09-09-2012, 01:24 PM
i am college student i want project report on above mentioned topic its an urgent
09-09-2012, 01:24 PM
i am college student i want project report on above mentioned topic its an urgent
01-03-2013, 09:49 AM
DIGITAL THERMOMETER CUM CONTROLLER
DIGITAL THERMOMETER.docx (Size: 462.17 KB / Downloads: 34) INTRODUCTION This standalone digital thermometer controls the temperature of a device according to its requirement. It also displays the temperature on four 7-segment displays in the range of –55°C to +125°C. At the heart of the circuit is the microcontroller AT89S8252, which controls all its functions. IC DS1821 is used as temperature sensor. IC DS1821 Dallas Semiconductor’s IC DS1821 is one-degree precision temperature sensor in a 3-pin pack like a transistor with single-wire communication protocol. It can operate as a standalone thermostat with user-programmable trip-points (set-points) or as an 8-bit temperature sensor with a single-wire digital interface. The open-drain DQ pin functions as the output for thermostat operation and as the data input/output (I/O) pin for single-wire communication. The single-wire interface lets user access the non-volatile memory (EEPROM) thermostat trip-point registers (TH and TL), status/ configuration register and temperature register. When configured as standalone thermostat, temperature conversions start immediately when power is switched on. In this mode, the DQ pin becomes active when the temperature of IC DS1821 exceeds the limit already programmed in the TH register, and remains active until the temperature drops below the limit programmed in the TL register. The DS1821 uses Dallas’ exclusive single-wire bus protocol that implements bus communication with one control signal. TEMPERATURE SENSOR FUNCTIONALITY The core functionality of IC DS1821 is its proprietary direct-to-digital temperature sensing, which provides 8-bit (1°C increment) centigradetemperature readings over the range of –55°C to +125°C. This circuit measures temperature by counting the number of clock cycles generated by an oscillator with a low temperature coefficient during a gate time-period determined by a high temperature- coefficient oscillator. The low-temperature-coefficient counter is preset with a base count that corresponds to –55°C. If the counter reaches ‘0’ before the gate period is over, the temperature register, which is preset at –55°C, is incremented by one degree, and the counter is again preset with a starting value determined by the internal slope accumulator circuitry of DS1821. The preset counter value is unique for every temperature increment and compensates for the non-linear behaviour of the oscillators over temperature. At this time, the counter is clocked again until it reaches ‘0.’ If the gate period is not over when the counter reaches ‘0,’ the temperature register is incremented again. This process of presetting the counter, counting down to ‘0,’ and incrementing the temperature register is repeated until the counter takes less time to reach ‘0’ than the duration of the gate period of the high temperature- coefficient oscillator. When this iterative processis complete, the value in the temperature register will indicate the centigrade temperature of the device. Circuit description Fig. 1 shows the circuit of the temperature controller using Dallas DS1821 temperature sensor. Microcontroller AT89S8252 is interfaced to DS1821 temperature sensor, three 7-segment displays and relay RL1. Port P1 of IC1 is used to output the data on the segment display. Port pins P1.0 through P1.3 and port pins P1.4 through P1.7 are connected to IC3 and IC2, respectively. ICs CD4511 (IC3 and IC2) receive the BCD data and provide the compatible code for 7-segment displays DIS2 and DIS3. Port pins P3.4 and P3.5 are used for ‘b,’ ‘c’ and ‘g’ segments of DIS4 through buffers N1, N2 and N3, respectively. Segments ‘b’ and ‘c’ become active when temperature exceeds 99°C. Segment ‘g’ becomes active when temperature goes below 0°C. This indicates ‘–’ sign for negative temperature. DIS1 is used in reverse direction for indication of °C. Segments ‘a,’ ‘b,’ ‘g’ and ‘dp’ (decimal point) are made permanently high with resistors R19 through R22 to indicate °C. Port pins P3.1 through P3.3 of IC1 are connected to S2, S3 and S4 switches for ‘up,’ ‘down’ and ‘display’ respectively. These pins are pulled high through a 10-kilo-ohm resistor. Switches S1 through S3 are used for setting/changing the temperature. When the set temperature is exceeded, the relay connected to port pin 3.7 through a transistor is latched on. Switch S1 is used as a reset switch. Power-‘on’ reset is achieved by capacitorC3 and resistor R4. Port pin P3.0 of IC1 receives the data from temperature sensor DS1821. Pin 17(P3.7) of IC1 is connected to the base of transistor T1 through buffer N4. The signal from port pin P3.7 drives relay RL1. Diode D1 is used as a free-wheeling diode and LED2 is used for relay-‘on’ indication. The device is connected through contacts of RL1. Resistors R5 through R22 and R26 through R28 limit the current through the 7-segment display. A 12MHz crystal is used for microcontroller clock. Fig. 2 shows thecircuit of power supply. The AC mains is stepped down by transformer X1 to deliver a secondary output of 7.5V at 300 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D2 through D5, filtered by capacitor C1 and regulated by IC6. Capacitor C2 bypasses any ripple present in the regulated output. Regulated 5V is used for circuit operation and unregulated 6V is used for the relay |
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