Using a Thermistor, RF Link, and Arduino to Report Temperature

The initial program simply reported back to the serial monitor about 300 feet away in our house at 1 second intervals. A serial RF link was attached to the arduino via the TX and RX connections. (The program has to be loaded before the RF link is connected, otherwise you can't download the program to the Arduino.) The APC220 RF link was used. In it's simplest form this is a 440 MHZ link (documented on net). No attempt was made to change the out of the box defaults.Similar to this version at DX.COM. The Beta parameter equation is used here to calculate temperature in Fahrenheit and Centigrade. General reference To do this, the series resistance equation was used to calculate the resistance of the thermistor. R1/(R1+R2)*Vin=Vout
This can be rewritten as R1/(R1+18000)*1023=count, where count is the output from the A/D converter, Divide both sides by 1023 where k=count/1023 then with R2=18000, This means R1=k*18000/(1- k). And yes dear Horatio, you can argue that there for that 0-5V 10 bit A to D converter that there are 1024 levels but the span is only 1023 not 1024, i.e. 5V is 1023 not 1024 as most authors assume. 
	
//July 8, 2015
//Redo of an earlier TI MSP430 Launchpad program, this VB program uses A0 input on an Arduino to monitor a thermistor. 
//in simple series with an 18k resistor. Raw data represents output from 10 bit A/D (0 to 1023). 
//Thermistor on Ebay: 100pcs 10k OHM Thermistor Resistor NTC-MF52-103J/3435 10K ±5% 3435±1% , $5.00
//Thermistor cost $0.05...
//========First Arduino program ===========
int a=0;
void setup()
{Serial.begin(9600);
}
void loop(){
a=analogRead(0);
Serial.print("T");
Serial.println(a);
delay(1000);
}

 


			'MSP430 and Arduino VB-6 serial communications program
'---------------------------------------------
Dim w, temp
Private Sub Command1_Click()
'Turn on or off using push button
 If MSComm1.PortOpen = False Then
   Command1.Caption = "Stop"
   MSComm1.PortOpen = True
'launch receive subroutine
   Call probie
 Else
   Command1.Caption = "Go"
   MSComm1.PortOpen = False
   Text1.Text = "---OFF---"
 End If
End Sub
'---------------------------------------------
Private Sub Form_Activate()
'Initialize the serial port....
t = 0
Picture1.Scale (0, 50)-(100, 0): Picture1.ForeColor = vbBlue
For s = 0 To 100 Step 20: Picture1.Line (s, 0)-(s, 50): Next
For s = 0 To 50 Step 10: Picture1.Line (0, s)-(100, s): Next
: Picture1.ForeColor = vbWhite
While Val(cp) = 0
cp = InputBox("Comm Port#", "Com#", "5")
Wend
   MSComm1.RThreshold = 1
' When Inputting Data, Input 1 Byte at a time
   MSComm1.InputLen = 1
' 2400 Baud, No Parity, 8 Data Bits, 1 Stop Bit (set to same baud rate as Java Gui)
   MSComm1.Settings = "9600,N,8,1"
' Disable DTR
   MSComm1.DTREnable = False
' Open COM3 ===== Check Com port by opening devices in control panel/Yours may differ
  MSComm1.CommPort = Val(cp)
  MSComm1.PortOpen = True
' =================================
'launch receive subroutine
Call probie
End Sub
'---------------------------------------------
Sub probie()
'Recieve subroutine - Wait for transmitted packets from shop  - (no error check for packet drop)
While MSComm1.PortOpen = True
' If comEvReceive Event then get data and display
    If MSComm1.CommEvent = comEvReceive Then
      Data = MSComm1.Input 'grab byte if port active
      If Data = "T" Then Text1.Text = "": woof = 1
      'Display Temperature
      If Data <> "" Then Text1.Text = Text1 + Data   'Display Temperature
' Find w, which is the thermistor resistance, by using the series resistance formula
      If Len(Text1.Text) = 4 And woof = 1 Then 
      r = Val(Right$(Text1.Text, 3)): k = r / 1023: w = k * 18000 / (1 - k):  burko
      Picture1.PSet (t, temp - 273): t = t + 1: If t > 99 Then t = 0
      woof = 0
      End If
     End If
     DoEvents 'Don't be too busy to check push button :-)
Wend
End Sub
'---------------------------------------------
'Private Sub Form_Unload(Cancel As Integer)
'close the form
End
End Sub
'---------------------------------------------
Sub burko() 
'This subroutine scales the thermistor curve to give F and C data.  Beta=3435, For a different thermistor, 
' use a different Beta value and center resistance value, 10k ohms at 298 Kelvin
'Uses the Beta parameter equation (see Wikipedia) to linearize the thermistor output and calculate the temperature
ntemp = 1 / 298 + (1 / 3435) * Log(w / 10000)
temp = 1 / ntemp: C = temp - 273 :'find and convert Kelvin to centigrade
 Text2.Text = Format(C, "###.#"): F = (9 / 5) * C + 32
 Text3.Text = Format(F, "###.#") :'find and convert Centigrade to Fahrenheit

