The idea is to interface an analog temperature sensor (TM35), which is directly calibrated to ºC or ºF temperature scale, to the input probes of the meter. The TMP35 output is calibrated to ºC and has a temperature-to-voltage scale factor of 10mV/ºC. This means the TMP35 output will be 255mV if the ambient temperature is 25.5ºC, and so on. We will construct a simple divide-by-ten network to scale the output voltage of the TMP35 down by of factor of 10 so that the voltage measured in mV will be numerically equivalent to the temperature in ºC. But before that it is very important to understand a little bit of the internal ADC structure of the multimeter.
Most of these inexpensive digital multimeters 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. It is important to note that the ICL7106 A/D converter does not use the negative terminal voltage of the battery as reference for A/D conversion. 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-. This is done so that the multimeter could measure negative voltages too. The multimeter 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 to be measured is supplied to the IN+ terminal. The input voltage applied at the IN+ terminal 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. This discussion leads to the following conclusions.
- TMP35 cannot be powered directly from the multimeter battery (9V or 12V) because the maximum operating voltage of the TMP35 is 5.5V.
- The ground of TMP35 should be the same as the reference voltage for A/D conversion otherwise the measurements will be wrong.
Based on the previous discussion, the TMP35 sensor ground pin will be connected to the COM lead (black probe) of the multimeter for correct ADC measurements. Now we need to find a positive supply for the sensor. For ICL7106 based multimeters, the COM lead is set at 3V lower potential than the positive terminal of the battery inside the multimeter. The minimum operating voltage of TMP35 is 2.7V. Therefore, the TMP35 sensor can be powered from the battery's positive terminal (as Vcc) and the COM lead (as ground). The output of the TMP35 is fed to the IN+ terminal (red probe) of the multimeter. Now if you put the multimeter knob to 0-200 mV range, the meter will show the output voltage (mV) from the temperature sensor which indeed is 10 times the real temperature (ºC).
The next step is constructing a divide-by-10 network to scale down the output voltage from TMP35 to 1/10. This can be done by connecting two resistors (1.8K and 200R) in series to construct a voltage divider network. The overall circuit setup of this project is shown in the picture above. An external DPDT slide switch is included in the circuit to turn the thermometer on and off. When the switch is turned on, two things happen:
1. The TMP35 gets power supply.
2. The divided sensor output is connected to the IN+ terminal of the battery.
If you set the multimeter to measure voltage ranging from 0-200 mV, the temperature in °C will be displayed on the LCD screen. When the switch is turned off, the sensor’s power supply is cutoff (which is important because we don’t want the sensor to draw current from the battery all the time) and the multimeter’s IN+ terminal is disconnected from the sensor so that the multimeter can resume its normal operation.
This additional circuit will therefore add a thermometer to your multimeter.
Placing the circuit inside multimeter
The TMP35 sensor and the voltage divider circuit are placed carefully inside the multimeter. Make sure there won't be any short circuit between the sensor and the multimeter circuit board. The pictures above show the placement of the sensor and the DPDT slide switch. The switch should be placed on a side such that it is accessible from outside.
After the sensor and the voltage divider circuit are placed inside and the appropriate connections are made, the meter is ready to display temperature. Turn the multimeter knob to 0-200mV range and move the slide switch to ON position so that the TMP35 sensor gets powered and its output is connected to the IN+ terminal of the multimeter internally. The multimeter will show the measured temperature in ºC.
I hope you enjoyed reading this!