DIY digital compass: electronics experts needed!
 

Hi,

OK, now that we have top-level conformation that the n900 has no digital compass:(, I've spent some time trying to figure out what it would take to build a digital compass and somehow connect it to the n900, so that once AR applications take off we can still use them with the n900.

I'm not skilled enough in electronics to figure this out by myself, so I hope some of you can offer some advice or ideas.

Basically I think we would need two things: (1) a small digital compass that could be held with one hand flush against the n900 case so that the n900 and the compass move together, and (2) some way of getting the compass data into the n900.

For (1), I found the following compasses that seem promising:
a. OS5000-US. Digital compass with USB interface. Best candidate, but expensive: $299, about half the price of the n900 itself! :eek:.
b. R117-COMPASS. Digital compass, but no USB interface, instead using a timing pulse and an I2C interface. Reasonable price: $57.00.
c. Dinsmore R1655 analog compass sensor. Provides analog signal only; needs analog-to-digital converter to be useful. The sensor specifications suggest some circuits for extracting the direction, and this project shows how to use a microcontroller to change the analog level to digital. It seems to be a bit involved. However, this sensor is cheap: $37.

Given one of these sensors, the next thing is to get the data to the n900. For that, I could imagine bluetooth or USB. Bluetooth would be nicer that the compass assembly could really be separate and only taken out of your pocket when needed; no need to plug it in to the n900. However I have no idea how complicated it would be to connect one of the above compasses to a bluetooth interface. Bluetooth also has the disadvantage of requiring its own power. USB would need a tethering cable or at least a plug, but might work easier, especially with the above compass (a) that already has a USB interface.

One bluetooth idea I had was to use the cheap Dinsmore analog sensor (c), hook it up somehow :confused: to a voltage-controlled audio oscillator that would produce a varying analog audio tone (mixture of the two sine waves that the sensor produces), connect that audio signal to an existing bluetooth headset's microphone, and grab the bluetooth audio signal on the n900 in software. Then on the n900, do signal processing on the (analog) audio signal to extract the orientation. This would be a wireless solution using an existing bluetooth headset with its own power supply, so no new bluetooth transmission circuit or power supply would need to be designed. It could also conceivably be made compact. Again, I don't know enough about electronics to know if this is feasible (in particular how exactly to connect the Dinsmore to a VCO and how to do the software signal processing).

Feasible? Any other ideas?

Thanks guys - the n900 looks super, but the lack of a compass is painful enough that I'm determined to find a DIY solution. After all, the n900 is aimed at hackers, right? :)

Re: DIY digital compass: electronics experts needed!
 

maybe

http://www.ocean-server.com/compass.html

They also have some sample source code for interfacing with the unit, although it is in C#

Their documentation says silabs.com has linux drivers...

Re: DIY digital compass: electronics experts needed!
 

oh, sorry, I see that the os5000 you have in your post is from what I posted above. I agree, best candidate

Re: DIY digital compass: electronics experts needed!
 

I have tried a few AR programs on my g1. They really arent all the amazing , but I think the hack to get this to work is a worthwhile pursuit from a "cool" point of view.

Re: DIY digital compass: electronics experts needed!
 

If your going to hack maybe start here http://www.dealextreme.com/details.dx/sku.11691 with a prebuilt compass its only 13 bucks. There are more expensive devices there also but this one seems reasonable. It has output the the screen I would think that leaching from the screen input and dumping it through the usb would be easier than the bluetooth suggestion.

If the device had a usb connector built in then it would always be in the same orientation for the n900 and you aug reality apps wouldn't get confused by movement of the external compass.

Epi

Re: DIY digital compass: electronics experts needed!
 

Thanks, looks interesting. However one thing I'm worried about is the accuracy of the compass in a cheap device like that. The Dinsmore sensors have two types: a cheap kind that outputs only 8 directions (N,NE,E,SE,S,SW,W,NW), and a more expensive kind that is analog and can be decoded to 1-degree accuracy.

Any idea if the clip-compass you linked is accurate to the degree, or if it's just 8 directions?

Thanks - I'll also start looking at consumer digital compasses to see if there are some that look hackable.

