Homebrew pit controller

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Re: Homebrew pit controller

Postby Chris__M » 20 Jun 2012, 10:59

nwaring wrote:The Linkmeter did start off using a WIFI shield but there was a good reason (can't recall what it was) as to why it was dumped in preference for the Linksys router... the next development will integrate with the PI, but that's still only about improving network connectivity.


I am guessing, based on my own decisions, but I would say (a) cost - the WiFi shields are (or at least were, when I looked) expensive (b) lack of compatibility. When I looked into WiFi shields, it seemed I had a choice of 3, which each required different code to drive them. Thus my original plan to design something I could share was immediately made difficult, by limiting the choice for others to the WiFi shield I happened to use.

On the other hand, I believe the LinkSys devices are readily available and as cheap as chips.
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Re: Homebrew pit controller

Postby aris » 10 Jul 2012, 07:23

I've had a Raspberri Pi for a couple of months now. Nice little unit, but if running wifi on it is not straightforward. Finding a USB wifi dongle with compatible ARM linux is simple enough, but the USB ports do not supply enough power to drive them - so you need a powered USB port. Also all the compatible dongles i've seen so far are quite large - i'm hoping to find a compatible one which is small, then work a plan around the power issue. One workaround that has been mentioned is to use a USB Y adaptor plugging into both USB ports, thus getting 200mw of power instead of just 100mw. It might also be possible to piggy back off of the RasPI's power supply directly too (with some additional hardware).

As for temperature probes. Yes, the RasPI doesn't have any analogue inputs - but it does have I2C inputs - and there are I2C temperature sensors readily available, and it is dead easy to interface them. Check here:

http://www.janwagemakers.be/wordpress/?p=214

Same chap also gives instruction on how to read temperature over USB - though a bit more involved (you need to program a PIC):

http://www.janwagemakers.be/wordpress/?p=182

Personally, i'd prefer to use a RasPI over Arduino - purely because of the versatility. I could run an entire web server on the RASPI , run cron jobs, do the whole thing in a simple scripting language instead of C, and have multi-gigabytes of storage too!

The Arduino does appeal too though - for a standalone device, analogue inputs, and a great user community.

I've not played with Arduino yet - but i'm very tempted to try one. I just fear it will become yet another time sink when I have so many other things to do :-(
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Re: Homebrew pit controller

Postby Chris__M » 12 Jul 2012, 09:33

Interesting about the Raspberry Pi l2c input - useful information.

If I was starting again now, I would have certainly considered the Pi, and I can see its attraction.

One note regarding the arduino. Because I went wired network instead of WiFi (due the the proximity of a LAN connection in my garage), the Ethernet shield I bought also contains a micro-SD card socket. So currently my arduino does have 2 Gb of storage available. web server routines for the Arduino are also freely available, although you are not talking about anything sophisticated.
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Re: Homebrew pit controller

Postby aris » 12 Jul 2012, 12:28

I've just ordered an Arduino. If anything, it just looks like so much fun :)

I'm going to have a play around with it - it has loads of both analogue and digital inputs. I'm wondering if I can't put two temperature sensors for the smoker itself, an external temperature sensor - then some additional sensors for the food.

All I need are some heavy-duty probes like the kind used on the digital remote meat thermometers. Perhaps I can adapt the probe on my Maverick meat thermometer.

Will take some thought an experimentation - but if we have the temperature at the bottom of the smoker (assuming a vertical type), and at the top - and the outside temperature, and in the inside of the meat, you should be able to make some good decisions on when to turn on a blower, and perhaps even predict when the food will be ready.

All pie in the sky at the moment of course - but it is certainly possible.
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Re: Homebrew pit controller

Postby aris » 12 Jul 2012, 13:14

I forgot to mention - getting relays compatible with the arduino is easy and cheap too. They should only use the digital ports. One to drive a blower - another to drive an alarm of some sort maybe?
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Re: Homebrew pit controller

Postby Chris__M » 12 Jul 2012, 15:19

Currrently I have connections for two probes to my Arduino - a pit temp and a food temp. But I worked out that I could have a pit temp and up to 3 food temp probes, without too many hassles.

