Thermostat project questions

[Jake 24]

Well I am starting to work on my thermostat

 

I have two separate units, a heat and cool unit

 

So there will be one output to run the heat, and one for the AC along with a fan signal

 

I have been going through some of the temperature sensor demos, and low power thermostat projects trying to get some ideas on how to attack this.

 

I thought about using the internal temperature sensor since I think it will be close enough for this project. 

 

I was thinking about having a LCD for the actual, target temperature, mode, and calling for heat or cool.

 

Eventually I want to set this up with bluetooth remote, but for now getting it to go will be the first step.

 

Have any of yall done a project like this?  Any tips?

 

I was thinking about sampling the temperature every 30 seconds,

 

If the system is in the off mode to go into low power state

 

I need to find the formula to send a F temperature value to the LCD

 

 

Thanks again!

 

 

[igor 163]

Haven't done one myself (though I have one on my todo list).

 

Few thoughts - be careful to fail safe.  (i.e. however your control system fails, make sure the overall system winds up in a safe state.)

 

Be careful to not short-cycle the air conditioner (may also be an issue with some types of heating element).

(Constantly turning some devices on and off may considerably shorten their lifespan.  So once turn it on/off, be sure it stays that way

for a reasonable period.)

 

If going to use internal temperature sensor, check how it affects measurements when go into/come out of low power state.

(How much power the processor is using will affect the temperature.)

 

I was thinking about sampling the temperature every 30 seconds,

 

 

How fast does the temperature change?

[Jake 24]

The current heater thermostat has about a 2 degree delta, all the cool down and firing controls are all in the unit which I'm leaving alone. The current thermostat only gives a signal to call for heat.

 

On the AC side it is about the same setup where thermostat has about a 2 degree delta.

 

I agree with you on the short cycling that can destroy components, I'm planning on timing the cycles to ensure that does not happen.

 

Temperature swings are not that big in the trailer, especially where the thermostat is going to be mounted.

 

Dealing with processor power affecting temperature has been my one concern, I am going to try to get the code rolling soon and use the UART to monitor the temp as it's coming in and out. I thought about using thermal paste or tape and having the chip against the aluminum body of the thermostat to help even out the temp. Looks like with the weather this week I may have some time to work on it.

[zeke 693]

@Jake: I've done similar projects using a couple different implementations. I've used:

1. The internal temperature sensor of the MSP430,
2. An external DS18B20 temperature sensor and
3. An external DS1620 thermostat chip.

[Jake 24]

I would like to try it out with the internal temperature sensor first, dependent on the accuracy.  I can live with a couple degree difference.  (home thermostats your lucky to get that accuracy)

 

Zeke, 

 

How did your projects come out?  would you run another one with the internal sensor? 

[zeke 693]

Probably not. The DS18B20 is my go to temp sensor now. It's just too easy for me to include it on a design now that I've developed code libraries for it.

 

I'd recommend taking a peek at that sensor. We have discussed it here quite a bit so there's lots of help available here when you need it.

 

As far as my projects go, I've completed several designs for clients who are now cranking out hundreds of units every month.

[Fred 453]

I've found internal temperature sensors to be a waste of time. They may accurately measure the internal temperature of the microcontroller but this varies too much compared to the external temperature. If your project is primarily about the temperature there's no point starting from somewhere broken.

 

By all means try it an see, but you'll probably find it's way out.

[Jake 24]

That's what I needed to hear! For $5 and one more component to deal with is no big deal. I'll get some ordered up today. Looks like with this weekend weather I should have some shop time!

[Fred 453]

@@Jake Of course while you're coding you can compare the internal and external temperatures to see how they differ. Try it with the MSP430 mostly idle and with it running at a high clock speed in a __delay_cycles loop. I suspect you'll see a significant (few degrees) difference.

[Jake 24]

@@Jake Of course while you're coding you can compare the internal and external temperatures to see how they differ. Try it with the MSP430 mostly idle and with it running at a high clock speed in a __delay_cycles loop. I suspect you'll see a significant (few degrees) difference.

 

 

I think until I get the external sensors in I am just going to use the internal sensor.  I am going to order a couple of the digital sensors Zeke had mentioned along with a couple TI analog sensors. 

 

Trying to get things going correctly has been challenging me

 

My though is the initial part of the program would be

 

 

Upon mode button press it will wake up the tstat

 

pushing and holding will toggle modes               Cool,heat,off

 

If it is in cool or hot it then goes to its respective program.

 

If you go to off it will go back to sleep after a minute.

 

Display mode on LCD (I may just do mode output to LED indicator for now)

 

Monitor temp sensor once a minute

 

 

 

 

Set target temp

 

one input would raise target temp

 

other input would lower target temp

 

display target temp on lcd  (lcd implemtation maybe later)

 

 

 

Cool mode

 

Select fan speed by push button and hold to toggle fan speed (3 speed)  Fan will be running as long as it is in cool mode

 

if temperature above setpoint by 1 degrees run AC compressor, shut off 1 degree below setpoint

 

 

 

Heat mode

 

If temperature below setpoint by 1 degree run heat until 1 degree above setpoint

 

Fan in heat unit runs along with heat unit ( I think my unit takes care of fan run before and after heat on and not the tstat)  I need to verify that but were on the road a bunch the next 5 weeks so I can't tear into it yet.....

 

 

I have been scavenging code off the initial MSP43 program and the low temp tstat implementation code but I do not think I am making any headway. 

 

Eventually I want to be able to run the temp to bluetooth to an android device so I dont have to climb down to adjust the tstat.....(lazy yes I know).....

[JonnyBoats 40]

This project does not use a MSP430, but it may give you some ideas: http://hackaday.com/2015/02/27/controlling-central-heating-via-wi-fi/

 

In particular you may be able to re-purpose ideas and code from the Open Thermostat Scheduler.

[Jake 24]

Probably not. The DS18B20 is my go to temp sensor now. It's just too easy for me to include it on a design now that I've developed code libraries for it.

 

I'd recommend taking a peek at that sensor. We have discussed it here quite a bit so there's lots of help available here when you need it.

 

As far as my projects go, I've completed several designs for clients who are now cranking out hundreds of units every month.

Zeke,

 

Do you have any code examples for the DS18B20 ? I'm finally getting some time to get after this again. I'm trying to use the ultra low power advanced code example and modify it to work with my requirements. It's an analog thermistor in the example. The digital sensor I think will be much simpler.

[zeke 693]

@@Jake,

 

Check out this thread to see how it's done on an F5529:

http://forum.43oh.com/topic/5908-console-with-one-wire-temperature-on-msp430f5529-lp/

 

Check out this thread to see how it's done on a G2452:

http://forum.43oh.com/topic/329-reading-the-ds18b20-temperature-sensor/

 

Let me know how you make out.

[Jake 24]

Thanks Zeke!