Temperature Controller

Temperature is the most often measured environmental quantity and many biological, chemical, physical, mechanical and electronic systems are affected by temperature. Some processes work well only within a narrow range of temperatures. So proper care must be taken to monitor and protect the system.

When temperature limits are exceeded, electronic components and ciruits may be damaged by exposure to high temperatures. Temperature sensing helps to enhance circuit stability. By sensing the temperature inside the equipment, high temperature levels can be detected and actions can be taken to reduce system temperature, or even shut the system down to avert disasters.

Some of the temperature control applications are Practical Temperature Controller and Wireless over Temperature Alarm Circuit Diagrams are discussed below.

Practical Temperature Controller

This type of controllers is used in industrial applications for controlling the temperature of devices.  It also displays the temperature on 1 LCD displays in the range of –55°C to +125°C. At the heart of the circuit is the microcontroller from 8051 family which controls all its functions. IC DS1621 is used as temperature sensor.

Practical Temperature Controller Circuit Diagram

The DS1621is gives the 9-bits of readings to show the temperature. User-defined temperature settings are stored in a nonvolatile memory EEPROM through 8051 series microcontroller .Maximum and minimum temperature settings are entered to the MC through a set of switches which are stored in the EEPROM -24C02.Maximum and minimum setting are meant for allowing any hysteresis necessary. Set button is used first and then the temperature setting by INC and then the enter button. Similarly for the DEC   button. A relay is driven from the MC through a transistor driver. The contact of the relay is used for the load, shown as a lamp in the circuit. For high power heater load a contactor may be used, the coil of which is operated by the relay contacts in place of the lamp as shown.

Standard power supply of 12 volt DC and 5 volt through a regulator are made from a step down transformer along with a bridge rectifier and filter capacitor.


Features of IC DS1621 are:

  • Temperature measurements require no external components
  • Measures temperatures from -55°C to +125°C in 0.5°C increments. Fahrenheit equivalent is  -67°F to 257°F in 0.9°F increments
  • Temperature is read as a 9-bit value (2-byte transfer)
  • Wide power supply range (2.7V to 5.5V)
  • Converts temperature to digital word in less than 1 second
  • Thermostatic settings are user definable and nonvolatile
  • Data is read from/written via a 2-wire serial interface (open drain I/O lines)
  • Applications include thermostatic controls, industrial systems, consumer products, thermometers, or any thermal sensitive system
  • 8-pin DIP or SO package (150mil and 208mil)

Wireless over Temperature Alarm

The circuit uses an analog temperature sensor LM35 duly interfaced to a comparator LM 324 whose output is fed to  a 4 bit input encoder IC HT 12E .The limit is selected with the help of a 10K  preset which is calibrated around its 270 degree rotation. The encoder IC converts this to parallel data to serial one which is given to a transmitter module for transmission.

Wireless Over Temperature Alarm Circuit Diagram

The RF module, as the name suggests, operates at Radio Frequency. The corresponding frequency range varies between 30 kHz & 300 GHz. In this RF system, the digital data is represented as variations in the amplitude of carrier wave. This kind of modulation is known as Amplitude Shift Keying (ASK).

Transmission through RF is better than IR (infrared) because of many reasons. Firstly, signals through RF can travel through larger distances making it suitable for long range applications. Also, while IR mostly operates in line-of-sight mode, RF signals can travel even when there is an obstruction between transmitter & receiver. Next, RF transmission is more strong and reliable than IR transmission. RF communication uses a specific frequency unlike IR signals which are affected by other IR emitting sources.

The transmitter/receiver (Tx/Rx) pair operates at a frequency of 434 MHz.  An RF transmitter receives serial data and transmits it wirelessly through RF through its antenna connected at pin4. The transmission occurs at the rate of 1Kbps – 10Kbps.The transmitted data is received by an RF receiver operating at the same frequency as that of the transmitter.

The receiver end receives this serial data and then feeds to a decoder IC HT12D to generate 4bit parallel data which is given to an inverter CD7404 to drive a transistor Q1 to actuate any load for warning purposes. Both the transmitter and the receiver are powered from batteries with reverse protection diodes and also to get around 5 volts out of 6 volt battery used.

HT12D is a 212 series decoder IC (Integrated Circuit) for remote control applications manufactured by Holtek. It is commonly used for radio frequency (RF) wireless applications. By using the paired HT12E encoder and HT12D decoder we can transmit 12 bits of parallel data serially. HT12D simply converts serial data to its input (may be received through RF receiver) to 12 bit parallel data. These 12 bit parallel data is divided in to 8 address bits and 4 data bits. Using 8 address bits we can provide 8 bit security code for 4 bit data and can be used to address multiple receivers by using the same transmitter.

HT12D is a CMOS LSI IC and is capable of operating in a wide voltage range from 2.4V to 12V. Its power consumption is low and has high immunity against noise. The received data is checked 3 times for more accuracy. It has built in oscillator, we need to connect only a small external resistor. HT12D decoder will be in standby mode initially i.e., oscillator is disabled and a HIGH on DIN pin activates the oscillator. Thus the oscillator will be active when the decoder receives data transmitted by an encoder. The device starts decoding the input address and data. The decoder matches the received address three times continuously with the local address given to pin A0 – A7. If all matches, data bits are decoded and output pins D8 – D11 are activated. This valid data is indicated by making the pin VT (Valid Transmission) HIGH. This will continue till the address code becomes incorrect or no signal is received.

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