It is a small thermometer with a 2×16 characters display that shows the current temperature and extreme values, i.e. minimum and maximum temperature. It’s used an AT90S2313 microcontroller connected through a 2-wire-bus a small eeprom to save the minimum and maximum values and to the temperature sensor, a DS1621.

 

 

 

The PCB is designed to fit behind the display so that it is possible to build up a really small device. This circuit should not be understood as a ready and working device, but as a platform to experiment, because there are some difficulties. Because of the placement of the temperature sensor on the pcb its measurement values are off by one or two degrees when the voltage regulator gets warm. Additionally the circuit - particularly the display - needs too much current so that it cannot be operated with small batteries over more than a few days. Additionally there are connectors for programming the controller, for connecting more I2C-Devices, a button, the supply voltage and to connect the circuit to the serial interface of a pc. The circuit can operate at 6V - 18V from a power supply or batteries, because a 5V voltage regulator is used. The display has a built-in background light, but I disabled it because it needs too much current that warms the temperature sensor. But it is possible to connect an external sensor to the bus to overcome this problem. There are two jumpers in the circuit. One is connected to reset of the controller and should be open. It can be shortened to reset the controller if it hangs (this should not happen...). The other jumper should be opened to program the controller in-circuit. It disconnects the display, the temperature sensor and the eeprom from VCC. After programming it must be shortened to let the thermometer work.

 

There are two versions of the firmware of the microcontroller. Written in Assembler for avra Assembler  and a version in C for avr-gcc - exactly for WinAVR - with the same functions.

Download source code