Re: DIY digital compass: electronics experts needed!
 

One of the other compasses that was around 20 or so had good reviews on accuracy. to see if it viable 20 vs 299 sounds like a win win situation........

Re: DIY digital compass: electronics experts needed!
 

Found an interesting link: Interfacing I2C sensors with the Nokia n800! The project page says "Attach any I2C client chip (thermo sensors, AD converter, displays, relais driver, ...) to your PC via USB". Compass (b) above that I linked to has I2C.

It says that you need to enable USB host on the n800 - which I think is not yet supported on the n900. Still, this looks like the most promising lead yet.

Re: DIY digital compass: electronics experts needed!
 

Now that the N900 supposedly has IR capability, that's one more connection type to consider. IrDA transmitter/receiver is typically directly connected to a UART, which is really easy to access in Linux.

You would need a clip to keep the IR LED aligned with the receiver on the N900, but software-wise it seems the simplest option. Hardware would consist of a simple 8-bit 8-pin microcontroller ($2 or less a piece) with an internal AD-converter and some support components. Atmel Tiny AVR series is a good fit, and the compiler suite (gcc) is free and runs on Linux. AVR gcc could even be made to run on the tablet if desired...

(Disclaimer: I'm an EE and do this stuff for living.)

Re: DIY digital compass: electronics experts needed!
 

Have a look at these, some are quite cheap http://www.ssec.honeywell.com/magnetic/products.html. For example the full 3 axis/I2C interface (it can give a 3D compass bearing) hmc5843 is only $20 here http://parts.digikey.com/1/parts/159...c-hmc5843.html
I something like that would be your best option, combined with the i2c to usb program above, or using one of the cheaper and simpler 2-axis magnetometers with a microcontroller that has an A/D converter (pic chips like that can cost about a dollar). Either option would probably be small enough that you could make it sit flush to the edge of the phone and attach via usb (so 3 axis would come in handy).

Re: DIY digital compass: electronics experts needed!
 

Thanks for the tip; searching on the hmc5843 led me to this breakout board which might be easier to work with: http://www.sparkfun.com/commerce/pro...oducts_id=9371

Also this guy is working on an I2C compass using the 5843 and has some notes on interpreting the output of the 5843.

I have to honestly admit this stuff is a bit over my head right now, but it certainly seems feasible. One thing that seems complicated is the orientation of the sensor. The vector math at the above link seems to indicate that an up-down tilt has no effect, but a sideways tilt would need accelerometer correction. Since the n900 has accelerometers this can probably be done, but I don't have a clear idea about exactly what would need to be done yet.

Anyone with more knowledge/experience in magnetometer circuits, feel free to share :)

Re: DIY digital compass: electronics experts needed!
 

The problem is that although you know the direction of north relative to the phone, you don't know how the phone's screen is orientated, so you can't show a 2D compass on the screen. Imagine holding your phone so that the camera is pointing due north. You would probably expect your phone to show N with an arrow pointing upwards, but to know what upwards means you either need to take information from the accelerometer, or tell you users to only use portrait mode, or to hold their phone flat.
Alternatively you could render a 3D arrow onto the screen which would avoid the whole issue.

To do the 2D case properly you need to consider 3 (unit) vectors : the vector from the compass, m, which points north, the vector from the accelerometer, a, which points down, and a vector pointing into the screen, s (along the z axis). What we want to do is pretend that a and s are the same. To do that you take the angle between a and s, the axis perpendicular to a and s and rotate m by that angle about that axis. Since naturally a and m are perpendicular, this should leave m as a vector in x-y as far as the phone is concerned, which then can be displayed (remember that x-y-z of the phone and compass are the same).

I bet that was clear as mud. And it won't work near the poles, or if you shake your phone. To fix it near the poles you would need to project m onto the plane perpendicular to a first.

I think the 3D option is best, especially when the hardware (omap3) is there to do it nicely. Also, the 3D vector option is actually exactly what you want for AR applications.

Re: DIY digital compass: electronics experts needed!
 

the accelerometer shows absolute orientation of the device.
all we lack is the direction.

