On a sunny day (27 Feb 2020 15:12:58 +0000 (GMT)) it happened Theo
 wrote in
:

>Jim Jackson  wrote:
>> The setup you describe provides a variable analog voltage output. Non of
>> the raspberry pi have the capability to directly read an analog voltage,
>> you need an add-on Analog-to-Digital-Converter (ADC) either as a "hat",
>> breakout board, or chip in a breadboard.
>
>You don't need an ADC, you can do it via timing a capacitor.
>
>Wire up something like this:
>
>GPIO0 -=thermistor=-----+---||----- GND
>            R           |    C
>                      GPIO1
>
>Set GPIO0 low, wait a while.
>Now set GPIO0 high and start timing
>When GPIO1 goes from 0 to 1, stop the clock
>
>If you know the time, capacitance C, the GPIO output high voltage, and the
>low-to-high threshold voltage for GPIO inputs, you can solve the capacitor
>charging equation to find R.
>
>Putting the midpoint into a comparator with a more precise threshold would
>help with accuracy over using GPIO1 directly.  Using its reference voltage
>as GPIO0 divided exactly in half via a potential divider would make the
>system independent of I/O voltage variations.
>
>Theo

Cool!

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