Interfacing a Pressure Transducer to a Protoboard

tae2010 · 2010-05-06 22:13

Hi there,

Ive been using a USB Protoboard for the past few weeks and Ive wanted to interface an MSI 0-100psi pressure transducer that I have. I know it runs on 5 volts, and I plan to use an external regulated power supply to drive it, but I wanted to know how I can interface the board to the signal pin ( the sensor outputs 0.5-4.5 volts). Also I want the board to be able to convert the read voltages into·pressure values·and display·the data·in the propterminal on regular intervals.

The long term goal of this project is to control when a solenoid valve turns on and off based on the pressure reading.

It seems like a simple thing to do, but if someone can point me in the right direction as far as wiring and code is concerned, it would help a lot.

Thanks

Leon · 2010-05-06 23:01

Use an ADC with a voltage divider. You will then need to scale the ADC readings to get the actual pressure.

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Leon Heller
Amateur radio callsign: G1HSM

computer guy · 2010-05-06 23:50

Like Leon said.

Red wire to +5V
Black to GND
Green to an ADC with a voltage divider.

The hardest thing will be to calibrate the ADC, maybe have an analoge pressure sensor connected as a reference whilst u adjust the ADC.
The coding should be easy.

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"powered by Propeller" domed stickers $1.50 - Find them here
Check out my Design and Technology project for my Higher School Certificate www.ecosureblog.net

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tae2010 · 2010-05-07 03:29

Thanks for the replies. Im not very familiar with ADC's, what model of ADC do you recommend then for the specs I have or is there one built into the board?

Post Edited (tae2010) : 5/7/2010 3:44:28 AM GMT

Leon · 2010-05-07 07:43

You might be able to wire the parts for the simple sigma-delta circuit (see Object Exchange) onto your board but I'd use the ADC chip and software that you will also find there.

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Leon Heller
Amateur radio callsign: G1HSM

computer guy · 2010-05-07 11:53

Parallax sells the 4 and 8 channel versions of the MCP3208. The 8 channel version can be found here www.parallax.com/StoreSearchResults/tabid/768/txtSearch/MCP3208/List/0/SortField/4/ProductID/573/Default.aspx

The code for interfacing it can be found here obex.parallax.com/objects/180/

I can't help you with connecting it up as I haven't used it before however DACs are pretty easy to connect.
Usualy just:

VDD to 5V
VSS to GND
Ref to a pot between 0v and 5v
Ch1 to the green wire from your sensor
Dout, Din, Clk and Cs all go to prop pins via a resister.

Please confirm this with someone else before conecting it though.

I hope this makes things clearer for you.

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"powered by Propeller" domed stickers $1.50 - Find them here
Check out my Design and Technology project for my Higher School Certificate www.ecosureblog.net

The Sarah Myatt Fund - Help Sarah and Her Family Fight Quadriplegia HERE

Leon · 2010-05-07 12:21

It will work at 3.3V The VREF input should be connected to +3.3V. An external reference could be used, but isn't really necessary in this application.

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Leon Heller
Amateur radio callsign: G1HSM

Post Edited (Leon) : 5/7/2010 12:26:30 PM GMT

tae2010 · 2010-05-08 01:28

So could I use a 10k ohm potentiometer as my voltage divider, where one end of the pot hooks to Vss, one to the ADC (Vin +), and the other to the green line of my sensor?

Based on my understanding, Vdd would hook into Vref and Vcc. /CS, CLK and D0 would all hook into pins on the board, and Vin - & GND would hook into Vss. (I'll probably end up using the 3202 since I dont need many analog inputs)

But then what will Din hook into?

Just trying to verify if Ive got this so I dont mess up my sensor.

Thanks.

Post Edited (tae2010) : 5/8/2010 2:17:02 AM GMT

Leon · 2010-05-08 16:29

Just use a couple of 1% resistors for the voltage divider. You will know exactly what division ratio you are getting, which isn't the case with a pot.

Post a schematic of how you propose to wire it up, it'll be easier to check it.

