Central heating thermometer with alarm

Central heating thermometer with alarm is controlled by processor Atmega8. Three digital sensors DS18B20 in housing TO-92 measure the temperature. Measured temperature range is from -55 to +125 degrees Celsius with an accuracy to 0,5 degree Celsius. The thermometer can also measure indoor and outdoor temperature. This thermometer may be useful to prevent glycol in central heating installation with solid fuel boiler from boiling. Boiling water would be quite undesirable since the installation can be done with PP STABI pipes.
The temperature measurement is presented on a LCD display 2*16 with HD44780 driver. The thermometer after exceeding the temperature 65 degrees Celsius enables alarm, next alarms are enabled after exceeding 75 degrees Celsius, 82 and the last one 88. Alarms are indicated by flashing diode with light power 2000mcd and by sound of varying intensity generated by two buzzers. The sound is louder and less friendly to the ear with alarm temperature. The program has set hysteresis in the range of 2-3 degrees Celsius, so during the fluctuations of the temperature the alarm would not turn on unreasonably. The exception is the first alarm, in which the difference is 15 degrees Celsius.
The alarm can be turned off in two ways.

• by holding the temporary button for 50ms and then the alarm will turn on after decreasing the temperature by the value of hysteresis
• it will turn off itself, when the temperature drops below the alarm threshold; then the values of hysteresis do not work and the buzzer activates after each exceeding the temperature

The code was written Bascom-AVR. It took about 44% of memory. It can be written in C as well.
Pictures and scheme:




Link to original thread (useful attachment) – Termometr wraz z alarmem do CO

Dear Vermes
Hi
Thank you for sharing your designs . i have seen some of your circuits ( i read some of your posts ) . and all of them was interesting .
Again Thank you .
Best Regards
Goldsmith