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Converting A Digital LCD Thermometer into A Single Range Current/Volt Meter

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esecallum

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Hello,

I am new here and i wonder if you could please help me with this project.

i would like to turn a very cheap digital LCD thermometer such as the one below into a very simple current measuring meter.

**broken link removed**

Now i know you can buy for about £5.99 simple digital panel meters which can measure voltage and using a simple resistor and V=IR ,and hence measure current.

However these are too large and use a lot of current with a short battery life as i am making a medical device which which sends micro currents through wrist artery electrodes and as the device is going to be wrist mounted everything should be small as possible and also the battery should last as long as possible.

I want to convert this LCD thermometer so that it can measure/monitor currents in the range of zero to 2 milliamp maximum as this will be the range required.

the actual currents used in the medical device will be around 100 to 200 micro amps range.

So I purchased this LCD digital thermometer as per specifications on the above link.

it uses 2 LR44 button 1.5 volt cells and uses only 10 microamps current consumption making it ideal !

also when i took one of the LR 44 button cells out it continued to work measure temperature!

the LR button cell gives 1.5 volts.

i also cut of the temp sensor and found it is a negative coefficient thermistor using a digital multimeter.i then reconnected it and took measurements as below for this thermistor.

THE MEASUREMENTS ARE AS BELOW.

TEMPERATURE READING IN DEGREES CELSIUS VERSUS THERMISTOR RESISTANCE IN KILO OHMS.


-47.8 394 K
-33.4 149 K
-25.2 99.2 K
-20.6 76.6 K
-11.8 50.1 K
+10.4 18.5 K
+23.0 11.8 K
+30.5 8.1 K
+52.1 3.88K

Now as you can see with temperature increasing the thermistor resistance decreases.

hence the LCD digital thermometer device is measuring resistance of the thermistor.

the question how can i turn convert this device to either measure voltage and or current?

Has anyone done this before?


i took the thermometer device apart and it uses a glued on microchip with no markings.

i did try to measure how much measure current was flowing into the thermistor but my meter could not detect any even though on the 200 microamp range and it has a resolution down to 0.1 micro amps.

i get the impression that it might be using some kind of H bridge configuration or null current flow to measure the resistance.

in any case do you know how to convert what seems to be a null current flow device measuring resistance into a current or voltage meter as indicated above.

Any help would be greatly appreciated.
 
It would be more useful to substitute a pot for the thermistor, and then using an accurate DMM, measure the voltage across the pot corresponding to different "temperatures". You cant change what the display reads, but you can figure out what voltage would make it read any given number on the display...
 
It would be more useful to substitute a pot for the thermistor, and then using an accurate DMM, measure the voltage across the pot corresponding to different "temperatures". You cant change what the display reads, but you can figure out what voltage would make it read any given number on the display...

tried that but no luck and the reason is that the voltage is applied to the sensor for about 0.5 seconds every 10 seconds and hence the very low power consumption indicated above.

i think it must be a bridge circuit.

do you know how these panel meters measure temperature? i mean the bridge method?
 
...do you know how these panel meters measure temperature? i mean the bridge method?

The underlying ADC is likely one of these. That is why it appears to "sample" the thermistor at a low duty cycle. Chances are that the thermistor is in a bridge, but the excitation for the bridge is applied only while it being read out.
 
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