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.
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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.
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.