i. Thermometer application with multiple temperature sensors

This application is relatively easy to build. The essence of the solution is an operational amplifier that works as an voltage range adapter between 10-bit A/D converter and thermistor with PTC (positive temperature coefficient). The circuit is composed of three parts. The first is a voltage regulator based on LM317T integrated circuit. A trimmer allows for precise setting for the regulator output voltage (V_dd) which should be 5 V.

The next part of the circuit is an operational amplifier TL061CP. One may also use TL062CP with two operational amplifiers in the same sized chip, more than one temperature sensors are to be connected. The amplifier is controlled by two (or three) trimmers. The first trimmer in the NI+ input is used to set the offset value of the output voltage that is measured by A/D converter at the reference temperature. The next trimmer is used for setting the level of amplification (gain). This determines the precision of the temperature measurement as well as the temperature range of the measuring circuit. If you intend to measure environmental temperature, you may aim to full range temperature interval from -30 °C to 60 °C. This is a span of 100 °C. The span of a 10-bit A/D converter is 1023 values. This gives you precision of 0.1°C per bit.

The third part of the circuit is a standard PIC18F2xJ50. The only difference is an input trimmer that allows for precise adaptation of voltage range of 0 … 5 V to 0 … 3.3 V. The latter is used by PIC18F2xJ50 microcontrollers. Please, see section 5 on more details how to build the microcontroller circuit.

Here are circuits:

It is possible to connect as much as 10 thermometer sensors circuits to one PIC18F2xJ50 microcontroller. The microcontroller on the photo below has just one such circuit connected to AN0 input.