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Sensor Interfacing 8051 Microcontrollers
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Atmel, NXP, Philips, 8051, 8052, 89C51, 89C52, 89S51, 89s52, 89C1051, 89C1051, 89C2051, AT89C4051, AT89S8252,l AT89C1051, AT89C2051, AT89C4051, P89C51RB+, P89C51RC+, P89C51RD+, P89C51RB2Hxx, P89C51RC2Hxx, P89C51RD2Hxx, P89C660, P89C662, P89C664, P89C668, P89C669, P89C51RA2xx, P89C51RB2xx, P89C51RC2xx, P89C51RD2xx, P89C60X2, P89C61X2,P89LV51RB2, P89LV51RC2, P89LV51RD2, P89V51RB2, P89V51RC2, P89V51RD2, P89V660, P89V662, P89V664.

There are many types of sensors are available into the market. They are used to measure or detect a huge variety of conditions including: temperature, pressure, level, humidity, speed, motion, distance, light or the presence or absence of an object and many other types. Following are the main types :  
  Temperature Sensors
• RTD
8051 Microcontrollers are the most powerful and intelligent device in today work because of their automation of systems, accuracy. and real real time applications. When they are interfaced with other device then they become masterpiece of art. Everywhere a single microcontroller has no mean. Basically they are CPU and other hardware components are required to work with them.
So Microcontrollers are most useful and reliable devices when they communicate with other devices, such as sensors, motors, switches, keypads, displays, memory and even other microcontrollers. So many techniques were developed to interfacing the equipment to the 8051 microcontrollers. Many interface methods have been developed over the last 20 years to solve the complex problem of circuit design such as features, cost, size, weight, power consumption, reliability, availability, manufacturability and real time.
Electronic sensors are physical devices that convert a physical parameter such as room temperature, blood pressure or wind speed into a signal that can be measured into electrical signal and interface to the microcontrollers. When the physical parameter is converted into electrical signal or pulse it is easily transmitted to the computer or microcontroller so that measurement can be done. When it is measured then system displays the electrical signal into display format on the computer software window or LCD, example temperature sensor. Every year hundreds millions of electronic sensors are manufactured by various companies. They are in domestic appliances, medical equipment, industrial control systems, air-conditioning systems, aircraft, satellites and toys. Sensors are smart in working, accurate and economical and easy to interface with 8051 microcontrollers 
RTD Ebooks Free  :  Also called Platinum RTD because it is typically made of platinum. A change in temperature causes a change in resistance of platinum. This change can be measured with an ohmmeter. In theory, any metal could be used to measure temperature. The metal selected should have a high melting point and an ability to withstand the effects of corrosion. Platinum has therefore become the metal of choice for RTD's. Its desirable characteristics include chemical stability, availability in a pure form, and electrical properties that are highly reproducible.
Interfacing C Code Thermistors : Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. Thermostats are inexpensive, easily-obtainable temperature sensors. They are easy to use and adaptable. Circuits with thermistors can have reasonable output voltages - not the mill volt outputs thermocouples have. Because of these qualities, thermistors are widely used for simple temperature measurements. They're not used for high temperatures, but in the temperature ranges where they work they are widely used.
Free Tutorials Thermocouples : Thermocouples convert temperature to voltage. They rely on See beck effect which states that a junction of different metals will generate a voltage that is proportional to the temperature of the metals. Thermocouples are low cost temperature sensors, they are readily available from multiple sources and they can measure a wide range of temperatures that can not be measured with semiconductor type temperature sensor. For example, they can be used to measure the temperature of the inside of a ceramics kiln which can reach 1200 Celsius.
Successive-approximation register counts by trying all values of bits starting with the most significant
bit and finishing at the least-significant bit. Throughout the count process, the register monitors the comparator's output to see if the binary count is less than or greater than the analog signal input, adjusting the bit values accordingly. This way, the DAC output eventually converges on the analog input signal and the result is presented in the Result register