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HIGH RESOLUTION PRECISION THERMOMETER USING NE555
goal this application note present realistic example thermometer using NE555. NE555 operating a-stable mode. frequency controlled resistance changes NTC-thermistor. frequency, well duty cycle, measured timer. NE555 output connected timer input capture pin.
Rev.
AN1755/0304
HIGH RESOLUTION PRECISION THERMOMETER USING NE555
DESCRIPTION NE555
NE555 monolithic timing circuit highly stable controller capable producing accurate time delays oscillation. time delay mode operation, time precisely controlled external resistor capacitor. stable operation oscillator, free running frequency duty cycle both accurately controlled with external resistors capacitor. more details NE555 datasheet.
ASTABLE OPERATION
circuit shown Figure (pin connected). triggers itself operates free running multi vibrator. external capacitor charges through discharges through Thus duty cycle precisely ratio these resistors. astable mode operation, charges discharges between VCC. self-triggered mode, charge discharge times therefore frequency independent supply voltage. Figure Circuit Diagram NE555 a-stable mode
VCC+
Output
NE555
Control Voltage 0.01µF
charge time (output HIGH) given 0.693 0.693 (R2) 0.693 2R2) discharge time (output LOW) Thus total period given
HIGH RESOLUTION PRECISION THERMOMETER USING NE555
Figure NE555 Timing Diagram
output
voltage
THEORY OPERATION NE555
general Rntc place well place assume Rntc place times needed charge discharge capacitor will 0.693 (Rntc 0.693 (R2)
result measurement should dependent value Rntc only. that should vary with temperature age. That capacitor eliminated from equations.The periods measured with timer. From formula calculate variables. Expressing from putting obtain Rntc (t1-t2) result depends precision time measuring (t1, tolerance only. depends neither supply voltage.
THEORY OPERATION
rising falling edges input signal captured micro periods measured with built-in timer. timer resolution (125 8MHz) sufficient capture these edges. Note: calculate equation multiples 125ns simplicity.
HIGH RESOLUTION PRECISION THERMOMETER USING NE555
DESIGN
design circuit, need choose right values These values determine frequency output signal (3). given temperature range resistor, chosen values 100nF Table Table theoretical values designed thermometer
Temperature (°C)
Rntc
27279 26134 25043 24003 23012 22067 21166 20306 19486 18703 17956 17243 16561 15910 15288 14694 14126 13582 13063 12565 12090 11635 11199 10782 10382 10000 9633 9282 8945 8622 8312 8016 7731 7458 7195
(ms)
4,662 4,583 4,507 4,435 4,367 4,301 4,239 4,179 4,122 4,068 4,016 3,967 3,920 3,875 3,831 3,790 3,751 3,713 3,677 3,643 3,610 3,578 3,548 3,519 3,491 3,465 3,440 3,415 3,392 3,370 3,348 3,328 3,308 3,289 3,271
(ms)
2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772 2,772
(tics)
37299 36665 36060 35483 34934 34410 33910 33434 32979 32545 32131 31736 31357 30997 30652 30322 30007 29706 29418 29142 28879 28626 28385 28154 27932 27720 27517 27322 27135 26956 26784 26620 26462 26311 26165
(tics)
22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176 22176
(tics)
HIGH RESOLUTION PRECISION THERMOMETER USING NE555
Temperature (°C)
Rntc
6944 6702 6470 6247 6033 5827
(ms)
3,253 3,236 3,220 3,205 3,190 3,176
(ms)
2,772 2,772 2,772 2,772 2,772 2,772
(tics)
26026 25892 25763 25639 25521 25406
(tics)
22176 22176 22176 22176 22176 22176
(tics)
Table Parameters their meanings
Parameter Rntc Comments Value NTC-Resistor particular temperature Example: Values time corresponding measured temperature, please refer equations (4), (5). Example: (25°C) 0.693 100nF 3.465 (25°C) 0.693 100nF 2.772 Values time 125ns timer tics MHz) Example: (tics) (25°C) 3.465 27720 tics (25°C) 2.772 22176 tics Time period timer Represents achieved resolution. It's difference values degree Celsius t1(n) t1(n+1). Example: (25°C) (25°C) (26°C) 27720 27517 tics
(ms)
(tics)
PRACTICAL ISSUES
possible implement this algorithm with family micro program memory required). general choose:
12-bit autoreload timer allowing configuration fcpu (~125ns), ST7LITE @8MHz 16-bit timer allowing configuration fcpu/2 (~250ns), ST72264 @8MHz
second case able time measuring timer cycle. Averaging measured results recommended needed. frequency quite stable. bipolar SA555, SE555 instead NE555 (the difference operating temperature range only). Recommended values 100nF CMOS TS555 should redesign resistor capacitor values match electrical characteristics.
HIGH RESOLUTION PRECISION THERMOMETER USING NE555
CONCLUSION
With this method able achieve high resolution temperature measurement. main advantages independence from variations capacitor supply voltage. Only precision used resistor affects final result.
HIGH RESOLUTION PRECISION THERMOMETER USING NE555
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