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Oh no, they all died

A project log for konijntje-thermo-humi

Gadget that shows room temperature & humidity on a LED-bar

Christoph TackChristoph Tack 09/03/2019 at 12:550 Comments

About 14 months now after creation, of all the boards I come across, none of them is still working.  A few weeks ago they started to behave strangely.  They immediately turned off on showing the humidity value.

What's going on?

First signs of life

The units appear to work well using a 3V lab power supply instead of the CR2032 cell.  This confirms that the hardware is still working correctly.  Only the power seems to be insufficient in some way.

Design mistakes

Standard CR2032 coin cells use Lithium Manganese Dioxide.  That's where the C in CR2032 stands for.  These batteries are designed for a maximum load current for about 1-3mA.  Drawing more current (e.g. to drive the single LED) will reveal the limitations of this battery chemistry.  They typically have an internal resistance of about 400ohms.

The current consumption should be reduced drastically.  Another option would be to change the battery chemistry to LiIon.  The LIR2450 would be a good option.  120mA current draws are allowed.  The battery is also rechargeable.  Be aware that the battery voltage is 4.2V down to 3.3V.  An LDO might be needed.  Digikey has plenty of coin cell holders available.

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