End Sub


But what if I want the house to poll the greenhouse, outdoors, and shop for three different values? Then do this:

		//Arduino program for remote polling of 3 thermistors
const int switchPin=3;
const int ledPin=13;
int a=0;
int switchState=0;
int sv=0;
//=====setup==========
void setup(){
pinMode(ledPin,OUTPUT);
pinMode(switchPin,INPUT);
Serial.begin(9600);
switchState=digitalRead(switchPin);}
//=====Program Loop=============
void loop(){
  sv=Serial.read();  //Tell me which thermistor data to send
//  digitalWrite(ledPin,HIGH);delay(100);
//  digitalWrite(ledPin,LOW);delay(100); 
  switch(sv){
   
case'0':   
    Serial.print("T");
      a=analogRead(0);
      Serial.println(a);//put 10 ms delay here
       digitalWrite(ledPin,HIGH);delay(300);
       digitalWrite(ledPin,LOW);delay(300);
    break;
    
case '1':
    Serial.print("U");
     a=analogRead(1);
      Serial.println(a);//put 10 ms delay here
      digitalWrite(ledPin,HIGH);delay(300);
       digitalWrite(ledPin,LOW);delay(300);
       digitalWrite(ledPin,HIGH);delay(300);
       digitalWrite(ledPin,LOW);delay(300);
    break; 
  
case '2':
    Serial.print("V");
     a=analogRead(2);
      Serial.println(a);//put 10 ms delay here
       digitalWrite(ledPin,HIGH);delay(300);
       digitalWrite(ledPin,LOW);delay(300);
       digitalWrite(ledPin,HIGH);delay(300);
       digitalWrite(ledPin,LOW);delay(300);
       digitalWrite(ledPin,HIGH);delay(300);
       digitalWrite(ledPin,LOW);delay(300);
    break; }
delay(100);}

The VB-6 program is shown below... VB6 beats VB.net for the same reason that Arduino beats TI Launchpad... simplicity!! 


'VB-6 Program for three thermistors as polled from home base with some error processing
Dim w, temp, q, t, da(1440), foo, dpks, data As String, dsource, ghu, ghl, shu, shl
'----------------------------------------------------------
Private Sub Command1_Click()
'Turn on or off using push button
 If MSComm1.PortOpen = False Then
 Timer1.Enabled = True
   Command1.Caption = "Stop"
   MSComm1.PortOpen = True
'launch receive subroutine
   Call probie
 Else
 Timer1.Enabled = False
   Command1.Caption = "Go"
   MSComm1.PortOpen = False
   Text1.Text = "---OFF---"
 End If
End Sub
'----------------------------------------------------------
Private Sub Command2_Click()
If Timer1.Enabled = True Then Timer1.Enabled = False
Beep
Picture1.Cls
DoEvents
Call pokadot
For num = 0 To 1440
Picture1.PSet (num, da(num))
Next num
End Sub
'----------------------------------------------------------
Private Sub Command3_Click()
If MSComm1.PortOpen = False Then MSComm1.PortOpen = True
Call probie
End Sub
'----------------------------------------------------------
Private Sub Form_Activate()
'Initialize the serial port....
t = 0
 ghu = 80
 ghl = 60
 shu = 80
 shl = 60
 Image1(0).Visible = False
 Image1(1).Visible = False
 Image1(2).Visible = False
 Image1(3).Visible = False
Call pokadot
DoEvents
cp = 0
While Val(cp) = 0
cp = InputBox("Comm Port#", "Com#", "10")
Wend
   MSComm1.RThreshold = 1
' When Inputting Data, Input 1 Byte at a time
   MSComm1.InputLen = 1
' 9600 Baud, No Parity, 8 Data Bits, 1 Stop Bit 
  MSComm1.Settings = "9600,N,8,1"
' Disable DTR & RTS
   MSComm1.DTREnable = False
'   MSComm1.RTSEnable = False
' Open COM3 ===== Check Com port by opening devices in control panel/Yours may differ
  MSComm1.CommPort = Val(cp)
If MSComm1.PortOpen = False Then MSComm1.PortOpen = True
' =================================
t = 60 * Hour(Time) + Minute(Time)
x = 6
Label9.Caption = "Started: " & Time & "  " & Date & "  " & InputBox("Location:", "Where")
'launch receive subroutine
Timer1.Enabled = True
End Sub
'----------------------------------------------------------
Private Sub Form_Unload(Cancel As Integer)
'close the form
End
End Sub
'----------------------------------------------------------
Sub burko()
ntemp = 1 / 298 + (1 / 3435) * Log(w / 10000)
temp = 1 / ntemp: C = temp - 273
 If dsource = 0 Then
 Text3.Text = Format(C, "###.#"): f = (9 / 5) * C + 32
 Text2.Text = Format(f, "###.#")
 