Re: DIY digital compass: electronics experts needed!
 

i just tried to close the back case of the n900 with a uk penny behind it
i could not find a way to close it without it popping back out - ie not much space.
the case is however fairly thick ruggedised plastic, so it may be possible to lap enough of an indent for the chip to sit ontop of the battery and within the case.

(instead of lapping, a small heated branding unit could make the required indent?)

not sure if the case itself is thick enough for this but if anything is wanted inside, i think in absense of nokian engineering, its down to hardware mods.

Re: DIY digital compass: electronics experts needed!
 

Agreed. Check out:

http://www.media.mit.edu/resenv/tricorder/tricorder.htm

It was done a while ago with an N770 and an expensive compass, but the idea is similar and it looks like the same hardware might work with the N900.

Re: DIY digital compass: electronics experts needed!
 

Sorry if I wasn't clear, I was replying to Otaku, and trying to make it clearer when/why an accelerometer is needed for a compass app. The rest of my post relys on the fact that accelerometer shows absolute orientation of the device :)

Unless we can find a spare I2C bus inside the N900 (seems unlikely without some help from nokia, and even if there is a spare I2C port in theory I bet the soldering would be pretty hard/impossible) there doesn't seem like much point in internal mounting. IMHO the best thing to do would be a flat circuit board, flush to the top of the N900 and attached with a usb plug on one side and the microcontrollers on the other. Unfortunately there would be a need for some other components since the compasses run at 2.5V (USB is 5V) and need external capacitors. If a 2.5V-capable microcontroller was used, that would make it 3 chips, including a voltage regulator. The compass is 4x4mm, a chip like the PIC18LF13K50 is 7.2x7.8mm in its SSOP form (it runs on 2.5V, and supports USB and I2C in hardware, so only a few lines of glue code should be needed on the chip. It costs a massive $2.50) and an appropriate voltage regulator like a TC1014 is 3x3mm. Along with the 3 required capacitors, which will take up a few more mm^2 it would all fit on a board of ~ 10x20mm and only be a couple of mm thick. That wouldn't be that much on the 18x60mm end of the N900. It would also be a soldering nightmare with all those surface mount components, but at least the board could be one sided. Also this method would give you the ultimate power saving option of simply unplugging it.

I put the model numbers down here in case I decide to have a crack at this one day.

Re: DIY digital compass: electronics experts needed!
 

Everything I wrote above may in fact be pointless, as I suspect that the N900 doesn't have usb-otg at all, since it looks as if it has a micro-B socket. USB-OTG devices are required to have a micro-AB socket. If that's the case, then no usb host mode.

Re: DIY digital compass: electronics experts needed!
 

my opinion is; use modules from spark fun as they are easy to use.most of the electronics is done for you. I would use
1 compass
2 bluetooth module
3 microcontroler card a very simple one is all thats needed
4 power supply
5 a baseball hat and some determination
It should cost you about 200 bucks but you would learn alot putting it all together

Re: DIY digital compass: electronics experts needed!
 

The OMAP3 supports OTG and Host. The n800 did not have a AB socket, but could do host mode through software switch. I don't use OTG on my n810, but instead the host mode software switch. So its probably fine.

Re: DIY digital compass: electronics experts needed!
 

What about the sparkfun breakout board for the HMC5843 compass chip?

http://www.sparkfun.com/commerce/pro...oducts_id=9371

It has the capacitors already mounted, with just 4 easy-to-solder lines for the I2C and power supply. The board is 12.7x12.7mm: it increases the size a little bit, but could probably make the soldering feasible for mere mortals such as myself.

Re: DIY digital compass: electronics experts needed!
 

Yea, that would also be a valid option, though if you want the easy soldering option, you would also have to get the big style microchip, and you still have to find a way to hold it all together, and it's getting a lot bigger. On the other hand I'm probably overestimating my own soldering skills too, the smallest thing I've ever soldered is a ps2 chip years ago. :o
As for making a separate bluetooth one, that sounds even harder to me, and more hassle getting batteries and chargers and stuff. Also it means that you would have to hold your phone and bluetooth box in alignment or it would be completely meaningless.
Since I've been assured that it will be possible to force the usb into host mode, that would be my choice of interface/attachment.

