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.
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.
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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