Red lead goes to the IN+ terminal of the multimeter

Most of the inexpensive digital professional-tool available in the market are based on ICL7106 chip, which is a low power A/D converter with a built-in 3 1/2 digit LCD display driver. The maximum voltage that can be applied between its power supply pins, V+ and V-, is +15V. However, the multimeters based on this chip are usually seen to be powered by either a 9V PP3 or a 12V A23 battery. Now if the LM35 sensor uses the same battery to power itself, then it will use the negative terminal of the battery as the reference point (ground) to generate the output voltage, which is proportional to the temperature. But the ICL7106 A/D converter does not measure the input voltage with reference to the negative terminal of the battery.

Instead, it uses a separate reference voltage (known as common terminal, or COM point) which is derived from the supply voltage and is set somewhere between V+ and V-. The best screwdrivers has two leads: red and black. The black lead goes to the COM terminal (which is the reference point) and the red lead goes to the IN+ terminal of the multimeter circuit. The voltage at IN+ is appropriately scaled (based on the selected range) through an on-board circuit before it is fed to the input of the ICL7106 A/D converter, which then measure it with reference to the COM voltage. The easiest way to find out the voltage of the COM terminal is to set the multimeter as voltmeter and connect the IN+ terminal (red lead) to the positive terminal of the battery as shown below.

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