Friday, September 29, 2023

Wireless Digital Thermometer For Multiple Sensors

By Arup Kumar Sen

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Construction and testing

An actual-size, single-side PCB pattern for the transmitter unit of the wireless addressable digital thermometer is shown in Fig. 4 and its component layout in Fig. 5. Similarly, an actual-size, single-side PCB pattern for the receiver unit is shown in Fig. 6 and its component layout in Fig. 7.

Fig. 4: An actual-size PCB pattern of the transmitter unit
Fig. 5: Component layout of the PCB shown in Fig. 4
Fig. 6: An actual-size PCB pattern of the receiver unit
Fig. 7: Component layout of the PCB shown in Fig. 6

Download PCB and component layout PDFs: click here

For connection of TWS1, use a four-wire cable to connect it to the PCB. Fix TWS1 on the front panel of the receiver unit.

Mounting of the sensor IC requires some special attention. If temperature of a solid surface is to be monitored, the sensor may be fixed to the surface by using metal clamps, or glued directly to the surface with high-temperature epoxy adhesive.

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For liquid temperature measurement, the sensor cannot be immersed directly in the liquid, as the liquid may be a conductive type, and in that case, the sensor’s leads would be electrically shorted. To solve this problem, the sensor may be mounted inside a sealed-end metal/glass tube. Connecting wires must go to the leads through high-temperature insulating sleeving. The tube may then be dipped into the liquid, or screwed through some threaded hole in the container of the liquid. Although steel gives very high ruggedness to construction, in general, a glass tube would be an ideal choice when temperature of a chemically reactive bath is to be measured.

During construction, special care must be given regarding the choice of some resistors and capacitors. Resistors R1, R2, R24, 25 and 26 must be of highly-stable and low-temperature co-efficient type, to make the units stable against time and ambient temperature variations. Metal-film type may be used for this purpose. VR1 is a 10-kilo-ohm, 40-turn trim potmeter. This should be of highly stable type.

Ground return leads of the sensors must be grounded close to ADC ground lead, otherwise erroneous results may be observed.

Calibration and adjustment

For proper operation of this wireless thermometer, reference current (to pin 4 of DAC0808 – IC22) of the receiver unit should be pre-adjusted. To do this, follow the steps below:

Connect a known voltage source (not exceeding +5V) to any input of the ADC, say, at IN6 (pin 4) of the ADC. Switch on the transmitter unit. Connect a DMM across R26 of the receiver unit. Set the range switch to DC 200mV range, positive lead to ground and negative lead to top of R26. Switch on the receiver unit. LEDs at decoder outputs should start glowing to indicate the received voltage data. If source voltage is 1.5V, status of LEDs should be as listed in Table I.

So, received voltage = (D× 5)/256 = (76×5)/256 = 1.50

where D is the weight of the binary numbers represented by LED7 through LED14.
Now, adjust trim potmeter VR1 to get 150.00mV on the dial of the multimeter. Connect another voltage source at the input and see that the multimeter shows it correctly. If required, re-adjust the trim potmeter. After proper calibration, enclose the circuit in two separate boxes with suitable connections of input and LED indicators.

How to use

1. Attach LM35 sensors (IC9 through IC16) of the transmitter unit to different subjects of interest, noting inputs (IN0-IN7) of the ADC to which these are connected individually.
2. Attach a digital multimeter to the output of the receiver unit with positive lead to ground and negative lead to R26. Set the multimeter’s dial to 200mV range.
3. Switch on both transmitter and receiver units.
4. Rotate the rotary thumbwheel switch to get the temperature of a particular subject. For example, if you like to get temperature of the subject attached to the sensor connected to IN6 input of the ADC, the thumbwheel switch ought to be rotated to position 6.
5. A temperature of 27.5°C would be displayed as 27.5mV (when value of R26 is around 500-ohm). If R26 is 5k, displayed value would be 0.275V when DMM range switch is set to 2V.


Although the system can be used best to measure temperatures in hazardous or inaccessible areas (like a radioactive zone), the same can also be used by a hospital doctor to monitor, from a fixed location, the body temperatures of multiple patients lying in different rooms without visiting each patient in person.

A hotel control room can monitor temperatures of all the rooms at the same time by using multiple units. The unit can also be used (with certain modifications) as a wireless digital voltmeter.

Arup Kumar Sen is a retired technical officer – II, S.A.I.F., Bose Institute, Kolkata


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