Digital thermometer with LED display (from the web)
Now we bring you two versions of a homemade digital thermometer with ICL7106 that I recently built. One version uses a LED display, the second version LCD display. In both versions, is used silicon transistor as a temperature sensor. The temperature is determined by the voltage drop, temperature dependence is approximately -2.2 mV / °C. Power can be either a 9V battery or a suitable power supply. Oscillator with R1 and C1 determines the sampling frequency – using 100k and 100p the frequency is 3 Hz. Theoretical temperature range is from -199.9 to 199.9 °C, the real temperature range is limited by the measuring transistor to approximately -65.0 To 150.0 °C. Resolution to 0.1 °C. If you want to use it as a room thermometer, it is not necessary to use the hundreds digit, (left) or minus sign. Also digit right from the decimal point may be omitted if 1 °C resolution is sufficient. A simple room thermometer thus requires only two-digit display, as on photo below.
I switched the thermometer BC327 for a KTY sensor. This required a couple of changes which I altered in this clickable circuit diagram. circuit digitale thermometer aangepast KTY C sensor. 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 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 7106 is a simple power supply without stabilization. The problem is solved by a low curren (1 mA) superbright display. 7106 is also easier to salvage from something (like digital multimetr) than 7107. I will not discuss the 7107 here. Info on both those integrated circuits can be found in their datasheet.
The thermometer must be calibrated. Calibration is performed using the trimmer P1 and P2 in two steps. The first step is to set the zero using crushed ice (the ice and water mix). Set P2 approximately in the center. Probe (Transistor) Place in a waterproof container and dip into crushed ice (ice pieces in water). After stabilizing the set P1 and to display 00.0 °C. In the second step P2 is set according to a known temperature, preferably in boiling water at 100 °C. Alternatively the P2 setting can be made according to another thermometer at room temperature. For accurate setting you can use multiturn trimmers.