Description of feature devices;
- Measures temperatures from -55 degrees C to +125°C
- Three LED's to indicate in what range the temperature
- User definable thermostat with high and low settings
- Output via a relay to control a heater element or a blower fan (or something else)
- Power supply......................4.5 to 5.5 VDC
- Power consumption ...........15mA
This schematic device uses a Dallas DS1621 temperature sensor which indicates the temperature of the device. The temperature sensor has a thermal alarm output, which becomes high when the temperature of the device exceeds a user defined value. When the temperature drops below a user defined value, the alarm output becomes low. In this way any amount of hysteresis can be programmed. The values are stored in a special register of the device that is nonvolatile. The signal of the alarm output is amplified by a BC557 PNP transistor, that it drives a relay that can switch a heater element or a blower on or off. The temperature settings and readings are communicated to/from the device over a simple 2-wire serial interface. An ATMEL 90S2313 microcontroller controls the serial communication to/from the DS1621.The microcontroller also controls three LED, only one of the LED's is on when the temperature is within a certain range. The range of the temperature in which the LED's are on can be set by the user in the program code. The circuit needs to be powered by a 5V power supply, which can be obtained from a wall-wart.