Re: DIY digital compass: electronics experts needed!
 

Oh, right - I didn't realize that the other chips you mentioned are also surface mount. Well, I did see this link the other day about DIY surface mount soldering using a toaster oven, but I'm not sure I'm ready to pop electronics into kitchen appliances yet (my wife might also have something to say about that...)

If/when I get around to trying this, I'll probably use the bigger chips to ensure greater chances of me actually being able to build the thing. I imagine that as long as the device is small enough to be pocketable, so it can be taken out when needed, it would still be usable - though of course, smaller would be better.

EDIT - This seems to be a feasible way to solder the surface mount chips that would be involved: http://www.sparkfun.com/commerce/tut...torials_id=100

Re: DIY digital compass: electronics experts needed!
 

Bad news. Looks like the n900, unlike its predecessors, does not have hardware support for USB host. Scratch the USB plans, looks like the DIY compass will have to use IR or Bluetooth to communicate with the n900.

Re: DIY digital compass: electronics experts needed!
 

That is only for external use cases. But the OMAP inside N900 DOES support USB OTG. So, this may still work.

Re: DIY digital compass: electronics experts needed!
 

I really think you're going to want an external bluetooth unit. You really don't want to be hacking your brand new (expensive) phone by carving away the inside of the plastic back, or something equally risky.

Especially since, as Bratag pointed out, AR apps just aren't there yet in terms of maturity or usefulness.

It will be fun to build, and fun to goof around with, but maybe a BT unit stuck to your N900 with some velcro is your best bet right now...

Re: DIY digital compass: electronics experts needed!
 

I also agree with using bluetooth.

I use a cover for my n800: http://www.amazon.com/gp/product/B00..._ya_oh_product

and I think it's reasonable to stick this device into a cover which would give you a known orientation.

My gut says you could do this for under a hundred bucks, if not half that, if you spin your own board. People charge you an arm and a leg for breakout boards.
I don't think spinning your own is as bad as it sounds, and I'm willing to buy an extra toaster oven for surface mount. :)

Though prototyping with the breakout boards makes sense to do first.

Biggest concern on my part is power which may make the widget infeasible to use.

Re: DIY digital compass: electronics experts needed!
 

Bluetooth sounds good, but what chips/components would you use to connect e.g. the HMC5843 compass chip (which has an I2C interface) with a Bluetooth module? SparkFun has a BlueSMiRF Gold Bluetooth module that is pretty small, but it seems to take RS232 as input, not the I2C that the HMC5843 has. How would you interface the two? I found an I2C-to-RS232 module, which could then connect the I2C device to the BlueSMiRF, but the adapter module is as huge as a brick. That company says they are considering making an I2C-to-RS232 chip, but they haven't yet. I also found a supposed I2C-to-Bluetooth module, but I couldn't find it for sale anywhere.

There also seems to be an RS232 compass module which could perhaps directly be hooked up to the BlueSMiRF. Minimum order quantity is 50, though.

Then there's also the issue of power - hooking up some batteries to this will increase the size even further...

I've got another really, really, REALLY low-tech idea about how to do this, but I'll only consider that if all else fails. Idea: use the low-res, user-facing camera to do image processing on an analog compass and determine its orientation :D This then becomes more a problem of mechanical rather than electrical engineering...

EDIT: Another potential compass module with RS232 interface that might be connectable to BlueSMiRF: TD-CMP03. Kind of large at 40x41mm. It has tilt compensation; I wonder if it can be held vertically upright against the N900 body, or if it has to be held horizontally. No word on its power consumption. Again, comment from electronics gurus would be welcomed... is it really as simple as connecting the RX/TX lines from the compass to a BlueSMiRF, and connecting the power?

Re: DIY digital compass: electronics experts needed!
 

Is this chip a requirement?

Yes, I mentioned that.

As to the actual hardware, again I don't know exactly without going through all the options, but magnetometer tied directly to a bluetooth modem is probably not the best one from a size / cost perspective. You are also looking at modules which are expensive and bulky.

I am suggesting a custom board if there was going to be use beyond R&D.

