Archive for January, 2014
Many a time I have found myself sitting at my workbench and desperately wished for a way of viewing the latest revision of an EAGLE schematic or board file on my iPad. But, alas, no such app exists in the App Store.
So I'm making my own. And it will be free on the App Store when it is finished, or course.
So far I can:
- Open schematic files from Dropbox
- Open schematic files directly or when they are contained in a zip archive from another app – Mail or Safari, for example
- Display the schematic (kinda the core feature, you might say). There is still some work to be done here before all shapes can be displayed properly.
- Pan and zoom using standard iOS gestures (pan, pinch)
The next features on the list are:
- Show/hide layers
- Component info popup on tap
- Displaying board files
If you guys have any wishes, feel free to post them in the comments.
Here's a screenshot of how it looks now:
My most recent project is an electronic to/from card for presents. A small device you can attach to presents showing "To" and "From" on a backlit 16 x 2 LCD display.
In order to save battery power, an accelerometer is used to detect motion and the LCD backlight is turned off after a couple of seconds with no motion and the MCU is put to sleep ("deep power-down", actually). When motion is detected, an interrupt is signalled by the accelerometer which wakes up the MCU which in turn turns on the LCD backlight.
The text shown on the LCD can be programmed using a USB connection and a Mac OS X application. The texts are stored in EEPROM.
There are two main design goals for this project:
- Make it cheap. These devices are meant to be attached to presents and then they are gone. Unless the recipient wants to use them, of course.
- Low power. The device should be able to run for a couple of days off two series-connected CR2032 batteries.
The low power consumption appears to be fairly easy to achieve: the LCD with backlight off, the accelerometer and the MCU in power-down mode all use very, very little power.
The low price, however, is more tricky. We'll see how it goes…
These are the main parts:
- MCU: LPC11U24 Arm Cortex M0
- Accelerometer: Freescale MMA8452Q I2C 3-axis accelerometer
- Display: Midas MCCOG21605C6W-BNMLWI 16 x 2 white-on-blue I2C display
The PCBs are designed in EAGLE and I will get them made at ITead. The top side will be hot-plate reflow soldered (mainly due to the 0.5 mm pitch QFN accelerometer) and the bottom side will then be hand-soldered.
Here's a rendering of the PCB courtesy of Mayhew Labs:
Until now I've been soldering SMD components by hand only (here's an image of my first hand-soldered SMD board). However, for my next project I will need to solder an MMA8452 accelerometer (datasheet) which is a 3 x 3 mm QFN package (0.5 mm pitch).
And while it is possible to hand-solder QFN packages (I've successfully hand-soldered an ADXL335 accelerometer in a 4 x 4 mm LFCSP package – which is a 0.65 mm pitch QFN), it is simply too difficult and too much bother (I'm sure people with better dexterity and eyesight than me can do it easily).
So: no way around it, time to start doing some reflow soldering.
For a hobbyist like me there seems to be three ways of doing it:
- Toaster oven
- Hot air rework thingie
- Hot plate
Originally I planned on using a toaster oven and this nice reflow controller from Beta LAYOUT. But after reading SparkFun's tutorial and the comments I decided on trying the simpler way first – which is getting a hot plate or electrical skillet.
They didn't have a skillet in the store and I couldn't be bothered to check other stores so I bought a simple and cheap hot plate.
And it works perfectly!
I got some (leaded) solder paste (like this one from Farnell) and applied the solder paste straight from the syringe needle (for the TQFP, SOIC and QFN packages I dragged a thin line across the pads), placed the assembled PCB on the hot plate and dialed it to the second-lowest setting and kept an eye on it to see when the solder paste started reflowing. After a minute or two, everything had reflowed and I took the PCB off the hot plate. This is what it looked like:
A lot of tutorials warn against using too much solder paste. If anything I should have used a bit more on the "big" components like the 1206 resistors and some of the 0805 capacitors. But both the SOIC and TQFP packages look nice and not a single solder bridge to clean up.
Of course, the real test is the QFN device. The pads are all nicely aligned with the PCB traces leading straight to the pads so it looks good, but we'll see when the board has been fully assembled (battery holder hand-soldered on to the bottom side of the PCB, for example) and the MCU has been programmed…
Even though it worked well enough, I will need to make two improvements:
- A thinner needle for the solder paste syringe. I will order these 25 gauge (0.25 mm) needles from Farnell to be able to apply the correct amount of solder paste to the pads.
- I will need an aluminum slab (about 15 cm square and 1 cm thick) to more evenly distribute the heat. Because the hot plate heats up pretty unevenly. According to this IR camera image, there is a difference of about 80 °C between the hottest and coldest spot on the hot plate:
A special thanks to the authors of the following articles – they were a great help: