Sensors are cool, but the really cool ones come in tiny packages that require breakout boards to be easily usable. I often buy (populated) breakout boards from one of the many hobby electronics vendors out there, but they are often in the $20 to $60 range (like this LSM303 breakout board), which is rather prohibitive.
Instead, you can obtain all components and make the breakout board yourself (almost) for free.
Looking for pictures? Here are all of them.
I ordered an LSM303DLM 3-axis magnetometer and accelerometer (i.e., six total axes of sensory input) as a sample from ST.
(Why? Because my omnidrive maze solver is currently too fat to navigate the maze without colliding into walls and corners. I thought I could solve this problem by adding a magnetometer to keep track of the robot’s heading.)
Initially, I modified SparkFun’s LSM303 breakout board to use 10 mil traces instead of 8 mil and planned to scavenge the 16 components (various capacitors, voltage regulators, and resistors) off scrap circuit boards. Then I realized that the board was unnecessarily complicated and cut it down to 5 components, resulting in a 0.5″ x 0.6″ board.
My EAGLE files can be found here.
There are tutorials available online for using a laser printer, magazine paper, and a clothes iron to transfer toner onto copper, so I will not describe it in detail here.
Pressure is key for successful toner transfer! Once I figured this out, everything else was easy. Obviously, the toner will not stick to the copper if it does not touch it in the first place!
Ironing the board is best done on a hard, flat surface like a wooden board. I waited for about 5 minutes with my upper body weight on the iron to melt all the toner. Then I pressed, rubbed, and pressed again to make sure that all of the toner had stuck to the copper. I took special care to press down on the corners of the board with the tip of the iron but was careful not to rub so hard that the toner would smear around underneath the paper.
Once I was sure all of the toner had stuck to the copper, I let the board cool and soaked it in water for 5 to 10 minutes. I found that if the toner had transferred properly, I could pull the paper off almost carelessly without pulling the toner with it, and the toner stayed on even under mild rubs of the finger (to remove any remaining paper).
Occasionally, it was necessary to make fixes using a needle. Paper does not dissolve away in the cupric chloride, so I cleaned this off, as well. A sharpie can be used as etch-resist to draw in missing traces, but I have found that it often flakes off in my cupric chloride etchant before the etch is finished.
Cupric chloride etch
I had ordered (forever ago) only one-sided copper board due to cost. For this reason, I designed the breakout board with a minimal number of vias, but I decided to try making a two-sided board by printing the component and copper layers and supergluing the two sides together. I drilled all five holes by first using a very small drill bit, lined up the holes, then drilled through with a #68 drill bit. If drilling by hand, it is much easier to avoid breaking drill bits if the bit is in the chuck such that only about 5 mm of the bit is sticking out of the chuck.
Surface mount components?
I do not have boxes of surface mount resistors and capacitors lying around, let alone reels of through-hole resistors. I do, however, have lots of scrap circuit boards scavenged from old monitors and computers that are covered in these. I used a hot air gun to collect them all and used a multimeter to find the appropriate capacitors. (The resistor values are labeled.)
Solder paste should make populating small boards like this easy, but not if you don’t have a stencil. Admittedly, this was my first time using solder paste, so I found it difficult to control the amount of solder I applied.
Anyway, I instead put little balls of solder on the IC pads by running normal solder (i.e., from a spool) over them and tinned the board. It was then a matter of carefully aiming a hot air gun at the bottom of the board until the solder melted (I thought it best not to heat the IC directly if I could avoid it). I placed a small smudge of solder paste on a corner of the PCB as I did this so I could tell when the solder had melted.
It took me two tries to get this right. On the first try, I let the board cool on its own, hoping the chip would seat itself properly (flat against the board and at right angles to the edges). Unfortunately, the chip ended up tilted and misaligned. On the second try, I quickly but carefully placed the board on a table and pressed down on the chip with tweezers (solder was still molten at this point) so it would seat horizontally on the board.
BEFORE soldering on any other components, I checked that nothing was shorted out. As an electronics newbie, I found it interesting to note that none of the pads of the sensor chip itself was continuous (as measured by a multimeter). This simplified the process of finding shorts, as having the chip soldered to the board made no difference regarding which traces should or should not be continuous.
I connected the via using a strand out of a piece of stranded wire.
An interesting problem when soldering through-hole pads on DIY circuit boards is that since the holes are not plated through, I have to be careful with how I solder on through-hole pins. Soldering a pin on one side of the board does not connect it to the other side. I spent an hour growing more and more afraid that I had toasted the chip before I realized I should remove the plastic part of the pins and solder the pins to the other side.
Categorised as: Projects