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Portable Thermometer

Joseph A. Zamnit

 

It’s usually is a good idea to check the temperature before setting off for an outdoor activity. Equally important is a temperature check while at the actual place. The former is easy to do using the local TV or the Internet but once you are in the bush or the countryside such a task becomes more difficult. The small circuit described here solves the problem. It is very easy to use and consumes so little current that it will work for the battery’s shelf life. The circuit uses a standard LM35DZ sensor (IC3) whose analogue output voltage is buffered by an LM358 (IC2A). The voltage

is read by the microcontroller and converted into a BCD value so it can appear on the multiplexed 7-segment displays. Thedisplay will switch off after approximately 30 s unless button S1 is pressed. In this way battery power is conserved.

Pressing the button again will show the temperature. In the prototype two 0.56” (14.2 mm)common-cathode (CC) green displays were used to show the current value of the temperature. The meter can show temperatures between 0 and 100 ºC. The first time it is used the meter has to be calibrated against a known reading. Preset P1 can be varied to change the value of the temperature by about 4 ºC. Press the button and then turn the preset until the correct value of the temperature is shown. The microcontroller used is a PIC16F684. This has been chosen because it has a number of inbuilt functions and most importantly an internal oscillator which obviates the need for an external crystal, freeing up pins for I/O activities. Two 7-segment displays are connected in a multiplexed fashion. The displays are alternately switched on and off by the BC547 transistors. Each display is blanked before displaying the value to prevent ghosting. A temperature sample is read every 30 s to prevent the value displayed from changing due to fluctuations in the temperature. An LP2950 is used to regulate the supply volt- age to 5 V. This is a low dropout regulator which can work down to 6 V hence juicing the battery for the last drop of energy. The thermometer may also be run from three AA dry batteries in series with no series regulator. The PIC software can be downloaded free of charge from the Elektor website. The archive file number is 080418-11.zip. The software was developed using CCS C.