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Leon Heller
Amateur radio callsign: G1HSM

tae2010 · 2010-05-08 21:38

Heres the diagram I was thinking it would look like. Im not sure about the D1 pin, and the POT. Based on a tutorial I found for wiring an ADC from Parallax they used a POT instead of a voltage divider. Since I didnt really need so many inputs as with the 3204 and 3208, I went with the 3202.

pressure%20sensor%20diagram.png

Leon · 2010-05-08 21:55

I don't understand why you want to use a pot, you know the input voltage range. You need decoupling for the ADC supply pins.

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Leon Heller
Amateur radio callsign: G1HSM

Post Edited (Leon) : 5/8/2010 10:01:38 PM GMT

tae2010 · 2010-05-08 23:33

Well I was thinking that a Pot could replace a voltage divider; its practically the same thing if you compare the two schematics. What do you mean by 'decoupling for the ADC supply pins'? If I did use resistors instead of the Pot, 1k ohm would be fine right?

Also does my wiring diagram look fine?

pressure%20sensor%20diagram2.png

Post Edited (tae2010) : 5/8/2010 11:47:51 PM GMT

Leon · 2010-05-09 11:16

You need a 100 nF capacitor across the supply and ground pins.

Use resistors; they are much more stable, easier to mount and cheaper.

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Leon Heller
Amateur radio callsign: G1HSM

tae2010 · 2010-05-09 21:28

Just for my understanding, whats that capacitor for? Supply as in Vcc right?

Thanks for helping me out.

Leon · 2010-05-09 22:06

It's called a decoupling or bypass capacitor, one should be used with all ICs. It goes between the Vcc and ground pins. Momentary high currents are required when the chip is operating, which are supplied by the capacitor, and prevented from affecting other parts of the circuit. It's a good idea to use an additional 10 uF capacitor as well, with analogue circuits like ADCs, to keep noise levels down. It'll work without them, but the readings might be very noisy. I often use a pot to check an ADC and the software, before I connect the actual sensor. It makes testing easier.

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Leon Heller
Amateur radio callsign: G1HSM

Post Edited (Leon) : 5/9/2010 10:13:27 PM GMT

tae2010 · 2010-05-09 23:54

So the 10 uF capacitor would hook in series with the 100nF cap?

Leon · 2010-05-10 03:41

No! In parallel.

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Leon Heller
Amateur radio callsign: G1HSM

tae2010 · 2010-05-10 05:52

Oh Ok, Thanks for the help. I'll be sure to post after finals, if this works out.

tae2010 · 2010-05-28 05:27

Well I managed to build the circuit, and I worked out the code for the 3202, but nothing is happening. Here is the latest setup Ive got. Im using a 1k ohm pot to vary the analog input to test the system. I'll include my code and my schematic if anyone can take a look to help me out. I dont have a clue why it wont work, its compiling, and it looks good to me.

http://lh3.ggpht.com/_JitqFAafVeQ/S_9VDWG-IoI/AAAAAAAAAGk/-JhztlgSt1c/ADC Circuit.png

And the code:


OBJ
 ' LCD     : "LCDRoutines4" 'for the LCD methods
   PST     : "Parallax Serial Terminal"
   Debug   : "SimpleDebug"
                                                     
CON                                                                   
  _CLKMODE=XTAL1+ PLL2X         'The system clock spec
  _XINFREQ = 5_000_000          
   chipDin =07    'to pin  5     
   chipClk =04    'to pin  7   
   chipSel =01    'to pin  1    
   chipDout=06    'to pin  6
   BitsRead=12

   Baud =  115200
   'relaypwr = 02
   'ledpwr = 05
   
VAR
  long stack2[noparse][[/noparse]25]
  word PotReading               
  word DataRead

  byte  rxString[noparse][[/noparse]50]  'Holds [noparse][[/noparse]50] incoming serial bytes
  Long  Temp

