Temperature Controller Circuit
Precision thermostat using op-amp comparator and thermistor sensor
Project Overview
This temperature controller uses a thermistor sensor and op-amp comparator to maintain precise temperature control. The circuit can switch heating or cooling devices automatically based on the setpoint.
What You'll Learn:
- Op-amp comparator circuits and hysteresis
- Thermistor characteristics and temperature sensing
- Voltage divider networks for sensor conditioning
- Relay control and switching circuits
- Noise filtering and stability considerations
Perfect for incubators, greenhouse control, aquarium heaters, and home automation systems.
Required Components
Components for this temperature control system
Circuit Schematic
Circuit Operation: The thermistor and 10kΩ resistor form a voltage divider that creates a temperature-dependent voltage. The LM358 op-amp compares this voltage to the setpoint voltage from the potentiometer. When the temperature exceeds the setpoint, the output drives the relay through a transistor to control heating/cooling devices.
Step-by-Step Instructions
Insert the 8-pin IC socket and wire power connections (+12V and ground)
Create voltage divider with thermistor and 10kΩ resistor for temperature sensing
Connect thermistor voltage to non-inverting input of first op-amp
Wire potentiometer as setpoint adjustment to inverting input
Configure second op-amp as comparator with hysteresis for relay control
Add transistor driver circuit for relay control with flyback diode protection
Connect green LED for heating indication and red LED for cooling indication
Install power supply decoupling capacitor across op-amp power pins
Calibrate setpoint potentiometer for desired temperature range
Test system with controlled temperature changes
Always use proper heat sinking for power devices and ensure adequate ventilation for the thermistor sensor.
Circuit Theory & Calibration
Thermistor Response:
NTC thermistors decrease resistance as temperature increases. The voltage divider creates a temperature-dependent voltage that the op-amp can compare.
Sensitivity: Approximately 50mV/°C change at room temperature
Calibration Process:
- Measure ambient temperature with reference thermometer
- Adjust potentiometer for relay switching at desired temperature
- Test with controlled temperature changes
- Fine-tune hysteresis for stability
- Mark setpoint positions on potentiometer
Performance Specs:
- Temperature range: 0°C to 100°C
- Accuracy: ±2°C (after calibration)
- Hysteresis: 1-3°C (adjustable)
- Response time: 30-60 seconds
- Relay rating: 10A at 250V AC
Troubleshooting Guide
Relay chattering
Add hysteresis to comparator or increase filtering capacitor value
Temperature reading incorrect
Check thermistor connections and verify voltage divider calculations
No relay switching
Verify op-amp supply voltages and check transistor driver circuit
System too sensitive
Adjust hysteresis or add more filtering to temperature input
Poor temperature stability
Improve thermal coupling of sensor and check for noise sources
Applications & Enhancements
Practical Applications:
- Egg incubator temperature control
- Greenhouse heating and ventilation
- Aquarium heater controller
- Home brewing fermentation control
- Laboratory equipment temperature regulation
Enhancement Options:
- Add digital temperature display with 7-segment LEDs
- Include multiple temperature zones with separate sensors
- Implement PID control for better stability
- Add data logging capability with microcontroller
- Include alarm for out-of-range temperatures
Success Criteria: System maintains temperature within ±2°C of setpoint with stable operation and proper visual feedback.