Microcontroller Based Electronic Thermostat Project

Electronic Thermostat

Mechanical thermostat has been around for a long time and has been used in industrial control, home appliances control and many other devices to measure and control the temperature of a certain processes. The sensor used usually is a bimetallic sensor that is make from two different metals that expand at different rates as they are heated up. These metal strips are bonded together and when the temperature rises, the strips will bend upward hence making connection to the contact of the circuit so that current can flow through the circuit.

As the temperature cools down, it will go back to its original position and disconnect the current from the circuit. By adjusting the strip and the contact, the temperature can be contolled. Most oven and air conditioners use this type of sensor. The mechanical thermostat is more widely used due to its lower cost compared to electronic thermostat.

The use of electronic thermostat is becoming more popular now as the cost of semiconductor continues to drop with the advancement of better fabrication and packaging processes. Many applications have switched to electronic control as the control of the temperature is more accurate, easier to control the desired temperature using digital technology, more reliable and interfacing with other devices.

This application note from Microchip uses a low cost 6 pin microcontroller in the design of electronic thermostat. The features of PIC10F204 are as shown below. One advantage is that it has the PDIP package which makes it easier for electronic hobbyists to do their own soldering.

• 256 Words Program Memory and 16 bytes Static RAM
• Wide Operating Voltage from 2.0V to 5.5V DC
• 3 I/O
• 1 comparator
• 25 mA source/sink current I/O
• 1 8-bit timers.

Among the learning experiences one gained from this projects are:

• Power supply is directly tapped from the AC lines voltage using a resistive based power supply. This makes the entire circuit live and one has to be careful when implementing this project. Ensure that no parts of the circuit is accessible to any user. Use a plastic enclosure to house the printed circuit board properly.
• The principles of triac is briefly discussed here. The use of zero crossing detection is useful as many applications use this method in their operations. Among them are light dimmer and motor control applications.
• Learn how to optimise the program code to make it efficient. Many programmers use long routines to accomplish a certain task when a few lines of codes would be sufficient. This experience needs to be learned as one hands on a project and repeatedly look into the code to make it shorter and efficient.
• Having learned the code, one can then modify and add temperature sensor to make it a close loop control. Display circuity and user interface can be added to the system by migration to a higher pin count microcontroller.

The full application note and source code of the Microcontroller Based Electronic Thermostat Project can be obtained from Microchip website.

ref: electronics-project-design.com/ElectronicThermostat.html

HVAC Thermostat

Introduction To HVAC Thermostat

HVAC thermostat has been one of the common device used in residential and industrial buildings to control the temperature of a space be it a warehouse, a room, a hall or an office. This thermostat project will focus on the heating control of a space that uses electric heater as its source of heating. It basically consists of a comparator that controls the ON and OFF of the electric heater based on the sensor temperature.

The control of the fan speed is usually hardwired with two speed or three speed motors and is incorporated into the thermostat. The temperature range of this thermostat is from 5 Celcius to 30 Celcius with a tolerance of approximately 3 degree Celcius. Hence, only non critical tolerance control of temperature control such as a room can be used.

Circuit Description

The circuit diagram shows the configuration of the HVAC thermostat. The LM358 Op Amp is used as a comparator to sense the inputs of the reference voltage (PIN 3) and room temperature (PIN 2). The thermistor used is a NTC (negative temperature coefficient) type where its resistance will drop when the temperature increases and vice versa. It has a resistance of 20K ohm at 25 degree Celcius. When the room temperature drops, the thermistor resistance will go up and hence the output of the operational amplifier will be low. This cause the relay to turn OFF and the heater will conduct until the temperature of the room rises again.

The circuit is calibrated using variable resistor VR1. Set the lever of the slide potentiometer or rotary potentiometer VR2 to 25 Celcius location. Place the thermistor at a space where the temperature is at 25 Celcius. By varying VR1, set the resistance at the position between the ON and OFF of the relay. Use a suitable contact relay rating according to the load of the heater.

Parts List

ref: electronics-project-design.com/HVACThermostat.html