 If f < Val(ghl) Then Image1(0).Visible = True Else Image1(0).Visible = False
 If f > Val(ghu) Then Image1(1).Visible = True Else Image1(1).Visible = False
 ElseIf dsource = 1 Then
 Text7.Text = Format(C, "###.#"): f = (9 / 5) * C + 32
 Text6.Text = Format(f, "###.#")
 If f < Val(shl) Then Image1(2).Visible = True Else Image1(2).Visible = False
 If f > Val(shu) Then Image1(3).Visible = True Else Image1(3).Visible = False
 
 ElseIf dsource = 2 Then
 Text10.Text = Format(C, "###.#"): f = (9 / 5) * C + 32
 Text9.Text = Format(f, "###.#")
End If
End Sub
'----------------------------------------------------------
Private Sub Text11_Click()
ghu = InputBox("Enter Greenhouse Cooling Set Point", , ghu): Text11.Text = ghu
End Sub
'----------------------------------------------------------
Private Sub Text12_Click()
shl = InputBox("Enter Shop Heating Set Point", , shl): Text12.Text = shl
End Sub
'----------------------------------------------------------
Private Sub Text13_click()
shu = InputBox("Enter Shop Cooling Set Point", , shu): Text13.Text = shu
End Sub
'----------------------------------------------------------
Private Sub Text8_Click()
ghl = InputBox("Enter Heating Set Point", , ghl): Text8.Text = ghl
End Sub
'----------------------------------------------------------
Private Sub Timer1_Timer()
DoEvents
Label6 = "Min.:" & t & ", Time:" & Hour(Time) & ":" & Minute(Time)
q = q + 1: Text1.Text = q
If q = 60 Then q = 0
End If
If q = 0 Then Call probie
If q = 20 Then Call gobi
If q = 40 Then Call mobie
t = 60 * Hour(Time) + Minute(Time)
End Sub
'----------------------------------------------------------
Sub pokadot()
Picture1.Scale (0, 50)-(1440, -40):
Picture1.BackColor = RGB(80, 0, 0): Picture1.Cls
 Picture1.Line (0, 20)-(1440, 30), RGB(0, 80, 0), BF
 Picture1.Line (0, -40)-(1440, 20), RGB(0, 0, 80), BF
Picture1.ForeColor = vbGreen
For s = 120 To 1440 Step 120: Picture1.Line (s, -40)-(s, 50): Next
For s = -40 To 50 Step 10: Picture1.Line (0, s)-(1440, s): Next
: Picture1.ForeColor = vbWhite
End Sub
'----------------------------------------------------------
Sub probie()
'Recieve subroutine
dsource = 0
foo = 2000: data = "": Text1.Text = "": r = 0:  MSComm1.InBufferCount = 0:
MSComm1.Output = "0" & vbCrLf
While MSComm1.PortOpen = True
foo = foo - 1
' If comEvReceive Event then get data and display
    If MSComm1.CommEvent = comEvReceive Then
      data = MSComm1.Input 'grab byte if port active
      If data = "T" Then Text1.Text = "": woof = 1
      If data = vbCr Then Beep
      'Display Temperature
      If data <> "" Then Text1.Text = Text1 + data   'Display Temperature
      If Len(Text1.Text) >= 4 And woof = 1 Then
      r = Val(Mid$(Text1.Text, 2, 3)): k = r / 1023: w = k * 18000 / (1 - k): burko
      Picture1.PSet (t, temp - 273), vbWhite: da(t) = temp - 273
      woof = 0: data = "": Exit Sub
      End If
     End If
     If foo = 0 Then dpks = dpks + 1: Text4.Text = dpks: Exit Sub
     DoEvents 'Don't be too busy to check push button :-)
     ss = ss + 1: Text5.Text = ss
Wend
End Sub
'----------------------------------------------------------
Sub mobie()
dsource = 1
foo = 2000: data = "": Text1.Text = "": r = 0:  MSComm1.InBufferCount = 0:
MSComm1.Output = "1" & vbCrLf
While MSComm1.PortOpen = True
foo = foo - 1
' If comEvReceive Event then get data and display
    If MSComm1.CommEvent = comEvReceive Then
      data = MSComm1.Input 'grab byte if port active
      If data = "U" Then Text1.Text = "": woof = 1
      If data = vbCr Then Beep
      'Display Temperature
      If data <> "" Then Text1.Text = Text1 + data   'Display Temperature
      If Len(Text1.Text) >= 4 And woof = 1 Then
      r = Val(Mid$(Text1.Text, 2, 3)): k = r / 1023: w = k * 18000 / (1 - k): burko
      Picture1.PSet (t, temp - 273), vbYellow: da(t) = temp - 273
      woof = 0: data = "": Exit Sub
      End If
     End If
     If foo = 0 Then dpks = dpks + 1: Text4.Text = dpks: Exit Sub
     DoEvents 'Don't be too busy to check push button :-)
     ss = ss + 1: Text5.Text = ss
     Wend
End Sub
----------------------------------------------
Sub gobi()
dsource = 2
foo = 2000: data = "": Text1.Text = "": r = 0:  MSComm1.InBufferCount = 0:
MSComm1.Output = "2" & vbCrLf
While MSComm1.PortOpen = True
foo = foo - 1
' If comEvReceive Event then get data and display
    If MSComm1.CommEvent = comEvReceive Then
      data = MSComm1.Input 'grab byte if port active
      If data = "V" Then Text1.Text = "": woof = 1
      If data = vbCr Then Beep
      'Display Temperature
      If data <> "" Then Text1.Text = Text1 + data   'Display Temperature
      If Len(Text1.Text) >= 4 And woof = 1 Then
      r = Val(Mid$(Text1.Text, 2, 3)): k = r / 1023: w = k * 18000 / (1 - k): burko
      Picture1.PSet (t, temp - 273), RGB(255, 130, 130): da(t) = temp - 273
      woof = 0: data = "": Exit Sub
      End If
     End If
     If foo = 0 Then dpks = dpks + 1: Text4.Text = dpks: Exit Sub
     DoEvents 'Don't be too busy to check push button :-)
     ss = ss + 1: Text5.Text = ss
     Wend
End Sub



