Arduino/ATMega based Digital Soldering station

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Jarod
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Arduino/ATMega based Digital Soldering station

Post by Jarod » Fri Jun 15, 2012 5:43 pm

Heres my idea: Build a digitally temperature controlled soldering station that will use easily available parts from other soldering stations, specifically I'm going to use handles and tips for a Haako 937 Station. (Ebay listing for handles with tips)

The plan is to build something similar to this: Ebay listing for Soldering Station Digital DC Controller

I am going to make several changes, first mine will be in an enclosure of some sort, I think a couple more memory buttons would be nice perhaps, and I'll use a 16x2 character display rather than the 3 digit display


Currently I have to figure out how the handle feeds the temperature info back to the station. and more importantly how can I get an arduino to use that.
I think its a thermocouple, which has a tiny tiny voltage output (10-30mV or thereabouts) that changes depending on the temperature, I saw a build by someone else that they had to use some method to step up the voltage to make the difference larger so the arduino could see the change in voltage. (1-3V type range)

I think they use a 5 pin 240 degree Din connector, somewhere i saw a pinout for what the Haako 937 connector does. 2 are 24V heat and 2 are for the thermocouple.
I can use either one of these connectors: 275-1025-ND or CP-7060-ND

I also need to build a basic mosfet based PWM setup to control the power output to the handle.

I've got a few mosfets and a few transistors so I'll have to build that while I'm waiting for the handles to arrive.

foxmiles
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Re: Arduino/ATMega based Digital Soldering station

Post by foxmiles » Fri Jun 15, 2012 11:39 pm

Sounds like a pretty good plan. The arduino does hardware pwm at about 490hz. Using a n-channel mosfet you can control the negative side. Make sure you have a heatsink on the mosfet.

You can probably use a opamp to boost the thermocouple voltage. If you estimate the peak voltage on the thermocouple you can make a resistor divider circuit and apply that to the aref pin on the arduino. It should give you 1024 points between 0 and the aref voltage. I don't know how well it works at very low voltages though. Probably need an opamp.
Use fairly high value resistors, there is practically no load at all on the aref pin. It also smooths out the voltage with a cap, too low values may introduce noise.
Thermocouple tutorial :
http://www.ladyada.net/learn/sensors/thermocouple.html
Opamp thermocouple amplifier:
http://www.bristolwatch.com/ele/thermal ... lifier.htm
Don't know what that digital chip costs but it looks like a really easy way to go.

I'd build up a pwm controller and measure the voltages. Do some sort of an emergency kill switch.
Digging with a spoon will accomplish more than leaning on a shovel.

Jarod
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Re: Arduino/ATMega based Digital Soldering station

Post by Jarod » Sun Jun 24, 2012 3:53 pm

I've found a few sites with some suggestions:

This one has been particularly difficult to find when i forget to bookmark it:
http://macoprojects.blogspot.com/2010/0 ... ation.html

Heres one on simple thermocouple amp circuits.
http://www.nhsouth.com/crafts/workbench/thermocpl.htm

Most everyone says an opamp is the worst way to go because the temperature is difficult to calibrate and could be off by as much as 10-20F...
Solder melts at around 300C if i'm correct. which means the iron is at 572F so being off by even 20F means my iron could be as low as 288C or as high as 316C... although I think that is a very significant difference I also think thats going to be much nicer to work with than a $10 firestarter soldering iron like I currently use.

Plus this whole project is my attempt to build a soldering station anyone can afford. if you immediately need to spend 10 to 30 bucks on a purpose built thermocouple amplifier chip, why not just spend 50 bucks and get the whole thing already built for you?

Its supposed to be cheap and expandable. build it then upgrade it later on as you decide you need more accuracy or a better display or better anything.

What I plan on building is going to be just about identical to the first link i mentioned in this post (this one at macoprojects) Except I'll be using an atmega328 because I have several available. He uses a LM358 Opamp, I have 5 LM358N dual Opamps I got off ebay for about $2.50

So, a parts breakdown so far:
  • 3x Haako clone handles: $12.69 (4.23 each)
  • 5x LM358N Opamps: $2.49 (.49 each)
  • 1x ATmega328p: $3.16 (I paid 1.98 each because i bought in bulk)
  • Arduino (for ease of testing and build. I'm not going to put the arduino in the actual device)
  • Total cost so far per iron: $7.88

Jarod
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Re: Arduino/ATMega based Digital Soldering station

Post by Jarod » Sun Jun 24, 2012 8:57 pm

I've started to test and build the opamp circuit, I'm making a schematic in kicad as I go, Currently I have basically a direct ripoff of the circuit at http://www.bristolwatch.com/ele/thermal ... lifier.htm except I've changed the variable resistor to a 1M as suggested on that site.