DAT       ''Line up all the expected Strings

 binaryvalue byte  "8-bit binary value: ", 0 

PUB Main

Go
                                                             
PUB Go
  'cognew(Cog_LCD, @stack2)        
  DIRA[noparse][[/noparse]chipDin]~~     '19 data set up to the chip   
  DIRA[noparse][[/noparse]chipClk]~~     '20 oscillates to read in data from internals                     
  DIRA[noparse][[/noparse]chipSel]~~     '21 osc once to set up 3202                                     
  DIRA[noparse][[/noparse]chipDout]~     '22 data from the chip to the Propeller                               
  repeat        
    DataRead:=0             'Clear out old data
    waitcnt(1000+cnt)       'Wait for things to settle down, but not needed
    outa[noparse][[/noparse]chipSel]~~         'Chip select has to be high to start off                        
    outa[noparse][[/noparse]chipSel]~          'Go low to start process
                                   
    outa[noparse][[/noparse]chipClk]~          'Clock needs to be low to lead data     
    outa[noparse][[/noparse]chipDin]~~         'must start with Din low to set up 3202                   
    outa[noparse][[/noparse]chipClk]~~         'Clock high to read data in
                                   
    outa[noparse][[/noparse]chipClk]~          'Low to load      
    outa[noparse][[/noparse]chipDin]~~         'High single mode                  
    outa[noparse][[/noparse]chipClk]~~         'High to read
                                           
    outa[noparse][[/noparse]chipClk]~          'Low to load               
    outa[noparse][[/noparse]chipDin]~          'Odd = channel 0          
    outa[noparse][[/noparse]chipClk]~~         'High to read
                                    
    outa[noparse][[/noparse]chipClk]~          'Low to load       
    outa[noparse][[/noparse]chipDin]~~         'msbf high = MSB first                
    outa[noparse][[/noparse]chipClk]~~         'High to read
    
    outa[noparse][[/noparse]chipDin]~          'making line low for rest of cycle
    
    outa[noparse][[/noparse]chipClk]~          'Low to load 
                            'Read the null bit, we dont need to store it
    outa[noparse][[/noparse]chipClk]~~         'High to read
   
    repeat BitsRead          'Reads the data into DataRead in 12 steps
      outa[noparse][[/noparse]chipClk]~         'Low to load 
      DataRead:=DataRead+ina[noparse][[/noparse]chipDout]  'Xfer the data from pin chipDout                           
      outa[noparse][[/noparse]chipClk]~~        'High to read  
      DataRead <<= 1         'Move data by shifting left 1 bit. Ready for next bit
        
    outa[noparse][[/noparse]chipSel]~~          'Put chip to sleep, low power
      DataRead >>= 1         'Shift data right 1 bit to cancel last "bad" shift
      PotReading:=DataRead   'Finished data read for display

    Printstr(string( "8-bit binary value: " )   )
    pst.bin(PotReading,8)

PUB PrintStr(str1)
pst.str(str1)

(Edit: for some reason the image wont appear, so I included the link of it)

Post Edited (tae2010) : 5/28/2010 5:44:01 AM GMT

Paul Sr. · 2010-06-05 21:33

How has your project progressed? I am interested as I have a couple of soil moisture sensors that would require almost the same setup!

tae2010 · 2010-06-06 06:20

Sorry Paul, I havent made any progress on this yet. I got stuck with the code that I uploaded, and the testing of that code didnt get me anywhere. Im going to try it again with some different code and see if I make progress that way. If anyone has suggestions to this code that I uploaded, I welcome them.

I'll keep you posted on any updates. At the moment, since I cant get the ADC and the code to work, Im working on the user interface part of the project.

tae2010 · 2010-06-14 17:58

Can anyone help me on this? I still cant get the code to work, but the setup is the one we talked about in this thread. I dont have the slightest idea why it wont work, Im not even getting any transmission over the USB to the computer.

ElectricAye · 2010-06-15 04:25

tae2010 said...

And the code:


OBJ
 ' LCD     : "LCDRoutines4" 'for the LCD methods
   PST     : "Parallax Serial Terminal"
   Debug   : "SimpleDebug"

In your Object block, should "Parallax Serial Terminal" be "Parallax_Serial_Terminal" or some other name that has no spaces? Maybe I'm wrong but I didn't think SPIN is very fond of spaces.

hover1 · 2010-06-15 12:20

PST : "Parallax Serial Terminal" is valid, spaces are OK.

