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Portable Thermometer Schematic Circuit Diagram

Smart Temperature Monitoring: A Portable Solution

Checking the temperature using a thermometer before embarking on outdoor activities is a prudent step. Equally vital is monitoring temperature while on-site. While this task is effortless with local TV or the Internet, it becomes daunting in remote locations like the bush or countryside. This circuit offers a simple yet effective solution. It’s user-friendly and consumes minimal power, ensuring it operates throughout the battery’s shelf life. The circuit utilizes an LM35DZ sensor (IC3) and an LM358 (IC2A) to buffer the analog output voltage. The microcontroller reads this voltage, converting it into a BCD value displayed on multiplexed 7-segment displays. To conserve battery power, the display turns off after approximately 30 seconds unless button S1 is pressed. Pressing the button again reactivates the display, showing the current temperature. In the prototype, two 0.56” (14.2 mm) common-cathode (CC) green displays were employed, capable of indicating temperatures ranging from 0 to 100 ºC.

Portable Thermometer Schematic Circuit Diagram

Calibrating the Meter for Accuracy

Before its initial use, the meter requires calibration against a known temperature reading. Using preset P1, one can adjust the temperature value by approximately 4 ºC. By pressing the button and manipulating the preset, the displayed temperature can be aligned with the correct value. The chosen microcontroller, PIC16F684, offers inherent functionalities and, notably, an internal oscillator, eliminating the need for an external crystal and allowing for additional I/O pins. The two 7-segment displays operate in a multiplexed manner, alternating between on and off states through BC547 transistors.

To prevent ghosting, each display is blanked before showcasing the temperature value. Temperature sampling occurs every 30 seconds to maintain stability against temperature fluctuations. A low-dropout regulator, LP2950, maintains a steady 5 V supply voltage, ensuring optimal battery utilization. Alternatively, the thermometer can run on three AA dry batteries in series without an intermediary regulator. The PIC software is available for free download from the Elektor website, accessible through archive file number 080418-11.zip. The software development employed CCS C for its creation.

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