Thermometer





How does it work?
This thermometer measure temperature from -8 °C to +30 °C exact to a 2 °C. In this device you can distinguish three blocks: dispaly(US3 & US4), supply(US1) & sensor (US2 & US5). As a themperature sensor I use LM35. It's very good for this application becouse it've got 1°C precision, it means that voltage on it output increase over 10mV always when theperature will get higher 1°C. In this device were used special aplication which cause increasing voltage over 100mV/1°C. It makes easier to scale display.
Potentiometer P1 is responsible for sensivity of LM35. US5 shift output voltaga about 1,2V. Without it and R1 would be imposible coretct measuring themperature under 0°C becouse under 0°C output of LM35 must be connected to the minus of supply, which in our case is GND.
As a display were used two LM3914 conected cascade to eachother. They drive 10 led every. They work in dot mode, it means that always only one diode is being lighted.
LM3914 it's a simple voltagemeter. It "shows" voltage between low reference voltage from pin 4 (Rlo) ang high reference voltage pin 6 (Rhi). LM3914 have got self reference sourse and smoothly leds current regulation.
In this device, for regulation high reference voltage are responsible elements R6 R4 and P3. Value of this voltage can be counted from this formula

Value of low reference voltage can be counted with this formula:

Supply UREFHI and UREFLO that are adequately upper and lower limit of measuring voltage. Thermometer has own power supply adaptor which gives stable 8V. This voltage is used to supply sensor and display.

Semiconductors: