mjvandermeulen
Posts: 2
Joined: Tue Jun 14, 2016 5:25 am

Temperature controlled slowcooker: What to Buy?

Tue Jun 14, 2016 5:49 am

I'm new to the RPi and I would love to make a temperature controlled slow cooker (Crock Pot).

I think I need:

- RPi (e.g.: https://www.amazon.com/CanaKit-Raspberr ... y+pi&psc=1)
- a thermocouple.
- a way to turn my slow cooker on and off (basically turning a power strip on and off programmatically).

What should I buy to get started with my project?

All help is greatly appreciated.
A very excited RPi newbie Maarten.
Loveland, CO, USA.

drgeoff
Posts: 9627
Joined: Wed Jan 25, 2012 6:39 pm

Re: Temperature controlled slowcooker: What to Buy?

Tue Jun 14, 2016 1:16 pm

You need more than a thermocouple. Those produce only a few millivolts and will need a dc amplifier followed by an A to D converter if you want the RPi to know the temperature.

You need a relay of some kind to switch the power. It also acts as an isolation between the RPi and potentially lethal (to the RPi and human users) AC mains that is powering the heating element.

However from a safety point of view, I don't condone beginners doing anything involving AC mains.

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penguintutor
Posts: 376
Joined: Tue May 08, 2012 9:11 am
Location: UK
Contact: Website

Re: Temperature controlled slowcooker: What to Buy?

Tue Jun 14, 2016 1:17 pm

There's a write-up on a Sous-Vide project at: https://www.raspberrypi.org/blog/sous-v ... -language/

I believe Rachel (from the Foundation) also created a Sous-vide cooker using Energenie remote control sockets.

Essentially you need

Input
In addition to the thermocouple then you need some way of taking the analog signal from the themocouple to the digital input of the Raspberry Pi. You could look at an analog to digital converter such as MCP3008 which is supported by the GPIO Zero.

processing
The Raspberry Pi along with some appropriate code. I suggest using the GPIO Zero library if possible https://www.raspberrypi.org/blog/gpio-z ... computing/

output
Some way of safely switching the power supply on and off. Energenie is good in the UK, but for the US you may want to look at some kind of remote control based plug. You could hack the remote control to connect a reed relay across the buttons (such as the one I did here), or perhaps one that uses infrared which can be controlled with an infrared transmitter circuit from the GPIO pwm port. The Wolfram example mentioned previously uses a relay board connected to the Raspberry Pi, but you need to know what you are doing with that as it involves connecting through the mains electrical supply [so best avoided].

mjvandermeulen
Posts: 2
Joined: Tue Jun 14, 2016 5:25 am

Re: Temperature controlled slowcooker: What to Buy?

Sat Jul 02, 2016 5:45 pm

Thank you for your input.

I'm tempted to go this way:

For input:
digital temperature sensor: https://www.adafruit.com/products/642.

For processing:
I've been using the library RPi.GPIO, according to Eben Upton's book (R Pi user guide), so I 'm not familiar with GPIO zero library. I noticed that no resistor is used with the GPIO zero example you linked to. I'm confused: Doesn't that create a short?

For AC control:
I'm going to play it safe with this: https://www.adafruit.com/products/268

Let me know your thoughts on my approach. (Basically following the adafruit lessons on temperature sensing and AC control, using their digital sensor and power tail ac switch)

My main concern/ question: This is pretty prefab out of the box. I like that for AC control, to keep my family safe, but for temperature sensing I'm wondering how hard it is to use an (analog) thermocouple with the MCP3008. How do I buy a food grade thermocouple?

Thank you so much!

Maarten.

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