But you never start the PST. You need to include:

PST.Start(Baud)

or

PST.Start(115_200)

Jim

ElectricAye · 2010-06-15 13:24

hover1 said...
PST : "Parallax Serial Terminal" is valid, spaces are OK.

....

Tar-nation! I didn't know that. Thanks for correcting me!

tae2010 · 2010-06-20 01:56

Well I got it to work at last and it outputs data to the serial terminal. Also Ive got a potentiometer hooked up to the 3202 to calibrate it, and that seems to be working. And its even outputting the binary like its supposed to! Also got the chip to output a decimal value of the voltage measured, but Ive got issues with how its calculating, so instead of it outputting a 5 volt measurement, its really outputting 2.0 as the highest. I followed a tutorial of how to calibrate on a stamp, but the code isnt the same, so its possible that Im coding this part wrong.

The major problem Im experiencing is that the decimal keeps including a -1 in the numbers to the right of the decimal place. Also the voltage isnt as high as its supposed to be. Im using a 1k ohm pot to change values, so is that why my voltage read is so low?

Heres the code, perhaps someone can tell me what Im doing wrong? As I always say, Two sets of eyes are always better than one.


Pub CalcVolts

v := 5*adcRead / 4095
r := 5*adcRead //4095
v2 := 100*r /4095
v3 : 10*r //4095
v3 : 10*v3 /4095

If (v3 >=5)
   v2:=v2 +1
If (v2 >= 100)
   v:= v+1
   v2:= 0

Return

All those variables are setup as Longs, except adcRead, which is setup as a word.

Thanks everyone for helping!

Post Edited (tae2010) : 6/20/2010 2:55:10 AM GMT

Harprit · 2010-06-21 13:38

So what did it take to get it working
What were you doing wrong
I'm interesting because you were using my code and I want to make sure there was no mistake in it.

Harprit

tae2010 · 2010-06-22 03:04

I added the serial start code to get the serial to receive data via the 3202 rather than outputting to an LCD screen. And from there I set it up to output as a binary number or a Decimal number.

Only reason it wasnt working before, is I forgot to start the serial connection. Its working now, Im just not sure if its working correctly. The max output voltage the Propeller is outputting is only 2volts, and Im supplying a 5V supply to the pot that is changing the analog input. The maximum the Propeller puts out should be around 2.5v. So I dont know what Im doing wrong in my setup or if Im missing a key bit of info from the 3202 spec sheet.

Harprit · 2010-06-22 14:46

Post your ckt diagram and complete listing and I will take a look
Harprit

tae2010 · 2010-06-23 05:17

OBJ
 ' LCD     : "LCDRoutines4" 'for the LCD methods
   PST     : "Parallax Serial Terminal"
                                                     
CON                                                                   
  _CLKMODE=XTAL1+ PLL2X         'The system clock spec
  _XINFREQ = 5_000_000          
   chipDin =10    'to pin  5     
   chipClk =12    'to pin  7   
   chipSel =13    'to pin  1    
   chipDout=14    'to pin  6
   BitsRead=12

   Baud =  115200
   relaypwr = 02
   ledpwr = 05  
   
VAR
  long stack2[noparse][[/noparse]25]
  word adcRead               
  word DataRead

  byte  rxString[noparse][[/noparse]50]  'Holds [noparse][[/noparse]50] incoming serial bytes
  Long  Temp

  Long v, r,v2,v3,v4


DAT       ''Line up all the expected Strings

        pin02 byte "PIN02", 0
        pin23 byte "PIN23", 0
        stop  byte "stop", 0
        exit  byte "exit", 0
        on    byte "ON", 0
        off   byte "OFF", 0
        WrongData byte "Don't Hand Me that Garbage", 0
        high byte "High", 0
        low byte "Low", 0
                                                        
PUB Go

 pst.StartRxTx(31,30,0,Baud)
 