Data sheets and arduino connections to the APC220 V3 RF module can be found online. There are two subroutines that compute the temperature and this could be done at the arduino by using the math.h include file. Instead, the analog to digital converter supplies and transmits a number between 0 and 1023. This represents in a linear way, the resistor ratio: R1/(R1+R2) which varies between 0 and 1, where R1 is the thermistor resistance, and R2 is selected arbitrarily to give maximum change at the temperature of interest (slightly below room temperature). R2 was selected as 18,000 ohms partly because this was close to the desired thermistor optimum somewhat below room temp. and partly because I had some high precision resistors in my toolbox with this value. So, if I divided the incoming data by 1023, I got the ratio R1/(R!+R2). You'll find this equation in the subroutine "probie." k=r/1023. Step one was to scale the analog reading to a value between zero and one.
Next set:

k=R1/(R1+18,000) and solve for R1.

k*R1+k*18,000=R1

k*18000=R1-k*R1

k*18,000=R1(1-k) ....or

R1=18000* k/(1-k) .... this is the thermistor resistance read by the arduino.
An arduino with higher resolution is available (12 bit), Arduino Due?? The Beta parameter equation then lets you calculate temperature. The beta value is usually listed when the thermistor is sold, very convenient. This is shown here as a photo from Wikipedia and appears in the subroutine "burko." It lets almost any thermistor to be read simply and without any sweat whatsoever, just a matter of typing carefully :) To get the resolution at a particular temperature, arbitrarily change the analog value by one unit, and then recalculate the temperature, the difference between that temperature and the read value is the temperature resolution. Pretty good, to be honest, tenths or hundredths of a degree usually. Some self heating occurs due to current flow. This can throw off the readings but my experience is that this is usually small and can be compensated for. .Here it is about 1.8 mW/degree Celsius ( https://www.mouser.com/datasheet/2/427/ntcle213e3-1762456.pdf ). Consult thermistor data sheets for this value under different insulation and/or air flow conditions... At 18k the power dissipated would be about P=V^2/R=6.25/18000 . And power would be I^2*R or an error of about 0.2 degrees C in still air.
Thanks for your attention, sincerely, Me....

That's all folks, but if you can put stuff down here that you don't want anyone to read.

Edward Kimble, PhD click here to e-mail me at: kimble@gunstar1.com
Edited March 19, 2021