Last Monday I shipped the first batch of orders for the Mixed Scale DIY DRO Kit, so those of you that have placed an order have likely already received it. I know that many of the people who ordered the kits have electronics experience and won't need a step-by-step assembly instructions. With that said, I designed the kit to be very beginner-friendly, so in this post I will go over the assembly process in some detail, so someone with limited electronics experience should be able to follow it and end up with a working DRO controller.
Required Tools and Materials
Soldering iron - quite obviously, to solder the kit you will need a soldering iron. It doesn't have to be particularly powerful but should have a tip that is small enough to solder IC pins individually. If you don't already have a soldering station, I recommed Hakko FX888D or similar.
Flux core solder - I recommend Rosin core 63/37 solder wire in 0.030" diameter for through hole assembly. 63/37 stands for 63% tin, 37 % lead. This particular alloy is "eutectic": it goes from liquid to solid state almost instantaneously. Another popular alloy, 60/40 has a "plastic" stage, where it's no longer liquid, but not yet solid, and when the pin moves during this stage, solder joint can develop cracks or voids. Unless you are good at soldering, I'd avoid lead-free solders, as they are a bit more challenging to work with.
Wire cutters - after the parts are soldered, the leads need to be clipped using wire cutters. Alternatively you can pre-cut the leads to the desired length with scissors before soldering.
Lead forming tool (optional) - if you want your resistors to be straight and neat, a simple lead bending jig will make the job much easier. Otherwise a pair of needle-nose pliers or sturdy tweezers will work as well.
Soldering is one of those things that looks easy, but if not done right will make the finished product flaky and unreliable. The two biggest issues you can have are solder bridges and colder solder joints.
Solder bridges are caused by applying too much solder and not enough flux. Fortunately they are relatively easy to detect using visual inspection and/or a multimeter.
Cold solder joints are much trickier to avoid and detect. They can be caused by both too much heat, and not enough heat. In the former case the flux is burned off and/or a brittle intermetallic layer forms. The join might look reasonably well on the outside but might crack later and cause reliability issues. The latter case can be avoided by heating the lead and the pad at the same time, and then touching the solder to the lead and making sure that it flows to the pad as well, instead of applying it to the soldering iron's tip. Moreover, use a lot of flux. It will look ugly and require extra cleanup but you will be happy you did.
|When soldering, heat up the pad and the lead at the same time|
General Assembly Tips
Putting the DRO kit together should be pretty straightforward, but before we begin, I'd like to cover a few basics. Some are more obvious than others:
First and foremost (and this one should be pretty obvious): you will be working with a hot soldering iron and molten metal that has potential of splashing into your eyes, so wear proper eye protection and avoid touching hot things.
When bending the leads of the ceramic capacitors (small yellow axial parts) and even resistors, make sure the lead doesn't bend too close to the body. These parts are prone to stress fractures that might be difficult to detect but will eventually render the part useless. In lieu of lead bending jig, hold the lead with a tweezers or needle nose pliers next to the body and bend the protruding part.
|To avoid micro cracks hold the lead with a tweezer|
Electrolytic capacitors, LEDs and ICs are directional, so make sure your orientation matches the silk screen (and the pictures). If installed incorrectly, ICs are a giant pain to desolder, LEDs won't work, and capacitors will explode.
The kit uses three kinds of resistors that attached to three different strips of tape. They have different values and should not be interchanged. The values are encoded using color-coded bands on the body of the resistor. Fortunately you can identify the resistors for this kit by count - there are 17 47 K resistors, 8 5K resistors, and 5 220 Ohm resistors. They are not directional.
Resistors and capacitors come attached to paper tape. Using scissors or wire cutters, cut them off the tape leaving the leads as long enough to be easy to work with. If you've bought or made a lead forming jig, bend the leads to the correct size. Otherwise use a pair of needle nose pliers or tweezers to bend the leads as shown below. The point here is to avoid putting stress on the component body, as it might cause micro-cracks that will lead to a component failure. If you don't have needlenose pliers, a wooden ruler can be used in a pinch as shown below.
|Old scissors make short work of cutting resistors off the tape|
Once the leads are formed, insert the parts into their locations and slightly bend the leads outwards on the back side, which will keep the parts from falling out, etc. This way you can insert several components and then solder them together.
|Bending lead out on the underside will prevent resistor from falling out|
2-row headers come as a single strip and need to be cut or broken apart. The style that I ordered for the kit has a thin connection between pairs, which should make cutting them with a knife easier. It also possible to break them apart, but I found that a chance of messing one up goes up, compared to cutting with a knife.
Finally, I'm listing the components in the sequence that was most convenient for me. The idea is to go from the pars with the lowest profile to highest. This way the board can be laid relatively flat while soldering the pins on the back side.
When laying out the board I tried to keep similar parts together as much as possible to make the assembly a bit easier and less error prone. Moreover, all resistors locations have the value indicated on the silkscreen, so hopefully there should be no confusion as to what goes where.
The only exception are the ceramic capacitors, which have to be located next to the parts for which they provide bypass path. We will install all of the resistors first, so it will be obvious where the capacitors need to go.