I am using standard Maverick probes, and recommend them. They work well, and they have a published specification (which I can find sometime, if you want, but you can probably Google for). The probe is connected (a) to ground (b) to an analogue port, balanced to the Arduino VCC line by a resistor.

As you say, you can control a blower easily. My case is a bit more complicated, in that - with a pellet grill - I am having to control a 240V auger motor. But I am doing that with a part bought off the shelf, that connects straight to one of the arduino output ports.
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Re: Homebrew pit controller

Postby aris » 12 Jul 2012, 18:28

Please post the maverick probe info - these probes are readily available on eBay which is a bonus.
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Re: Homebrew pit controller

Postby Chris__M » 12 Jul 2012, 20:59

I have got it somewhere (or a link), but can't find it offhand.

However, what I have got are the values to plug into the Steinhart-Hart equation, which some kind soul before me derived from the raw data:

A = 2.3067434E-4;
B = 2.3696596E-4;
C = 1.2636414E-7;

How to use this? Rather than bog you down with the code for my own device (which (a) is specific to a pellet grill, and (b) makes no attempt to control temps automatically - it reports temps and then lets me remotely tweak the rate of pellet feed), the following was my very first test routine to read the probes. It shows how to deal with the info, which you can then adapt as you need.

Disclosure: most of the following code is not my own, I borrow and adapt without shame.

/*
AnalogReadSerial
Reads an analog input on pins 0, 1 & 2, prints the result to the serial monitor

This example code is in the public domain.
*/

void setup() {
Serial.begin(9600);
}

void loop() {
delay(1000);
int sensorValue1 = analogRead(A0);
Serial.println("Probe 1:");
Serial.println(thermister_temp(sensorValue1), DEC);
int sensorValue2 = analogRead(A1);
Serial.println("Probe 2:");
Serial.println(thermister_temp(sensorValue2), DEC);
int sensorValue3 = analogRead(A2);
Serial.println("Probe 3:");
Serial.println(thermister_temp(sensorValue3), DEC);
}

int thermister_temp(int aval) {
double R, T, Rknown;

// This routine uses the following values calculated from the thermister data sheet

// A = 2.3067434E-4;
// B = 2.3696596E-4;
// C = 1.2636414E-7;

// This is the value of the resistor used in the voltage divider. Set it to whatever you use
// Many people are using 22K Ohm with success, I had 10K in my kit, and it doesn't seem to harm anything.

Rknown = 10000;

// Do the log once so as not to do it 4 times in the equation

R = log((1 / ((1024 / (double) aval) - 1)) * (double) Rknown);

// Compute degrees C
T = (1 / ((2.3067434E-4) + (2.3696596E-4) * R + (1.2636414E-7) * R * R * R)) - 273.25;

// un-comment next line to return degrees F
return ((int) ((T * 9.0) / 5.0 + 32.0));

// un-comment next line to return degrees C
// return (T);
}

This is the webpage that I borrowed the above code from. It is also for an arduino controlliing a blower, so there may be more code there you can use.

http://hruska.us/tempmon/
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Re: Homebrew pit controller

Postby aris » 12 Jul 2012, 21:36

Thanks!

What temperature sensors are you using? All the ones I've seen max out at 125c and do not seem to have heat resistant leads.
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Re: Homebrew pit controller

Postby Chris__M » 12 Jul 2012, 23:14

I am using standard Maverick ET73 probes, which is what those values are for. These have metal braided leads and - according to Maverick, are good to up to at least 410 F - "Do not use the probe sensors and wires above 410°F (210°C) because doing so will deteriorate the wire."

As it happens, my Traeger has frequently gone above that (but not by much, 450-470 F), and I don't bother to remove the pit probe. It has been in place for over a year without problems. Even so, they are cheap to replace - just over 8 quid each from http://www.smokingbbq.co.uk/spare-parts-21-c.asp
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