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?
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
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.
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.
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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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.
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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,
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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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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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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