  'cognew(Cog_LCD, @stack2)        
  DIRA[noparse][[/noparse]chipDin]~~     '10 data set up to the chip   
  DIRA[noparse][[/noparse]chipClk]~~     '12 oscillates to read in data from internals                     
  DIRA[noparse][[/noparse]chipSel]~~     '13 osc once to set up 3202                                     
  DIRA[noparse][[/noparse]chipDout]~     '14 data from the chip to the Propeller                               
  repeat        
    DataRead:=0             'Clear out old data
    waitcnt(1000+cnt)       'Wait for things to settle down, but not needed
    outa[noparse][[/noparse]chipSel]~~         'Chip select has to be high to start off                        
    outa[noparse][[/noparse]chipSel]~          'Go low to start process
                                   
    outa[noparse][[/noparse]chipClk]~          'Clock needs to be low to lead data     
    outa[noparse][[/noparse]chipDin]~~         'must start with Din low to set up 3202                   
    outa[noparse][[/noparse]chipClk]~~         'Clock low to read data in
                                   
    outa[noparse][[/noparse]chipClk]~          'Low to load      
    outa[noparse][[/noparse]chipDin]~~         'High single mode                  
    outa[noparse][[/noparse]chipClk]~~         'High to read
                                           
    outa[noparse][[/noparse]chipClk]~          'Low to load               
    outa[noparse][[/noparse]chipDin]~          'Odd = channel 0          
    outa[noparse][[/noparse]chipClk]~~         'High to read
                                    
    outa[noparse][[/noparse]chipClk]~          'Low to load       
    outa[noparse][[/noparse]chipDin]~~         'msbf high = MSB first                
    outa[noparse][[/noparse]chipClk]~~         'High to read
    
    outa[noparse][[/noparse]chipDin]~          'making line low for rest of cycle
    
    outa[noparse][[/noparse]chipClk]~          'Low to load 
                            'Read the null bit, we dont need to store it
    outa[noparse][[/noparse]chipClk]~~         'High to read
   
    repeat BitsRead          'Reads the data into DataRead in 12 steps
    
      outa[noparse][[/noparse]chipClk]~         'Low to load 
      DataRead:=DataRead+ina[noparse][[/noparse]chipDout]  'Xfer the data from pin chipDout                           
      outa[noparse][[/noparse]chipClk]~~        'High to read  
      DataRead <<= 1         'Move data by shifting left 1 bit. Ready for next bit
        
    outa[noparse][[/noparse]chipSel]~~          'Put chip to sleep, low power
      DataRead >>= 1         'Shift data right 1 bit to cancel last "bad" shift
      adcRead:=DataRead   'Finished data read for display

      
    'pst.str(string( "12-bit binary value: " )   )
    'pst.Bin(adcRead,12)
    'pst.str(string(pst#NL))

    'pst.str(string( "Decimal Equivilant: "))
    'pst.Dec(adcRead)
    'pst.str(string(pst#NL))

    CalcVolts
    
    pst.str(string( "Voltage Equivilant: "))
    pst.Dec(v)
    pst.str(string("."))
    pst.Dec(v2)
    pst.Dec(v3)
    pst.Dec(v4)
    pst.str(string(pst#NL))
    
    waitcnt(clkfreq/2 + cnt)
    '0 to 4095 for 12bit

PUB CalcVolts

v := 5 * adcRead / 4095
r := 5 * adcRead // 4095
v2 := 100 * r / 4095
v3 := 100 * r // 4095 
v4 := 10 * v3 / 4095 
 IF (v4 >= 5)
     v2 := v2 + 1 
 IF (v2 >= 100)  
     v := v + 1 
     v2 := 0 

RETURN

Thanks for taking the time to take a look at whats wrong. That last bit of code I used one of the tutorials supplied with the Digital (ADC) package that you can buy. Except that tutorial was for the Stamp, so I had to change the code to get it to work on the Propeller.

Harprit · 2010-06-23 08:42

Send me your mailing address so I can send you what I found out.
Its easier for me to send you my hand written stuff that way.
I will make up your ckt and get you a solution/notes of some sort.
This seems manageable. Might take a day or two.
Then you can post it all for all to see.

H

Interfacing a Pressure Transducer to a Protoboard

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