Building North Paw V1.5
Step 1: Electronics
1: Assembling the circuit board …
|The little electronics bag includes all of this.
For this set of instructions, you won’t need either the ribbon cable snap or the motors, so set them aside in a safe place.
|You’ll need a solding iron and some solder.
If you’ve never soldered before, check out this awesome Soldering is Easy! comic tutorial. That’ll be enough to get you past Step 1, but in Step 2 you’ll need more advanced soldering skills. Practice makes perfect!
|The North Paw board is pretty well labeled as to where the various parts go, but there are several tricks, so be sure to read at least the section about the compass jacks. From here forward, if I refer to a place on the board, I am assuming that the board is oriented as shown in this picture: with the sensebridge logo upright and at the top.
Start soldering with the shortest components first… this makes it easy to turn the board over and solder on the bottom, where nothing is in your way. So we start with the resistors. Two of them are the same, and one is different, just look at the colored stripes. The one that is different is the 330 ohm, it goes near the top left corner of the circuit as shown. All three resistors are shown in this picture overtop of the area that they go on the board, see the next step for more instructions about the “reverse” resistor.
|The first 10k ohm resistor goes on the underside of the board, as shown. That’s the meaning of “reverse” in the silk-screen. Insert the leads from the bottom, then bend them a little outward so that they stay in place by themselves. This makes it easier to solder them.|
|Once you have soldered them, clip the extra lead length off. Don’t do it as shown – instead hold the lead as you snip, so that it doesn’t shoot off into someone’s eye :-). You’ll want to clip these ones especially close to the board because later the ATMEGA will be on top of them. Solder the other two resistors in as well.|
|Solder in the JST connector. The holes in the PCB are a little bit small for this part, so it may take some pressure to get the leads through, but I assure you it is possible. Once through you can solder them from the bottom as usual.
(Some older kits have a 3xAAA enclosure instead of the LiPo battery. If you have this kind, you’ll simply be soldering the leads through the same holes as the JST would go. The red lead needs to go into the hole marked with the +. I recommend however that you delay doing this until near the end – otherwise the 3xAAA is going to be a constant pain as you flip the board over and over.)
|Find the USB jack and solder it in now. Use the single 1×3 jack as a prop if necessary to keep the USB in as you solder it. It requires a lot of solder to make the big holes full. There are three little pins that you will not solder at all – these are the data pins, which our circuit does not use. Do not attempt to solder them – because there are no pads, it will not work. We use the USB only for the 5V power, to charge the LiPo.|
|The 1×3 female header jack goes on the bottom of the PCB, facing out to the hole where the compass module will sit. Place it from the bottom then solder from the top.|
|The other half of the compass module jack is a 2×3 female socket. To use it you must first clip off the smaller, inner set of leads. This is so that when you solder it in from the top, only the outer row solders into the board – the inner row is already taken by the 1×3 female socket you just soldered in. Clip the inner leads off carefully, you don’t want to accidentally clip off any of the outer leads!|
|This is what it should look like when you are done.|
|Place the 2×3 into the board and then slot the HM55B compass into the jack. The ROHS mark should go towards the small part of the board (where the green LED will be). If there is no ROHS mark, look for the three little resistors in a row, that side goes towards the green LED.|
|Once the compass is mounted, you’ll see that the 2×3 is actually held out a little distance from the PCB. Square everything up as best as possible, then solder the 2×3 headers to the board.|
|There are three capacitors in the LiPo kit (only 2 in the 3xAAA kit). The large one goes in the hole nearest the USB jack (and is not used in the 3xAAA version). The positive leg goes in the hole nearest the USB jack. If one leg is longer than the other that’s the positive leg. If they’re the same length, there will be a tiny + mark on the front, above the positive leg.
The other two capacitors go in just above the compass socket and beside the ATMEGA at the bottom right, and their orientation does not matter.
|Once they are all in, bend their leads as with the resistors to hold them in place, then solder and clip the leads.
Apparently here I soldered in the switch, without taking in any photos. It’s very similar to the USB jack in procedure. Sorry for the lack of photo, but hopefully by this point you can handle it on your own. Thanks to Sai for pointing out how it is magically soldered in by the next step
|The little blue part with three leads is an 8MHz resonator. It goes between the ATMEGA and the motor socket, just below the sensebridge logo. Solder from the bottom as per usual.|
|The red LED goes next to the MAX1555 (the tiny surface mount part that came presoldered on the board, or alternately if you have the 3xAAA kit, the spot with the little pads that don’t have holes) . The flat side goes to the right, towards the ATMEGA, just like on the silk screen. Another way of saying that is the the long leg goes into the hole near the USB jack. Solder it in and clip the leads as usual.|
|Place the green LED into the holes next to the switch, with the shorter leg closer to the switch. For most kits the flat side of the LED matches the silkscreen on the board. Some kits have a green LED in a nonstandard shape where the flat side is actually rotated 90 degrees from the usual location on the short lead. So, for these, the flat side will face towards the compass (HM55B). Depend on the short leg if you are in doubt. Solder and clip.|
|Both the ATMEGA and the TPIC are regular PDIP parts, which typically means that the leads are angled outward a tiny bit compared to the standard footprint holes. If you grab the edges of the part and press the leads into the table, you can bend them all in at once. I usually use two hands; the important thing is not to press on the leads on the top, as if you press on just a few of them you will bend those leads more than the rest and that will be trouble. Repeat for both sides of each device.|
|Place the ATMEGA and the TPIC into their holes. Note the orientation: the little circular indent on the ATMEGA should face down, while the indent on the TPIC should face to the right. The silkscreen has little matching half-circle marks as well.|
|Flip the board over, and solder down two corners of each device first. This will securely hold them in place as you solder the rest. Be sure not to hold the iron on the pins for longer than necessary – these more complicated chips are temperature sensitive. But don’t worry too much – I’ve never actually had one fail.|
|The round barrel object is a big 100uF capacitor. It is polarized, meaning that it must be wired in a particular orientation. The stripe on the barrel is the negative side. You can think of the stripe as a big “negative” dash: -. The negative strip goes towards the ATMEGA, equivalently, the short leg goes into the hole labeled with the negative sign. This cap stores the energy necessary to get the motors started, so that their big draw on starting doesn’t brown-out the rest of the circuit.|
|Finally, the big 5×2 keyed motor socket goes in. Solder happily since after this component, Step 1 is done! Before proceeding on to the next step you should look over all your solder joints and make sure they are all nice and shiny and complete. You may also wish to clean your circuit if there is any flux on it (this will depend largely on what kind of solder you used and how much). Typical procedure involves isopropyl alcohol and a toothbrush.|
North Paw V1.5, for help contact firstname.lastname@example.org
First published May, 2010