Posted on Feb 4, 2014

This circuit is composed of a temperature sensor, amplifier, V/F converter, three digit binary coded decimal (BCD) counter, time base and seven segment led displays. In addition to the 9400 V/F converter, other ICs need for this project include the LM334 temperature sensor, LF353 dual op amp, NE555 timers, 74LS00 NAND gate, MC 14553 three digit

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BCD counter, MC 14543 BCD to 7 segment decoder/driver/latch, and three seven segment LED displays with three PNP switching Transistors. The output of temperature sensor changes linearly as a function of temperature. The output is connected to a summing amplifier, which is used to calibrate the output of the temperature sensor for a desired temperature type (K, C or F) and an intended range. That is, to display the temperature in K, C or F, you have to adjust the 10K POT according to suit the voltage appears at the output of the summing amplifier. Since the output of the temperature sensor is directly proportional to the temperature changes. The output of the Inverting amplifier is the input of Voltage to Frequency converter, therefore the output frequency of the converter is directly proportional to the output voltage of the inverting amplifier. The output frequency of the converter is then ANDed with the gating signal to produce the clock signal for the tree digit BCD counter. The BCD output of the counter drives the three bit LED display sequentially via the BCD to 7 segment decoder and the temperature is displayed on the LED. The temperature sensor LM334 is a three terminal adjustable current source whose current can be programmed from 1 A to 10mA with one external Resistor. The three terminals are labelled +V, R and -V. The pin out is shown above.

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