Temperature Meter Powered by COM Port

The project aims to create a simple four channel temperature meter that can be connected directly to the COM port of the computer without the additional power supply.

The temperature meter is built around an ATtiny15L microcontroller where a 10-bit ADC is built-in. It takes the power of the circuit from the COM port ER and RS signal lines. Up to 5mA of current can be supplied by the high level lines which have a rating of 6V to 12V. The L series microcontrollers are low power and are not enough for this application.

The core of the AVR is clocked by only internal RC oscillator and any other clock source cannot be used while the ADC has a bandgap reference and differential amplifier as its front end. Available for timer/counter is the 25.6MHz clock source that 16x multiplied from core clock. This makes the ATtiny 15L provide good analog I/O capability.

There are four very small 103AT precision thermistors used as temperature sensors which could give pretty good precision at room temperature. An Excel file is used for logging the temperature data while using the firmware for Attiny15L and VBA project.

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