Troubleshooting my Hot Air Rework Station (Updated)

[tineras]

I thought I'd try out a cheapo hot air station that has good reviews on Amazon and I ordered one of these (http://www.amazon.com/gp/product/B004ZB9D4O/ref=oh_details_o05_s00_i00) last month and have been constantly getting an over temperature error. It was more rare at first, but now the hot air side simple doesn't work. There is an LM358N op amp circuit that is telling the micro what the temperature is; telling it whether or not to turn on the element. The input to the micro (output of the LM358N) is pegged at ~3.6V and does not change when the temp changes. I believe I have drawn the circuit correctly below and would like to figure out how I can fix the unit or replace parts to make it better.



More pictures here: https://picasaweb.google.com/102677756008875158290/HotAirReworkStation?authuser=0&feat=directlink

The temp sensor in the tip is reading about 1.4Ohms at room temperature. I tried putting a pot in there instead and it didn't make any difference. I can force the heating element ON by cutting off the input from the opamp and putting my own pre-set voltage on there.

Drawing out the schematic is as far as I got last night. I'm hoping to sit down tonight to try and sort it out a bit more, but I thought I'd ask you guys what you thought.

Thanks!

[Kevin.D]

quote - "I can force the heating element ON by cutting off the input from the op-amp and putting my own preset voltage on there."  ?

But you said the op-amp output is already hard on. (3.5V is the max for a lm358 with a 5 V supply) .
So what is the V  on the MC pin itself when op amp output is 3.5V . (after R14 and R15 )
Is the electrolytic cap between these two  leaky and pulling the output V down through r14. ?

[Kevin.D]

oops sorry,  my bad ,it's gotta go low  . 

[tineras]

quote - "I can force the heating element ON by cutting off the input from the op-amp and putting my own preset voltage on there."  ?

But you said the op-amp output is already hard on. (3.5V is the max for a lm358 with a 5 V supply) .

True. I'll elaborate. I believe the micro has a maximum allowed input voltage (aka max temp) and it will turn heating element off as a safety precaution. I didn't check to see what that max voltage was, but for example, if I put 1V into that temp sens input of the micro, it will operate normally and the heating element turns on. I'm at work now, but will take measurements this evening when I get home and answer your other questions.

I measured the ~3.6V on the input of the micro (after the resistors) and not directly from the opamp output.

[tineras]

 

Well, after poking, prodding and measuring for a little while it was misbehaving, the stupid thing started working and has been working non-stop all night. I'll have to revisit this when it stops working again.

[tineras]

Here are some pictures of noise starting from the input of the opamp (A) going to GND (D). The noise (I assume) is coming from the air pump because when it's disconnected, I don't see it when I probe any of the points (A,B,C,D). I'm placing the ground lead of my scope close the microcontrollers ground which is reasonably far away from the ground shown in the drawing below.

I assume this is not good, but don't know if it's the root of my problem. But I figured I'd try and minimize this noise before I move on.

EDIT: I noticed the noise on the '-' input as well after properly setting the trigger. Though I don't know how often the noise is happening. It may be so infrequent that it's not as significant. Not sure how to tell with my scope.

Any input would be greatly appreciated.