Mydin-UseCase-Basic-Thermostat

Basic Thermostat

This project implements a basic temperature control system from a 3 modules MyDin made of:

• DINCASE-MB3PICO : controler based on Raspberry-Pi Pico
• DINCASE-2R-BP3MOD : 2 relays backplane
• TMP-DS18B20-WP : a DS18B20 OneWire temperature sensor.

The temperature sensor takes the room temperature and the heater is controled by the relay 1.

• LED 1 reflect the Rel 1 state (so heating state).
• LED 2 is lit while a FORCED forced is currently active
• LED 3 will blink once to acknowledge T° increase.
• LED 4 will blink once to acknowledge T° decrease.
• BUTTON 1 reverse (force) the current state of the relay.
  When the relay is ON then it switch it off.
  When the relay is OFF then is switch it on.
• BUTTON 2 cancel the forced state.
• BUTTON 3 will decrease the setpoint T° of 1 degree.
• BUTTON 4 will increase the setpoint T° of 1 degree.

Current temperature, setpoint temperature and relay state is displayed in the REPL session.

Hysteresis

A thermostat allows to define the target temperature called the setpoint (eg: 25°C) and use an hysteresis range (EG: 2.5°C) to avoids continuous state change around the target temperature.

With the hysteresis, the heater starts heating from 25-2.5 = 22.5°C and will stop heating when the temperature raised above 25+2.5 = 27.5°C. The temperature will slowly decrease and the heater will start again when temperature falls under 22.5°C.

Forced Mode

From the previous example here upper

Lets say the heater already reached 27.5°C and the heather stopped.

The temperature is now 25°C and you are already frozen... unfortunately the heater will not restart before the temperature falls under 22.5°C.

Pressing the Forced button will restart the heating process and the forced state will automatically disabling when the temperature is reached.

Remarks:
Forced state can also be used during a normal heating process to stop it. In such case, the forced state will get disabled when the lower temperature is reached.

The code

Here is the code of thermostat2.py available in the repository.

 1 from mydin import configure
 2 from mydin.pico import Pico3Mod
 3 from mydin.backplane.relays import TwoRelay3Mod 
 4 from hyst import HeaterTh
 5 import time, sys, asyncio
 6 
 7 # Which Controler + Backplane to use
 8 din = configure( Pico3Mod, TwoRelay3Mod )
 9 thermostat = HeaterTh( 25, 2 ) # Thermostat
10 forced = None # Forced state (High=True,Low=False,None=None)
11 last_print = 1
12 
13 # === OneWire bus & DS18B20 ===================
14 # When running DinControler asynchronously, a task 
15 # is created to capture the DS18B20 temperature! 
16 # The AsyncIO task is scheduled every  60 seconds 
17 # (see pico.py; DS18B20_UPDATE_TIME)
18 
19 async def loop( din ):
20   """ called again and again (like Arduino) """
21   global thermostat, forced, last_print
22   if din.temp_ds18b20==None:
23     await asyncio.sleep( 2 )
24     return
25 
26   # User Force state change ?
27   if din.was_pressed( 0 ):
28     forced = not(din.rel1.value())
29 
30 
31   # Ask to reset forced state ?
32   if din.was_pressed( 1 ):
33     forced = None
34 
35   # Increate / decrease setpoint
36   if din.was_pressed( 2 ):
37     thermostat.setpoint -= 1
38     print( 'setpoint @ %i °C' % thermostat.setpoint )
39     din.led2.on()
40     await asyncio.sleep_ms( 300 )
41     din.led2.off()
42   if din.was_pressed( 3 ):
43     thermostat.setpoint += 1
44     print( 'setpoint @ %i °C' % thermostat.setpoint )
45     din.led3.on()
46     await asyncio.sleep_ms( 300 )
47     din.led3.off()
48 
49   # IF forced THEN apply forced state
50   if forced != None:
51     heat = forced
52     # Should we auto-reset the forced state
53     if (forced == thermostat.update( din.temp_ds18b20) ):
54       forced=None # Reset forced state
55 
56   # IF not forced THEN follow thermostat
57   if forced==None: 
58     heat = thermostat.update( din.temp_ds18b20 )
59   
60   din.led1.value( forced!=None )
61   din.led0.value( heat )
62   din.rel1.value( heat )
63 
64   # print status once every 2 seconds
65   if time.ticks_diff( time.ticks_ms(), last_print )>2000:
66     print( "[%s] %s °C - %s - %s" % (thermostat.setpoint, din.temp_ds18b20, "Heat" if heat else "...",  "(forced)" if forced!=None else "") )
67     last_print = time.ticks_ms()
68 
69   await asyncio.sleep_ms( 100 )
70 
71 
72 din.setup( setup=None, loop=loop )
73 din.run()

Improve it

In house, thermostat are associated with a scheduler used to define one or more day-time hours range for heating (the remaining being the night-time).

Some advanced thermostat can defines a day temperature and night temperature.

In such case, the forced mode could be used to switch between day-time and night-time (with the corresponding temperature).