I've powered the heater element and received voltage through my voltmeter first, then I connected it to one of my arduinos analog input pin and set up a small program to feed the analog input value to the serial port.

This worked very nicely, the iron heats up very quickly, using just 12V from an old transformer it was hot enough i didn't want to touch it (nearly burned my finger) within about 20 or 30 seconds.

Now the next things I have to do is figure out a way to calculate the analog input value as temperature and put out a pwm to keep the temperature the same, then add a way to adjust it. or perhaps do that at the same time.

EDIT: forgot the schematic so far.
Attachments
g2656.png
Schematic so far.

foxmiles
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Re: Arduino/ATMega based Digital Soldering station

Post by foxmiles » Mon Jun 25, 2012 7:24 pm

A quick and dirty way to get going would be to have the arduino do a serial print of the temperature value. Then you can see what the things doing without messing with lcd's.
EDIT: read the temp with an infrared thermometer for a rough measurement. Capton tape a glass thermisitor to it for a more accurate measurement not sure of the top rating though. They use them on the reprap extruded, so they can get pretty hot

Add some averaging code and a ladder climbing algorithm to maintain the set temperature.
Digging with a spoon will accomplish more than leaning on a shovel.

Jarod
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Re: Arduino/ATMega based Digital Soldering station

Post by Jarod » Mon Jun 25, 2012 7:29 pm

thats kinda what I did. however I'm gonna have to figure out how to make a temperature out of a 0-1023 value.
I figure I'll put a thermocouple in boiling water and hook it up to the arduino and have it log the value, then I'll know what 100C is in 0-1023 value.

Jarod
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Re: Arduino/ATMega based Digital Soldering station

Post by Jarod » Mon Jul 02, 2012 5:25 pm

So heres where I'm at at the moment. at the analog pin value of about 550 i can start melting solder at the tip. which means the tip is around 190c -200c because i have 60/40 rosin core solder that melts at that temp.

however once power is removed I can still keep the solder liquid until around 450, so that means the tip dropped below 183C because that's when 60/40 solidifys according to wikipedia (I'm sure that's 100% accurate)

I tried to measure the temp with a infared thermometer, that got me a value between 645 and 660 on my arduino was 374F which is 190C... I think my lil craftsman thermometer is getting a tad inaccurate with the combination of abuse and its old age as the cause.

I may try the thermistor and kapton tape, just have to find somewhere to get kapton tape lol. I've got 2 glass thermistors that you gave me, i'm not sure what kind of temp they'll withstand, I'm going to look that up before I try it for sure. also have to figure out a way to measure the temp. i'm thinking i can use the sanguinololu since it has a thermistor monitoring function in the repetier host software so i could monitor it.

I also found a nice power supply to use, 19V 3.16A HP laptop supply.

Jarod
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Re: Arduino/ATMega based Digital Soldering station

Post by Jarod » Mon Jul 02, 2012 7:31 pm

I hooked up a lcd, pot, mosfet and power supply, heres what I came up with:

at 420 (analog in value) the solder melts very nicely without much pressure.
at 410 the solder melts but requires a little pressure, and only melts where the iron is.
at 405 the solder will not melt at all.

so between 410 and 420 is approx 190C

to test this i hooked up a mosfet to a pwm pin on my arduino, hooked up a pot to an analog in pin and set it up to map the pot value to the pwm value. this way i could control the power somewhat precisely.

I set up the lcd to display the value coming from the pot and also the output from the thermocouple.

I then played with the pot until the value stabilized on the thermocouple and then i could adjust it up and down quite easily from there on.

heres a few photos of the setup.
Attachments
IMG_1317.JPG
another view of the wiring and the board.
IMG_1316.JPG
value on left is the thermocouple output (this was right after a test during cooldown) and the value on the right is the value of the potentiometer
IMG_1315.JPG
I need a bigger breadboard...

Jarod
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Re: Arduino/ATMega based Digital Soldering station

Post by Jarod » Wed Jul 04, 2012 2:05 am

Made a minor mistake on the first schematic, haven't got it completely up to date with what i've built so far but heres the progress on the schematic so far.
Attachments
Solderingstation.png

foxmiles
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Re: Arduino/ATMega based Digital Soldering station

Post by foxmiles » Wed Jul 04, 2012 6:19 pm

Very cool. Now you just need to write the code so the pot sets the desired temperature, maps the temp to the analog value, monitors the thermocouple and adjusts the pwm to maintain that value. Actually not that hard at all.

I said ladder climbing algorithm earlier. I meant hill climbing. here is an article.
http://echorand.me/2010/03/14/hill-clim ... on-method/
Digging with a spoon will accomplish more than leaning on a shovel.

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