Redboard & 3.3volt snsors

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LloydF
Posts: 3
Joined: Thu Nov 22, 2018 11:03 am

Redboard & 3.3volt snsors

Post by LloydF » Thu Nov 22, 2018 11:19 am

Greetings,

I have Sparkfun's REDBOARD
K TYPE THERMOCOUPLE
" CONNECTOR
" BREAKOUT board

My question is this, can I use my REDBOARD which has 3.3volt power out and in the description says
"0-5V outputs with 3.3V compatible inputs" with this sensor and other 3.3volt products?
To a complete newbie this is a daunting question.

Ive called Sparkfuns tech support and have not gotten a specific "Yes" or "No".

Any help greatfully appreciated..
LLOYD

paulvha
Posts: 297
Joined: Sat Nov 11, 2017 2:39 am

Re: Redboard & 3.3volt snsors

Post by paulvha » Fri Nov 23, 2018 10:16 am

I am struggling to understand the question and thus to provide help.

Normally a K TYPE Thermocouple provides output in the order of mV and need to have a separate amplifier (like the https://www.sparkfun.com/products/13266).

The Redboard has a ATMEGA328P processor. The analog reading is in 10 bits. So value is measured in steps of 5 V / 1024 ~ 4.8mv. Not knowing with K-type Thermocouple you have, I looked at https://www.thermocoupleinfo.com/type-k ... couple.htm.
You can see that 4.8mV will be reached around 118 *C. (very HOT).

So Sparkfun's answer is correct, Yes you can connect, but first of all only if you work with HIGH temperatures you will get a reading AND also extremely rough ( you would measure the next point at 230 *C). As such I would say : NO. Get an extra amplifier, the for-mentioned board or build one yourself ( e.g. with https://www.sparkfun.com/products/306)

NOT knowing what you want to measure, but if it is between -55 to 125°C (-67°F to +257°F) temperature, I would advise you get DS18B20 ( like : https://www.sparkfun.com/products/11050)
regards,
Paul

LloydF
Posts: 3
Joined: Thu Nov 22, 2018 11:03 am

Re: Redboard & 3.3volt snsors

Post by LloydF » Fri Nov 23, 2018 2:52 pm

Paul,
The temp I wish to read 225 degrees F with a range 20 F above or below.

The thermocouple I have is the SEN-13715 Stainless steel thermocouple from Sparkfun

I have exactly the amplifier you listed, the 31855k by Sparkfun.

To quote the Redboard hookup guide on the Sparkfun site " There are six analog inputs on the Analog header. These pins all have a digital to analog convertor which can be used to read voltage between 0 - 5volts. These are usefull if you wish to read an analog sensor."

If it says it can read analog voltage between 0-5volts I am not understanding why it wouldnt read the amplified signal from the 31855k

The Redboard has a 3.3 volt power supply

LLOYD

lyndon
Support Volunteer
Posts: 1632
Joined: Thu Jul 17, 2008 6:37 pm
Location: Minnesota, USA

Re: Redboard & 3.3volt snsors

Post by lyndon » Fri Nov 23, 2018 4:27 pm

Have you actually tried it?

paulvha
Posts: 297
Joined: Sat Nov 11, 2017 2:39 am

Re: Redboard & 3.3volt snsors

Post by paulvha » Sat Nov 24, 2018 1:44 am

Why does the Redboard not has a 5V power supply? The processor will need that.

You can power the 31855K also with 5V. The output of the board is NOT analog, but a SPI interface. Hence it should be connected to pin 10 to 13 (connecting the right points to each other).

The hookup guide provides direction, although I would not use A0 and A1 for GND and 5V (that is applicable to the board they used) but just connect the 31855K to the GND and 5V pin on the redboard. ( https://learn.sparkfun.com/tutorials/ma ... 1506862864)

regards,
Paul

LloydF
Posts: 3
Joined: Thu Nov 22, 2018 11:03 am

Re: Redboard & 3.3volt sensors Follow Up

Post by LloydF » Sun Nov 25, 2018 5:42 pm

Paul,
The Redboard works fine with a Thermocouple and the Max 31855 breakout board. I would assume ( but only assume) it will work with other 3.3 volt sensors.

When I downloaded the library for the 31855k there is EXAMPLE code written as well. I used this with no changes whatsoever. It worked perfectly. I dont understand two thing though.
1) The code makes a note saying it was written for a Pro Nano. I used a Redboard. On the SPARKFUN hookup guide there is a Code To Note saying to configure the pins / edit that line . I didnt and it still worked.

2) the code says const uint8_t VCC=14; and a GND=15;

I looked up the Pro Mini pin layout and nothing labels a pin 14 or a pin 15. The board has a physical lables of GND and VCC printed on it.
There are 2-13 numbers and an A0 through A3.


SO.... how does the code come up with a 14 and 15?

Any answer or place to look this up on my own would be a help,

LLOYD

paulvha
Posts: 297
Joined: Sat Nov 11, 2017 2:39 am

Re: Redboard & 3.3volt snsors

Post by paulvha » Mon Nov 26, 2018 2:05 am

it is indeed the same..The pin 14 and 15 have the same function as on the pro nano (just keep counting the pins, they did not include a pin-number)

pin 14 on the schematics for the Redboard is GND (not VCC).
Pin 15 is AREF (reference voltage for the analog input). Internally in the 328P processor that is connected to AVCC, which in turn is connected to the +5V.

So I am surprised as well that it would work. The driver accepts any pin for GND or VCC (it just set the pins as output and pulls GND-pin low and VCC pin high). However in the case of pin 14 and 15, these are hardwired and can not be changed with software.

Did you connect VCC to pin 14 and GND to 15 or directly to +5v and GND ? if you did connect to pin 14 and 15 I wonder about the results you got and happens if you connect VCC to pin 15 and GND to pin 14?

Personally I would not have used AReF output for powering the 31855K

Regards,
Paul

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