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First SMT board: a 5-day photo adventure!


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DAY 5:
Oh yeah baby.  It even works, too.

Third picture shows the aftermath of my difficulty with the CPU chip. It had to be reworked 3 times to get the alignment right  (of course the other chips were easy). Despite being a bit freaked I might have killed the chip with heat, it still works!

This project has been a lot of fun so far but there is still much more to be done!  Just last night I managed to start drawing pixels on the 128x64 LCD screen  (if you can find the block of 2x10 through-holes, that is the connector to LCD).

There are three PIC microcontrollers that need to be filled with code.. this project has now entered MY realm  (muahahah)...




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It is a controller to be placed inside a cheapo 1200 watt toaster oven for (future) reflow baking.

- Thermocouple connector (uses MAX6675 conditioner IC for the accurate temperature reading)
- On-board thermal sensor for monitoring temperature of controller board/area
- MOSFETs to switch both an oven cooling fan and a controller area cooling fan
- Low current output to an optoisolated TRIAC switchboard (the smaller board in the photo of toner transfer) for independent switching of two 120VAC 10A heating elements (top/bottom of oven)
- 20-pin header for connecting to a 128x64 1-bit monochrome LCD (w/ EL backlight). menus and temperature graphs should look nice
- Output to 8ohm speaker for the bleeps and bloops of menu actions and oven events
- Four button inputs for menu navigation (Up/Down, Go, and Cancel)
- 4kb EEPROM for storing new target heating profiles or recording temperature profile for accuracy comparison
- RS-232 (DB9 plug) for connecting to PC software, to transfer settings or temperature profiles
- Three independent controllers (Peripheral control, CPU, Graphics rendering/LCD driver)  split up tasks for smooth operation

I used one of these butane microtorches with heat blower tip for doing the reflow on the whole board

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  • 4 weeks later...

I finished construction of the oven-  everything is installed and ALL parts are working  (whew...).

The front panel looks kind of crappy, I was messy with epoxy, but the side panel looks great-  and it's hinged so I can open easily to reprogram the microcontrollers and when closed will stay shut with help of a simple magnet.

Pictures or demo video will be coming soon :)

There were several last-minute problems but managed to fix them all:

Problem: One fan could not be switched
Solution: The Drain leg of SOT23 MOSFET was not actually soldered down (looked ok but failed continuity test).  Used soldering iron tip and touch of solder to fix.

Problem: Upper heater output LED was dim and only delivered 6mA when on.  It should have been bright LED and delivered 20mA to the port.
Solution: SOT23 transistor was damaged.  Replaced and it is working.

Problem: TRIAC switchboard did not work to switch 120vac.
Solution: There was a flaw in the design that caused short circuit (on logic side, NOT 120vac side).  Controller board had common +5v and switched GND, but switchboard had common GND and assumed +5v to switch it's optocouplers. Had to cut a couple traces and install an extra piece of wire to make common +5v and independent GND.

The last big problem is not so huge, but I have to order some new parts to make the project resiliant.  The two TRIACs I ordered are rated at 12.7A,  they aren't on a heatsink,  and I am switching 10A loads each.  They work but if I let them on long enough to get the oven's heating elements red hot, the TRIACs overheat and will not turn off.  (Once they cool down again they are fine, which is good for now).

Solution is to order TRIACs rated for 25A  and probably use heatsinks.  I hate to pay $7 for shipping on $2.50 of parts  so I am waiting until I have partslist for another project.  I can still get this oven up to proper temperature by pulsing power periodically to give TRIACs time to cool down  (last test I did was outside in 50 degree weather..  took about 5 minutes for inside the oven got up to 470 degrees farenheight).

All in all, I'm very happy with how the project has turned out so far, and learned a lot too!  All that is left is to fix the overheating TRIACs and write the code for the menus and plotting temperature and etc etc  (that is the fun stuff!!)

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  • 1 month later...

Well, I still haven't ordered replacement TRIACs yet-  need to come up with a parts list for the next project.  However, I did use the oven just a couple nights ago to reflow 14 LEDs and some chip resistors/caps for a  moving lights/persistance of vision  sort of project  (maybe more on that later?)

Included are pictures of the finished oven, and the boot-up screen on the LCD



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