Design

The circuit design is based on the circuit by danyk. I switched the thermometer BC327 for a KTY sensor. This required a couple of changes which I altered in the circuit.

In addition I had to change the values of a couple of resistors. Not shown in the diagram, but I inserted a 10uF electrolytic capacitor parallel to the KTY thermometer between 9V+ and the end of the 1K resistor. This makes the transition of the displayed digits a bit smoother.

The LED version is usually constructed with the circuit ICL7107, which has a higher output current, but this circuit requires symmetrical stabilized power supply. Advantage of ICL7106 is a simple power supply without stabilization. The problem is solved by a low current (1 mA) super bright display.

Calibration

The thermometer must be calibrated. Calibration is performed using the trimmer P1 and P2 in two steps. The first step is to calibrate the zero degree using crushed ice (a mix of ice and water. After stabilizing set the P1 variabel resistor to a a display value of 00.0 °C. Secondly, expose the P2 variable resistor to boiling water at 100 °C. Adjust the setting such that the display reads 100.0 °C.