Anyways, I'm going to wait for the n900 to be in my hands just in case this is some kind of bizarre easter egg (wasn't the radio tuner an easter egg in the n800?) In mid october maybe I take more time to look at it if nobody beats me to it..

Another question I have is if there's an existing library to take raw counts from the digital compass and turn it into a heading. I know there are differences also between true north and magnetic north and I don't know the magnitude of those differences.

Re: DIY digital compass: electronics experts needed!
 

You can call it like that. The Bluetooth module included (a quite common one, by Broadcom) also had FM tuner. Free, it just was by default disabled due to lack of driver IIRC. The likelihood of such happening with digital compass... well, I wouldn't hold my breath.

Re: DIY digital compass: electronics experts needed!
 

Sigh. I'm way too obsessive.

Let me know if I screwed anything up. Attached is costed bom + misc for board / prototype in OO and csv format.

Re: DIY digital compass: electronics experts needed!
 

Seems quite reasonable. Take a look at the RN-41 BT module, it has a built in antenna and is cheaper at digikey. I haven't looked at the datasheet but they claim a 30mA power draw when transmitting, which is less than you have estimated: http://www.rovingnetworks.com/rn-41.php

While it may be slightly easier to prototype with complete modules like arduino you will have to start again when you move to your final design. If you use one of the common, low end microcontrollers from Microchip (PIC) or Atmel (AVR) you should find very cheap programmers and free development tools exist along with loads of example code for i2c and serial comms. HTH

Re: DIY digital compass: electronics experts needed!
 

I think meant to put down the rn-21 actually because physical integration looked easier. It still needs an external antenna according to the DS. I think I pulled 100mA off the datasheet somewhere but maybe it was the wrong one, thanks for pointing that out.


The arduino has the same chip I picked out, the ATmega328. In fact given it's creative commons, starting with an arduino schematic and adding bluetooth / magnetometer to it may be the best solution.

The one thing which is different from board to proto is the bluetooth module, and I am unsure that is a big deal. In fact I think for prototyping it may be better to use what was used with the n900 push http://blogs.nokia.com/pushn900/wp-c..._guidev1.0.pdf

It is going to be in the sheet but not in the prototyping total.

I am going to update the original sheet. I would have used a smaller battery but there is nothing on sparkfun between 100mAh and 1100mAh right now.

Re: DIY digital compass: electronics experts needed!
 

So, do you think we could get money from Nokia to build what they should have put in in the first place? :P

Of course push n900 is for flashy, not for "I want to get navigation directions when I'm walking and have to stop at a crosswalk" which I see as being the first basic app.

Re: DIY digital compass: electronics experts needed!
 

Well, if that's the main goal....I think I just solved your problem!

Behold the latest in high-tech navigation gadgetry:

http://www.dealextreme.com/details.dx/sku.8388

(since satire doesn't come across in writing...please note this is intended to be VERY tongue-in-cheek)

Re: DIY digital compass: electronics experts needed!
 

I know someone who lost several job interviews before he got a GPS due to showing up hours later, and still needs a GPS for routes he has used dozens of times ... a standalone compass would not help him much. :)

Re: DIY digital compass: electronics experts needed!
 

Feel free to take this as a joke-post too, but...

With an old-fashioned compass, I guess one could build a contraption that would hold the compass in front of one of the cameras and use image recognition to get the bearing...

Re: DIY digital compass: electronics experts needed!
 

Well, I wasn't suggesting that the compass would replace the GPS..rather the bolt-on electronic compass that is being proposed in this thread ;)

Re: DIY digital compass: electronics experts needed!
 

I think it would take him at least 30 seconds to reorient himself every time he stopped. :)

Of course this might be a more interesting way of using the compass:

http://www.exothermia.net/monkeys_an...aptic-compass/

Image processing an analog compass would work if the compass is at a known orientation, but would be a hassle to use.

Re: DIY digital compass: electronics experts needed!
 

Well, lucky for your friend the N900 is also a phone so he can call a taxi rather than getting lost :p

Re: DIY digital compass: electronics experts needed!
 

The other thing being, now I can just listen to the robot voice tell me where to go on the headphones and not have to pull out the n900 at all :)