BYGGESÆT: DIGITAL TERMOMETER
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Et førsteklasses digitaltermometer med flydende krystal udlæsning. Måleområde -25gr. til +150gr.C. Nøjagtighed +/- 1 grad. det lave strømforbrug på ca. 1,5 mA gør det muligt at forsyne termometret fra et 9V batteri. Måler luft- og væsketemperatur. Velegnet i hjemmet, såvel som til laboratoriebrug. How it Works In order to understand the operation of the digital thermometer it is necessary to know a few basic things about signal converters. The converter used in this project is an Analogue to Digital Converter (ADC) and is also known as a double slope integrator. It has the following important features: Very high inherent accuracy. No need for high accuracy external components. Indifference to external noise. No sample and hold circuit required. The IC as it is can be used as a voltmeter, as an ammeter or as any other instrument for measuring analogue quantities provided they can be converted to voltages. In this case as we are interested in measuring temperatures we take advantage of the changes of the internal resistance in a common transistor with temperature. The transistor is connected as a diode, (Base and Collector are shorted together) and the reason for choosing a transistor over a diode, is that transistors are much more reliable for this type of use and also have a more linear response over a wider temperature range. The resistor R7 together with the capacitor C5 set the time base for the internal clock and with the values given here the clock «ticks» at about 48 KHz which means that the circuit takes about three different readings per second. The capacitor C4 provides a reference voltage and its value should not be changed because it will affect the accuracy of the instrument. This capacitor is kept charged at a constant voltage which is used as reference point by the comparator. The reference voltages are set by the values of R4 and R5 and the settings of the trimmers P1 and P2, which also control the minimum and maximum temperatures that the thermometer can measure accurately. The signal from the sensing element (Q1) is applied directly to the IN LOW pin of the IC (pin 30). The capacitor C2 is performing the function of self resetting the circuit before a new measurement can be taken. The resistor R3 together with C1 form the integration circuit of the input voltage and at the same time prevent any division of the input voltage making the circuit faster and more reliable and also reduce the possibility of measuring errors. The resistor R6 takes the out of range signal from the IC to the base of the transistor, so that when the measurement is out of the instrument’s range on the display will appear the number 1 and the third dot, which means that the temperature measured is beyond the instrument’s preset limits. From the pins 2 - 20 and 22 - 25 of the IC is driven directly a liquid crystal, 3 1/2 digit, display of the type DO51030 - RO40 or equivalent. Finally the transistor connected between the pin 37 of the IC and the pin 36 of the display lights the dot between the two last digits on the screen, giving this way readings having an accuracy of 0.1 C. The circuit operates from a miniature 9 V battery, which because of the very low consumption of the instrument will last for a quite long time.
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