IEEE Envision Project Expo 2024
Electronic thermostat
Mentors: Jayant Jha, Chiranka K, Bhavy Parashar
Mentee: Pranav Kashyap
Brief Summary: This project creates a simple electronic thermostat to control room temperature using transistors. It employs an NTC thermistor and a 12V DPDT relay. When the temperature is low, transistors activate the relay to turn on the heater. Capacitors stabilize the system. If the temperature is above the set value, a red LED indicates standby; below, a green LED shows the heater is on.
Aim: The primary aim of the electronic thermostat project is to design and implement a circuit that regulates room temperature by controlling a heating device using transistors.
Introduction:
In this instance, the electronic thermostat is built to operate as a heater whenever the temperature of a room is less than a preset temperature. Central to this project is the use of a thermistor NTC (negative temperature coefficient) as the primary sensor.
The circuit is designed to activate the relay and illuminate a green LED when the temperature is lesser than the set value, indicating the requirement for heating. On the other hand, when the ambient temperature exceeds the set value, the relay remains inactive, and a red LED illuminates
Critical to achieving precise temperature control is the adjustment of the potentiometer. Calibration of the circuit involves positioning the NTC inside a glass tube and adjusting it alongside a mercury thermometer in various temperature conditions, including ice water, ambient temperature, and near a gas burner.
The circuit's functionality relies on the cutoff and saturation states of transistors, particularly transistor T1, which saturates when the NTC resistance is high (indicating a low ambient temperature). Once T1 saturates, it triggers transistors T2, T3, and T4, ultimately activating the relay.
The relay, typically a double-contact type, serves two key functions: switching the LEDs and activating the heater or load. Additionally, capacitor C1 ensures stability by minimizing sudden changes in the NTC's resistance.
Methodology:
10k NTC Thermistor:
The NTC is the temperature sensing component in the circuit. The NTC resistance, which has an inversely proportional relationship to ambient temperature, makes it suitable to be used as temperature sensing.
In our circuit, the NTC thermistor is used to detect changes in the ambient temperature. When the temperature is low, the resistance of the NTC thermistor is high, indicating that the ambient temperature is below the set value. Conversely, when the temperature increases, the resistance of the NTC thermistor decreases, indicating that the temperature is above the set value.
Fig1. resistance-temperature graph of a NTC thermistor (source:https://www.northstarsensors.com/calculating-temperature-from-resistance)
Control circuits:
The control circuits in our electronic thermostat play a critical role in managing the heating device based on the temperature sensed by the NTC thermistor. Transistors, especially T1, are vital components. When the ambient temperature is low, T1 saturates, initiating relay activation and keeping the heating device on. This relay, often a double-contact type, switches power to the heater and controls the LEDs. For example, a red LED indicates heating activation, while a green LED signifies standby mode. The control circuits ensure the heating device activates when needed to raise the temperature and deactivates to prevent overheating, maintaining precise temperature control.
Fig2. schematic diagram of circuit
Relay Functionality:
The relay serves as a pivotal component in our electronic thermostat circuit, performing two essential functions: switching the LEDs and activating the heating device or load. When the relay is activated, it connects the power supply to the heating device, allowing it to turn on and raise the room temperature. Additionally, the relay switches the LEDs to indicate the status of the thermostat. For instance, when the heating device is activated, a green LED lights up, signaling that the temperature is below the set value. Conversely, when the heating device is off, a red LED indicates that the temperature is above the set value, and the thermostat is in standby mode. This dual functionality of the relay ensures effective temperature control and provides clear visual feedback to the user.
Simulation:
The model has been simulated by using LTSpice, and data collection was done with the help of excel.
The model below illustrates the electronic thermostat circuit designed for temperature control.
Fig3. LTSpice model of circuit
( R10 represents the NTC resistor, R11 represents the potentiometer and R7 represents the relay)
Initially, I varied the potentiometer and NTC resistor to find the exact values at which the relay voltage enters the cutoff region.
Fig4. resistance values when relay is in cutoff region
This allows me to identify the ideal NTC resistor to be used in the circuit such that temperature sensing and control remains precise. Once this data was collected and graphed, I found a NTC resistor which has a resistance value of 7.6k at 25°C is suitable for this circuit. Giving me a resistance-temperature graph as below.
Fig5. Temperature-Resistance curve of the ntc resistor
With all the data at hand, I can now create a working simulation of the electronic thermostat. The main principle behind this circuit is that the potentiometer is set to a particular value such that, if the temperature detected by the NTC is lesser than or equal to the set value, the relay will switch on, if not, it will remain switched off.
Assuming that you want the thermostat to operate when the temperature is less than 15 degrees, we first look at the data taken from Fig5, and identify the value of NTC resistance when the temperature is 14°C. The resistance value is close to 9925 ohms. From Fig4. we know that when ntc resistance is 9925 ohms, the potentiometer should have a resistance of 10000. Once we set the potentiometer to have a resistance value of 10000 ohms, it can be tested that any temperature value less than 14°C causes the relay to switch on.
Fig6. Circuit simulation when temperature is >14°C.
The graph at the right indicates the voltage across the relay when the ambient temperature is greater than 14°C(resistance value is greater than 9925 ohms). If the cutoff voltage is kept at 5V for the relay, it can be seen that the relay is switched off at this current point of time.
Fig7. Circuit simulation when temperature is <14°C.
Fig7 shows that when the temperature is greater than 14°C, the voltage across the relay is greater than 5 Volts, causing the relay to switch on, and the heater to start working.
Fig8. Circuit simulation when temperature is <14°C.
Fig8 further shows the working of the circuit as the temperature decreases, showing that it reaches 11V, and it still operates
Conclusion:
By utilizing components like the NTC thermistor and transistors, the thermostat accurately switches the heating device on and off based on ambient temperature. The methodology employed, including potentiometer adjustment and circuit calibration, ensures precise temperature control.
References:
1)https://www.homemade-circuits.com/simple-thermostat-circuit-using/
2)https://makingcircuits.com/blog/how-to-make-a-simple-thermostat-circuit-using-transistors/
3) EPCOS NTC Thermistors - Standardized Resistance/ Temperature Characteristics (B572.. and B573.. Series)
Report prepared on May 5, 2024, 10:31 p.m. by:
Report reviewed and approved by Aditya Pandia [CompSoc] on May 9, 2024, 10:49 p.m..
