rPi 3 w/thermistor

Hi, I am trying to use a 2 lead thermistror (not a temp/humidity sensor) with
RPi 3B+. Is it possible to wire this without a breadboad?
I cannot find any instructibles or any rpi python code examples anywhere. What
I did so far is connect the ground to one lead, vcc to the other lead using a Y
union and put an inline 1-K ohm rewsitor to a third lead as signal [GPIO 23).
Is there any code magic that can read the digital signal and convert it to
tempature?
Thanks in advance
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Reply to
Gregg Somes
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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.
Googling
raspberry pi using a thermistor to measure temperature
gave a few hits such as ...
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Happy reading.
Jim
Reply to
Jim Jackson
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
Reply to
Theo
IIRC Thermistors can be quite non-linear. Check the datasheet for the thermistor you are using. Over your specific temperature range it may be linear enough.
Reply to
Richard Owlett
Low to high threshold is probably more temperature dependent than the thermistor!
A better bet is to construct an RC oscillator with the thermistor as part of the R and measure frequency, but this is not a simple thing - you need a fair few components
The best bet is to buy this:
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I2C interface ..
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Reply to
The Natural Philosopher
Pi's don't have any analog to digital convertors. The simplest solution is most likely to be a 1-Wire temperature sensor:
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Plenty of code out there to work with them (and many of the other 1-Wire devices).
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Dave.
Reply to
Dave Liquorice
One wire? BULL! I quote the referenced pdf "... one data line (and ground)". And as a PRACTICAL point you need another wire to power the device. That makes it a *THREE* wire device.
Reply to
Richard Owlett
Yebbut there is only one DATA wire. :-)
You are probably not old enough/in te wrong country to have listened to the exploits of Larry the Lamb and Dennis the Dachsund in Toytown, on the radio.
There was a character called IIRC the Inventor who complained 'the trouble with this wireless is that it involves such a lot of WIRE!!!"
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Reply to
The Natural Philosopher
On a sunny day (Thu, 27 Feb 2020 09:08:45 -0400) it happened "Gregg Somes" wrote in :
First question is what do you want to measure if it is temperature below / above some point MAYBE, but thermistors are highly non-linear, so you want to either measure voltage across your resistor and then calculate the resistance / and then whatever that represents
AFAIK raspi has no analog input, so you need what is called an analog to digital converter (board?).
The GPIO as _digital_ input sees a 'logic zero' below some voltage and a 'logic one' above some voltage. Th exact voltage can vary, also depends on temperature and production spread, but is somewhere between 0 and 3.3 V
So UNLESS you want o measure a dead cold versus a red hot thermistor is not of much use.
An external ADC (analog to digital converter) also will need a stable external reference voltage I have done all sort of things with temperature sensors using Microchip PICs as ADC.. connected to serial port... but that needs some electronics and programming knowledge,
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Of course there are other ways, for the fun .. I wrote a program that converts a digital clock display read by a camera to time as text, you could use it to read a multimeter :-) Anyways to interface and ADC to GPIO takes some electronics knowledge and some programming knowledge, preferably C,
But maybe there exists a board 'HAT?'for that?
Reply to
Jan Panteltje
On a sunny day (27 Feb 2020 15:12:58 +0000 (GMT)) it happened Theo wrote in :
Cool!
Reply to
Jan Panteltje
Of course there dies, Or rather an 12c interface board which is handier since you can mount it where you want to measure temps..
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Reply to
The Natural Philosopher
ty
eadboad?
ROFL
o
I was in college by time it ended. But on the wrong side of the Pond.
!!!"
ansmitted
Reply to
Richard Owlett
They|some can be 'parasitically' powered from the data line's pull up ...
so one wire ... and a ground that is everywhere - ish :-)
Avpx
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Reply to
Nick Norman
Yeah. BUT the spec sheet shows it as a three terminal device.
Reply to
Richard Owlett
- as in DS18B20, parasitic temperature chip, most easily used as a 3-pin TO-92 package...
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From RISCOS 5.27 on a BeagleBoard-xM and Raspberry Pi2B 
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Reply to
Mark J
On Fri, 28 Feb 2020 11:39:05 GMT, Jan Panteltje declaimed the following:
Presuming common CMOS thresholds of 30 and 70%: 2.31V is HIGH. What the circuit does between those thresholds is indeterminate (I'd hope it holds the last valid state until the far threshold is crossed).
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Reply to
Dennis Lee Bieber
# A fairly presumptuous presumption.
2.31V
No, it is perfectly possible to have the output between '0' and '1'. At hear all digital circuits are in fact analogue, just as all analogue circuits at the quantum level are in fact digital :-)
It does not.
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Reply to
The Natural Philosopher
that is exactly what does NOT happen. the logic sate will flip at some in-determinant point between the two thresholds. The thresholds are simply points at which the state is guaranteed.
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Reply to
Alister
This is mostly true, but not when the device specifies a schmitt trigger input, when indeed the output is held until the second threshold is reached.
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W J G
Reply to
Folderol
far
And even when it has flipped it can flip back again if the signal remains between the thresholds. Hence the requirement for pull up/down resistors.
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Cheers 
Dave.
Reply to
Dave Liquorice

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