Showing posts with label Negative temperature Coefficient. Show all posts
Showing posts with label Negative temperature Coefficient. Show all posts
Tuesday, June 21, 2011
Heat Sensor Circuit
This is the simple heat sensor circuit. It can be used to control any device using heat sensor. In this circuit a thermistor and a resistance is connected in series. This arrangement makes a potential divider circuit.
Here the thermistor is Negative Temperature Coefficient type. So when the room temperature is increased its resistance decreases simultaneously and more current flows through the resistor and the thermistor. We find more voltage at the junction of the resistor and the thermistor.
Our thermistor resistance value is 110 ohms. Suppose the resistance value becomes 90 ohms after heating the 110 ohms thermistor. Then the voltage across one resistor of the voltage divider circuit equals the ratio of that resistor’s value and the sum of resistances of the voltage across the series combination. This is the concept of voltage divider.
The final output voltage of the voltage divider circuit is now applied to the npn transistor (BC548) through the base resistor (3.3K ohms). Here the emitter resistor is replaced with a zener diode. Emitter voltage is maintained at 4.7volt with the help of zener diode. This voltage is used to compare voltage. Transistor conducts when base voltage is greater than the emitter voltage. Transistor conducts if it gets more than 4.7volt of base voltage. Then the circuit is completed through buzzer and it gives sound.
Wednesday, May 25, 2011
Design a Temperature Indicator
This circuit is a temperature indicator circuit or differential instrumentation amplifier using a transducer bridge. This circuit is calibrate in degrees Celsius or Fahrenheit. In the circuit used buffer in and points for exact voltage of and points. Because gain is always 1 of buffer circuit. Then the output voltage of buffers is input voltage of differential amplifier.
The differential amplifier is difference voltage of and points using 741 Op Amp. When temperature is increased then resistance is also decreased and output voltage is decreases and when temperature is decreased then resistance is also increased and output voltage is increases.
The temperature indicator is a circuit that indicates of temperature in degrees Celsius or Fahrenheit. The temperature is inversely proportional to the resistance or transducer.
Fig.01 - Temperature indicator
In the circuit used as the transducer in the bridge circuit is a thermistor and replaced output voltmeter to temperature indicating meter. Then temperature indicating meter is calibrate in degrees Celsius or Fahrenheit.
The bridge can be balanced at a desired reference condition, for instance 250C. As the temperature varies from its reference value, the resistance of the thermistor changes and the bridge become unbalanced. This unbalance bridge in turn produces the meter movement.
Friday, February 11, 2011
Temperature Meter
The use of temperature devices in temperature measurement and sensing have made tremendous progress in the last few decades. There are a few types of measurement solutions that you can implement in your projects. The use of thermistors or thermocouples are the two most widely used devices in measurement solutions. The recent decade has seen the use of integrated circuits devices in many temperature control related systems because they are much smaller, provide a more accurate measurement and simpler to be integrated to other digital control devices.
Most of the digital temperature sensor system has a built-in communication bus to enable it to communicate with the master control IC. The most used communication interface is called I2C, a simple bi-directional 2-wire bus that was developed by Philips Semiconductors in the 1980's. Since then, many devices has this built in communication protocol that enables all devices that have this feature to be linked together without any other additional components. The I2C interfacing standard has become a world standard that are used in more than 1,000 integrated circuits.
The I2C standard basically define the start, stop, device selection addressing and data transfer interfacing protocol. The hardware consists of 2 I/O lines called SDA and SCL lines.
START Condition
The Start Data Transfer is initiated when there is a change of state of SDA line from HIGH logic to LOW logic while the SCL line is at HIGH logic. This is the START condition.
STOP Condition
The Stop Data Transfer is initiated when there is a change of state of SDA line from LOW logic to HIGH logic while the SCLline is at HIGH logic. This is the STOP condition.
DATA Transfer Condition
The data transfer is done between the START and STOP conditions with the data being transferred when SCL transition fromLOW to HIGH logic. Data is read when SCL is at HIGH logic. SDA line data will only change when SCL line is at LOW logic.There is no limit to the number of data bytes transferred and is determined by the master device. Acknowledgement of successful transfer of data is done between the master and the slave devices at regular interval.
Digital Temperature Sensor Applications
If you are into designing of thermostat controls for various buildings, industrial controls or home appliances, you maywant to consider using the TMP100 digital temperature sensor from Texas Instruments. This device can be connected tothe microcontroller using the SCL and SDA lines.
The features of the TMP100 sensor include:
* Low Quiescent standby current of 0.1uA means if you choose a proper microcontroller, the device using battery powered could last for years compared to the use of thermistor.
* Temperature range from -55 °C to 125 °C.
* Wide Power supply range from 2.7V to 5.5V.
* Accuracy of +/- 2.0 °C.
* Resolution up to 0.0625 ° C.
The typical application of the TMP100 digital temperature sensor is as shown in the diagram below.
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