Combination of adafruit projects... will they work?
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Combination of adafruit projects... will they work?

by unclebob on Wed Dec 12, 2012 11:08 am

Good Morning/Afternoon,

I am looking to use a Pi to replace my home heating thermostat/programmer, using a 8 or so LM35's and the same number of outputs (possibly as relays connected to standard zone valve or solenoids to work with 22mm copper piping).

In the future this will possibly be expanded to also monitor the temperature of a solar water heater, and control mixing of water with a domestic hot water tank though this shouldn't be to much different to the room stat I am looking to do now. I.e if temperature of HWT is less then set amount and water of solar heater is greater then that of HWT, let the solar water circulate, if its not, then fire up boiler...

Looking through the tutorials, there are a number of projects that i think will cater for the hardware side of my requirements...

they are:

LCD+buttons http://learn.adafruit.com/adafruit-16x2-character-lcd-plus-keypad-for-raspberry-pi
IO Expander (prob 16) http://learn.adafruit.com/mcp230xx-gpio-expander-on-the-raspberry-pi
Real time clock http://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi
ADC using LM35 instead of the POT http://learn.adafruit.com/reading-a-analog-in-and-controlling-audio-volume-with-the-raspberry-pi/connecting-the-cobbler-to-a-mcp3008

Use of the LCD will be to display temperature of various sensors, current time, and the buttons to adjust the current selected "room" and adjust the predefined temperature.
IO expander to give me sufficient I/O connections for the 16 required for the heating system (8 in and 8 out)
RTC to keep time in case of power outage etc and not being able to connect to internet to get the current time.
The ADC to convert the voltage from the LM35 to a digital value...

Question are:

a)Will all of these work together from i2c side?
b)would it be best to use a number of ADC's connected to the IO Expander for the inputs from the LM35? ( ie PI -> expander -> ADC -> LM35)
c)Any suggestions for method of controlling the relay or solenoid? Couldn't find any tutorials for this sort of thing...
d)Any other hints or tips to bring the above together?

Basic setup will be as follows:

R-Pi, in its own box (hidden away near the boiler)
connected to:

the LCD on its own in a box in a convenient location (eg mounted in hallway)

8 or so LM35's in their own enclosures in various rooms

Overall the system will have a by-pass to allow a minimum temperature to be maintained should the R-Pi setup fail.
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Re: Combination of adafruit projects... will they work?

by adafruit_support_bill on Wed Dec 12, 2012 11:18 am

Analog sensors like the LM35 are susceptible to noise with long cable runs. You might want to consider something like the DS18B20 instead: https://www.adafruit.com/products/374.
All those components should work fine on the I2C bus.
For your relay/solenoid control, this circuit will work: http://playground.arduino.cc/uploads/Main/relays.pdf
You probably will need the TIP120 for a heating system relay. And don't forget the kickback protection diode.
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Re: Combination of adafruit projects... will they work?

by unclebob on Wed Dec 12, 2012 11:55 am

adafruit_support wrote:Analog sensors like the LM35 are susceptible to noise with long cable runs. You might want to consider something like the DS18B20 instead: https://www.adafruit.com/products/374.
All those components should work fine on the I2C bus.
For your relay/solenoid control, this circuit will work: http://playground.arduino.cc/uploads/Main/relays.pdf
You probably will need the TIP120 for a heating system relay. And don't forget the kickback protection diode.



Thanks for the reply adafruit_support!

Any ideas when a tutorial for the DS18B20 may be written? or is it some time in the future but not determined?
I assume then i can do away with the ADC? or is that still needed?
Given that individual rooms are to be monitored, would it be best to put one DS18B20 per GPIO rather then use on one wire run?
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Re: Combination of adafruit projects... will they work?

by adafruit_support_bill on Wed Dec 12, 2012 12:02 pm

Several DS18B20s can share a pin. You wouldn't need the ADC.
We don't have a tutorial yet, but some folks over at RaspberryPi.org have got it working (search for ds18b20 on this page) http://www.raspberrypi.org/phpBB3/viewtopic.php?t=6649
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Re: Combination of adafruit projects... will they work?

by unclebob on Wed Dec 12, 2012 12:57 pm

Thanks for the support!

Also do you know if there is a distance limit for the mcp230xx expander boards? do they need to be close to the pi or can they be say 5m/10m away without any problems?
I ask because people have had some problems with the DS18B20s over longer distances and was thinking maybe i can bring the temp sensor run shorter by bringing the mcp230xx closer, so for example have one in the middle of the house on the ceiling of ground floor, and the second in the ceiling of the first floor, with the DS18B20s then running from these positions.

For anyone else interested, According to the thread mentioned, 1 wire is now supported by the current version of raspbian
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Re: Combination of adafruit projects... will they work?

by adafruit_support_bill on Wed Dec 12, 2012 2:05 pm

I2C is designed for short distances, but it people have gotten it to run up to a hundred meters. Cable capacitance is the main limiting factor, so you want to keep that low. You can also slow-down the clock speed of the bus. For your application, the slower speed should not be a problem.

I suspect that the 1-wire interface could be extended in the same way, but I don't know of any examples.
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Re: Combination of adafruit projects... will they work?

by unclebob on Thu Dec 13, 2012 6:53 am

Thanks for the info,

I'll have to have a play with various lengths between each device once I've ordered and assembled.

Do you know if you have a UK supplier who sells all of the interested kits?
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Re: Combination of adafruit projects... will they work?

by adafruit_support_bill on Thu Dec 13, 2012 8:06 am

In the UK we have Oomlaut, SK Pang, Phenoptix & Proto-Pic. I don't know what they would have in stock.

There are others in Europe: http://www.adafruit.com/distributors/
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