Temperature Meter Tear-down and Elastomeric Connector

 Temperature Meter Tear-down and Elastomeric Connector

The cheap temperature meter, the mysterious elastomeric connector, and reincarnation.

This part explores bare-metal working, the other two in later parts

~ Rajeev Mehndiratta

Image: 1 thermometer front case, elastomeric connector and crystal display

What's a digital thermometer?

It has a 150mm long pen-type probe, few buttons, a display, and is powered by a tiny button cell. It is a versatile temperature meter but mostly used in the food industry and your kitchen could have this.

One popular model that pops out in the web search is TP-101. It is available on almost all 

the e-stores for worth INR 150, near $2. But I don't have this exact model but is almost 

similar to TP-101 in specifications and features. This has been labeled 'Eurolabs' waterproof 

digital thermometer. Both are made in china.

Image: 2 TP-101 model thermometer Source: web

Specifications

The user manual narrates it can measure a wide range of temperatures from -50° C to +300° C (-58°F to +572°F.

• Accuracy: ± 1° C

• Sampling Rate: 1 Second

• Battery: 1.5V LR44

• Ambient Temeprature: -10° C to +50° C

Bare-Metal

It has a tiny 40mm x 15mm PCB with COB, chip on board, mounting. Few SMD resistors and few more SMD capacitors. A 2.2uF/50V electrolytic capacitor surprises with its voltage rating of 50V. I understand might be a non-availability of the lower voltage rating  capacitor or no major price difference between the lower and high voltage at the time of  assembly, but both reasons are difficult to digest.

It runs on a 1.5V LR44 button cell. No transistor or other active component can be seen on either side of the board. Understandably everything is accommodated inside the COB.

Two micro tactile switches for on/off and changing temperature mode.

It has eleven copper pads to connect with the LCD through the elastomeric connector.

A battery placeholder and sensors connections complete the PCB.

 See image 3 and 4 front and backside of the PCB.

Image: 4 Back side of the PCB

Sensor

Honestly, I ransacked the whole internet in search of the type of sensor used in the device 

to sense the temperature. But I could not get any answer to my queries. Digging more deeper

 I found one post that itself was in dilemma whether it could be a thermocouple or thermistor. 

So, I decided to sacrifice one sensor probe! :)

It has one Glass Type NTC, negative temperature coefficient, thermistor at the tip and 

connecting wires are covered with a transparent insulated rubber tube. See Image 5.

  Image: 5 NTC Thermistor

NTC

An NTC stands for Negative Temperature Coefficient. As they have a negative

 temperature coefficient the resistance decreases with the rise in temperature and increases with the fall in temperature.

See the below equation relationship between resistance and temperature:

ΔR = - kΔT

where,

ΔR, change in the resistance

ΔT, change in the resistance

k, temperature coefficient

Kindly note that the temperature coefficient k for NTC is negative because resistance decrease as temperature increases.

Image 6 showing resistance of the thermistor at near 29° C and image 7 shows 

resistance at 37° C.

Image 6 showing resistance of the thermistor at near 29° C and image 7 shows 

resistance at 37° C.

Working

As I mentioned earlier there are no active components are mounted on the board so COB 

is the big boss. As COBs don't have numbers and datasheets like ICs the working can only 

be speculated based on the experience, so don't claim it to be absolutely correct. 

At the same time, you can trust it is not absolutely incorrect! No contradictions!

Change in resistance is converted into corresponding voltage with a voltage divider 

or Wheatstone bridge. This analog voltage is converted into digital format with 

ADC and displayed on the LCD.

The button cell provides the power of 1.5V to run the circuitry. Please see the block

diagram.

Goodbye!

Rajeev Mehndiratta

15th September 2021

15:23 HRS