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Electronic Thermometer Project by LM35 and LM3914

Electronic Thermometer Project by LM35 and LM3914

As shown in the schematic, temperature sensor of our electronic thermometer is LM35DZ. There are some kinds of LM35 IC, since it is cheap and easy to find we used LM35DZ in this project. It measures from 0°C to 100°C with a very linear output graph.
2 numbered (middle) pin of the sensor is connected to the 5 numbered pins of LM3914 ICs. So every IC determines how many leds of bargraph will bright due to the analog signal received from the sensor. 2.2 microfarad tantal capacitors are connected between the 2 and 3 numbered pins of LM3914. Resistors in the circuit have %1 tolerance values

For proper operation, voltage values of the nodes shown in the circuit must be adjusted to 90mV, 190mV, 290mV and 390mV by using multiturn trimpots. This adjustment determines the range of the shown values for each led bar display. For instance first display shows from 10°C to 19°C, second display shows from 20°C to 29°C and so on

To test the circuit, you can use a mercury thermometer as shown in the image. We can see our thermometer gives right results

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