overshoot and ripple — the core experimental result of the project.

flowchart TD
  S["Timer tick (every 100 ms)"] --> R["Read MAX6675; check open-thermocouple flag"]
  R --> Q{"Sensor fault?"}
  Q -->|"Yes"| X["Latch heater OFF; error code + buzzer"]
  Q -->|"No"| M{"Control mode?"}
  M -->|"On-off"| OO{"PV below setpoint - 1 deg C?"}
  OO -->|"Yes"| H1["Heater 100 percent"]
  OO -->|"No"| H2{"PV above setpoint + 1 deg C?"}
  H2 -->|"Yes"| H3["Heater 0 percent"]
  H2 -->|"No"| K["Hold last state"]
  M -->|"Proportional"| P["Duty = Kp x error, clamped 0 to 95 percent"]
  H1 --> A{"Overshoot beyond setpoint + 15 deg C?"}
  P --> A
  K --> A
  A -->|"Yes"| Z["Buzzer ON"]
  A -->|"No"| L2["Log sample to EEPROM; update LCD"]
  Z --> L2
  L2 --> S

Logging. Every 30 seconds the firmware writes the sample (setpoint, process value, duty) into the next free 24C02 page over bit-banged I2C; the UART menu command replays the log as CSV at 9600 baud so a PC terminal captures the run for plotting.

• Full PID with integral and derivative terms and a simple tuning procedure, compared against the proportional-only result.
• Second thermocouple at the heater wall to estimate dead time and improve the loop.
• An RTC chip (DS1307) so the log carries real timestamps for audit-style records.
• Bluetooth HC-05 replacing the cable for the CSV dump, or a phone dashboard.
• A supervised 230 V version driving a proper laboratory oven through a rated SSR.
• Setpoint profiles (ramp-and-soak) stored in EEPROM for curing or drying-style processes.