Vladimir Mitrovic (Croatia) The thermometer shown here is "micro" not only because it is built around the ATtinyl3 microcontroller, but also because it can be built as a miniature device when built from 5MD components. The temperature is measured by a DS18520 high precision 1-Wire@ digital thermometer from Maxim. The program inside the ATti- ny134 microcontroller initiates a single tem- perature conversion, waits until the conver- sion has finished, then reads and displays the result. the temperature can be read by count- ing red and green blinks from a two-colour LED. For example, 2 red and 3 green blinks will be produced if the temperature is 23'C. Blinks are easily readable because each blink lasts approximately 135 ms and is followed by a 400 ms pause. The same LED pair is used to display other events, too: 1. When the temperature is negative (centi- grade value), an R-G-R-G sequence with no intermediate pauses stands for the "-" sign (red and green blinks are clearly visible); 2. 0"C is displayed as a 1 second long sequence ofshort red and green blinks (red and green light blend together); 3. A communication error is displayed as a 1- second long red light. As indicated in the circuit diagram, two differ- ent two-colour (red + green) LED types may be used: 3-terminal (with common cathode) or 2-terminal (with red and green LEDs in anti- parallel connection). The ATtiny program is the same for both versions. Since LEDs con- sume most of the power, choose an appro- priate value for R2 to suit your own needs. A 100 O resistor results in an 8 mA current flow through the LED that's switched on. During the display period the LEDs are on at a 25% duty cycle, with 1 second conversion peri- ods between two display sequences to reduce the average LED consumption to roughly 1.5 mA. This may be considerably lowered if two separate red and green low-current LEDs are used for display. But even with 20-mA LEDs, the circuit may be powered by a small 3 V Lith- ium cellfor a good period. Although in theorythe instrumentcan meas- ure temperatures between -55"C and +125C, in practice it seems prudent to stay within the -1 5oC to +50oC range. The D51820 can be detached from the rest of the circuit foras faras the l-wire protocol allows; a 3-m (10 ft.) connecting cable was tested and everything worked well. lf the sen- sor is properly insulated, you can measure the temperature of water or other non-aggressive liquids. But the most common use of the pro- posed circuit is to build a small and simple thermometer with low power consumption, which will be at hand and functional when- ever you need it. With lP1 closed the readout is in 'modulo 5' mode: each blink of the red LED now equals 5, while the green blinks are still unity. Thus 4 red and 3 green blinks will occur if the tem- perature is 23'C. lf JP2 is closed, the microcontroller goes into power-down mode if the temperature is meas- ured and displayed forthe firsttime. This option consumes minimum power. To repeat the meas- urement, switch off the thermometer, wait for .l to 2 seconds and switch it on again. The program developed for the project is called 'EE_micro_T.bas' and was written in BascomAVR for compiling and turning into object code. A small extract is shown here. The complete program is a free download [t]. Those without access to an ATtinyl34 programmer or BascomAVR may buy their ready-programmed lC through the same web page. Readers preferring Fahrenheit readout should modify the BascomAVR program accordingly. (o9o634) Software
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