This one is using a 16F876 PIC, MCP1047A temperature sensor ( X2 ), MCP1541 voltage reference and MCP6022A opamp.
The display is a 2 row HD74780 based 2X16 char, SII L1652BIJ2 but any other display based on HD74780 can be used. The Hardware is quite simple but some important points must be checked in order to keep the noise to a minimum and do not influence the A/D readings. Each IC is using a 0.1UF capacitor mounted as close as possible to the Vcc/GND pins of the IC. The ground circuit should have the shape of a star with a central point. The Vcc circuit should follow the same pattern where possible. Any kind of loops in the power circuit must be avoided.
The temperature sensor used has an analog output, with 500mV at 0C ( zero degree Celsius or centigrade ) and a sensitivity of 10mV/Centigrade. The output of the sensor is filtered in order to eliminate the noise and applied to the A/D inputs of the integrated 10Bits A/D contained in PIC 16F876. As voltage reference a MCP1541 is used . The typical output voltage is 4.096V – this is buffered with a MCP6022 Rail-to-Rail AO and used to power the temperature sensors. Also a resistive divider is used to obtain 2.56V as reference voltage for the A/D. For the 2.56V reference voltage used and 10Bit A/D the temperature in centigrade is simple to compute as T=Reading-200/4.
Two sensors are used – one for Outside temperature and one for Inside temperature. MCP1047A sensor is a small SMD device so it was a little bit difficult to handle it…the solution was to mount the sensor on a small piece of PCB, connect a shielded wire to it and seal everything in a piece of heat shrink tube and some silicon. Like this I have obtained two water proof sensors. For the outside sensor I hare replaced a piece of the shielded cable with three thin wires (30AWG) in order to make this wires pass thru the window.
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