47 KOhm Resistors
There are 17 47KOhm resistors on the board. 16 of them are located on the left side of the board and provide virtual ground and pull-up/pull-down for the comparators. The last one is next to the MSP430 microcontroller and pulls-up the reset pin, so the chip can be re-programmed if needed.
|All 47 KOhm resistors installed (17 positions)|
5 KOhm Resistors
There are 8 5 KOhm resistors on the right side of the board, next to the microcontroller. They pull-up the comparator outputs to 3.3V, since LM339 is an open-drain chip.
|There are 8 5 KOhm resistors (blue)|
220 Ohm Resistors
There are five 220 Ohm resistors, located in the lower-left corner of the board. First three (on the left) provide current limiting for the LED. The two right-most ones are used to divide 3.3V into 1.65 V for the 1.5 scales. If the board will be used with 5V glass or magnetic DRO scales instead, these two resistors can be skipped, so the rail can be used to provide 5V to the scale Vcc pins. The most convenient way to do so is to solder a jumper as shown below.
|Five 220 Ohm resistors added (bottom right)|
If you intend to use the kit only with 3V scales, these two resistors can be skipped altogether.
0.1 uF Capacitors
There are 9 of these capacitors sprinkled throughout the board. Their function is to provide a bypass for high-frequency noise. As I mentioned before, it's very important to make sure that there are no cracks in the body from bending the pins. The board will function with some of the capacitors missing/broken, but will be more susceptible to ambient noise.
|All 0.1 uF capacitors inserted into their places|
Comparators are the two of the shorter ICs that came with the kit. These parts are directional, so make sure that the notch on the IC is facing the top of the board, matching the outline shown on the silkscreen.
From the factory the leads are bent slightly outwards to aid automated insertion, but will not go into the holes easily. The easiest way to deal with this is to gently press the leads together with your fingers until they bend slightly.
Another thing to keep in mind is that these ICs are sensitive to electrostatic discharges and should be handled in ESD-controlled environment. This is usually impossible to do at home, so try to discharge yourself before touching the leads of an IC.
The kit includes a socket for the microcontroller. Although it's completely up to you whether or not you want to solder the chip directly into the board, or socket it, I recommend using the socket. It might come handy later, if you decide to (or need to) replace the microcontroller.
The microcontroller is directional; the socket is not. Nevertheless, install the socket with the notch facing the top of the board (matching the silkscreen).
Don't insert the chip into the socket yet, though, since will make soldering the power jack more fiddly.
|Notches on the ICs should all be pointing up|
There are four 2x3 headers and one 1x4 header on the board. The kit comes with a strip of 2-row headers that needs to be cut apart into appropriately sized chunks.
Installing these headers is a bit of a pain, since they won't stay in the board, so the easiest thing to do is insert one at a time, turn the board over (while holding it in place) and solder a single pin. The board can then be turned over, and if the headers isn't straight, it can be easily straightened.
Alternatively, if you know how your board will be used, you can skip the headers altogether and use wire jumpers instead. You can refer to the "" post for jumper settings.
Tip: resistor lead clippings are just the right size for this.
Location of the power jack is pretty obvious (in the lower right corner of the board). Soldering it in might be a bit fiddly, since the holes are rather large and the part won't want to stay put. It might be easiest to use a piece of tape to hold the jack in place, solder one of the pins, adjust the part if needed and solder the other two pins.
There are three locations for these capacitors. The parts are directional, and will explode if installed backwards, so make sure that the light bands (negative side) are facing to the left.
|Electrolytic caps and LEDs are directional|
The three LEDs are located on the bottom edge of the board. They are directional, so make sure that the tab matches the silkscreen. The order is completely up to you. I prefer Red, Yellow, Green (left-to-right). This way probe will be red, heartbeat will be yellow and power will be green.
The voltage regulator is located next to the power jack. It should be installed with the tab facing outwards.
IMPORTANT NOTE: The higher your input power supply is, the hotter this part will get. At 13V it will give you a nasty burn quickly. I recommend keeping the input power as close to 5V as possible, or installing a heat sink. A piece of aluminum (preferably with some ribs) bolted to the tab should do the job.
The kit comes with a HC-05 BlueTooth transceiver. The transceiver has six pins, but only four pins in the middle are used. Depending on your installation you can either clip off the unused pins and solder the module directly into the board or use jumper wires and mount it away from the board.
|Only inner 4 pins are used; outer pins can be clipped off|
Depending on the size of the box you've chosen, the BT module can be mounted standing up or flat. In this case I've used a pair of liers to bend the leads straight and mount the pert flat to keep the profile lower (I will be shipping this board and standing up the part is too tall for a small box), but quite obviously the layout takes more space that way.
|Mounting BT module flat will keep the profile low but takes more space|
If everything went well, the board should be ready for the first power up. It might be a good idea to leave the MSP430 chip out initially. When the power is applied the power LED should be lit solid (assuming the LED is installed correctly). If that is not the case, unplug the board immediately and inspect it for the shorts.
If the test goes well, it's time to insert the microcontroller and configure the jumpers. For more info on the speciffic settings please refer to the "Do-it-yourself DRO Mixed Scale Controller Kit" post
Once done, power up the board agai. Fraction of a second after the power is applied, heartbeat LED will start flashing once per second and an LED on the BT module will start blinking slowly. At this point the board is ready.
|Finished kit with jumpers set for 1.5V scales or calipers|
Building the kit is fairly straightforward, but requires some soldering skills. If everything goes according to the plan, the whole process takes around an hour, give or take. As long as the components are installed correctly and have good solder joints, you should not expect any problems.