Making a Soldering Station

[Noxer]

I have decided to make my own soldering station because I think that I will never buy some of the expensive one like Weller. My whole project will be based on an Arduino because I have never programmed something else. Here is the idea:
I will buy a Hakko 907 cheap of Ebay. Now I am trying to make a compatible an accurate controller for it. I thought to use the PWM output of the Arduino to trigger an optocupler that will trigger a triac. The triac will interrupt the supply of the heater. The sensor of the iron is an PTC resistor.
In the attached picture you can see my whole idea. I've also thought to add an LCD monitor to view the current and goal temperature I also need to ad something to adjust the goal temperature.
My question now is what have I done wrong in the design and how to continue designing my project and my Arduino code. 
P.S.
S2=24V
S1=?
R4=?

[sleemanj]

Head over to dangerousprototypes.com, in the forum search for soldering iron controller. 

[dannyf]

I thought to use the PWM output of the Arduino to trigger an optocupler that will trigger a triac.

How about something far simpler:

1. get a laptop power supply (19-20v, 3a +) to power the handle.
2. use a mcu (arduino or not) to control a mosfet that turns on / off the handle;
3. use a pot to set the desired temperature and use the adc to read the handle's temperature sensor;
4. if the handle's temperature is less than the desired temperature, turn on the handle; otherwise turn off the handle.

You may use a led to indicate if the handle is on/off.

All can be done quite simply.

After that, you can refine the algorithm to make it fancier (for example, adding pid, or pwm, or auto sleep, or display, etc.).

[dannyf]

How about something far simpler:

BTW, you can do all of that via a comparator, or if you desire fancy, an analog computer.

[mamalala]

No need to use PWM. The triac can sswitch off only if there is no current flowing, so you are bound to the mains frequency anyways (well, actually twice that frequency, since you have two zero crossings per cycle).

Never, ever connect any voltage higher than what you use as VREF/AVCC for the ADC to an analogue input (or generally, never anything higher than the supply voltage in regards too any pin). Look at your circuit and guess what will happen if someone pulls the plug of the iron while the circuit is still powered up. Right, you get 9V into the ADC input pin.

You will also need some signal conditioning for the sensor, the way you hooked it up now will give far too little dynamic range to make any useful temperature control possible.

Greetings,

Chris

[kxenos]

What kind of sensor are you using? I think it's not going to be that simple. If it is a thermocouple you have to have an amp, better still with zero temp. compensation. You can use a maxim 31844 or something like that, that outputs in I2C bus. On the heater side, what's the plan? I don't see a ZC detector going in the uC. Are you planning to just on-off the triac? If so use an opto with ZC built-in. I don't remember if 304x or 302x or 306x is the type with the built-in ZC. It would be better if you could control the turn-on timming and not just skip AC cycles. On the software side, what's the plan? Are you planning to use a PID algorithm?

[dannyf]

What kind of sensor are you using?

The OP mentioned the 907 handle.