Computing stuff tied to the physical world

Archive for July 2009

Back in August

In News on Jul 16, 2009 at 00:01

This is to announce that I’m suspending this weblog for a month to take some time off. No new posts in the coming weeks, but you are of course welcome to browse around in everything that’s been posted so far.

This is my 217th post, i.e. over 7 months of daily posting about all sorts of embedded and wireless fun stuff since this weblog was started. A lot of different topics have been covered, from getting the wireless radio to work, via a couple of excursions into robotics, to the latest reflow developments here. I hope you’ve enjoyed all this as much as I’ve enjoyed diving in and writing about it all. I’m sure there’s a lot more fun waiting down the road, and am looking forward to get back to this after the summer.

Here’s my very latest progress on the new JeeLink. I don’t have all the parts yet, so this isn’t a complete unit, but you can see where it’s all going:


As you can see, there are a couple of solder bridges, i.e. too much paste. But the “big” chips both do appear to have lined up quite nicely and the simple parts are all fine. All I need to do is get the solder paste amounts down next time. Man, this stuff sure is tiny.

Enjoy, and watch out for those soldering fumes! ;)

Building the JeeNode v3

In AVR, Hardware on Jul 15, 2009 at 00:01

The v3 boards are finally ready! Here’s a step by step overview on how to assemble a JeeNode v3 kit:


You’ll need some basic skill at soldering but don’t worry, as there are only a few parts to solder on. The best way is to start with the lowest-profile part, that way you can place things flat on the table and press down to keep the parts in place. So let’s start with the 10 KΩ resistor:


Turn the board around and solder the leads:


Then carefully cut them off:


Continue in a similar vein with the four 0.1µF ceramic capacitors:


Next the IC socket, which is a bit more work. The best thing is to solder two diagonally opposite pins, then check that the socket is pushed in all the way:


If not, reheat and fix it. Then solder the remaining 26 pins:


The 10 µF electrolytic capacitor is polarized – there is a “+” marking to indicate which way it should be soldered in. I normally bend the leads first and then make it lie flat on the board:


Next the voltage regulator IC. This one needs special attention because it has to be mounted differently from what the marking on the board says. The reason is that the board was designed for an LP2950, but the kit includes an MCP1702 which has a different pinout. Here is how it should be mounted:


The 16 MHz resonator is next, it’s the tallest part on the board:


You’re almost there now. The radio module is a surface mounted module, which needs a slightly different approach. Put a bit of solder on one pad, then place the module over it and reheat to stick it in place:


Once correctly positioned, add solder to each of the remaining pads to make shiny round joints:


The 6-pin male FTDI connector can now be soldered on, I usually mount it sideways, but the choice is yours:


A simple wire acts as antenna for the radio – attach it to pin 1 of the radio module. You can bend it as needed afterwards. I used a red wire, even though the kit probably has a yellow one.

One more step and you’re done: add the four 6-pin port headers.

Good, the soldering is over. Now bend the pins of the ATmega microcontroller slightly inwards so it fits into the socket. Make sure you only press it firmly down after all the pins are in the proper position.

Voilá! Your finished JeeNode:


If you have a USB-BUB adapter, you can now plug it in and try out the board (note the 3.3V jumper – the JeeNode uses 3.3V logic signals). The ATmega that comes with the kit is pre-loaded with the RFM12demo sketch to get you up and running in no time:


That’s it. Congratulations with your new JeeNode v3!

Reflow controller

In AVR, Software on Jul 14, 2009 at 00:01

Yesterday, I presented my new reflow system. Here’s the temperature graph from a sample run:


The purple steps are the different phases:

  • Preheat (50) is whatever time it takes to reach 140°C.
  • Soak (100) is a 30-second temperature ramp up to 170°C.
  • Dwell (150) is defined as whatever time it takes to reach 220°C.
  • Reflow (200) is defined as 20 seconds at 220°C.
  • Cool down (250) is where the buzzer goes off and I open the lid.

The green bands indicate when the heater is on. The blue line is the target temperature which the system is trying to reach. The red line is the actual temperature. As you can see, the heater stops well before target temperatures are reached, and does a pretty good job of ending up in the desired range.

All in all, this appears to be ok. The 700-watt heater isn’t quite hot enough to ramp up 3°C/sec, more like 2°C/sec at best. Since the grill isn’t heating up quickly enough, the soak phase ends up taking 60 seconds instead of the planned 30, so the target stays pinned at 170°C a bit longer. This could probably be shortened by aiming directly for 170°C, but I don’t think this phase is critical. My only concerns are that it took about 80 sec to “dwell” from 170°C to 220°C and that the system is above 200°C for some 60 sec. This was shorter in other trials, but as you can see the heater is turning on a few times to nudge the system towards 220°C as things slow down a bit.

This plot was produced by a really nice free package for the Mac, called Plot (how original). The readings were obtained by logging the sketch output from the USB port and reformatting it to tab-separated numeric values.

My first reflow

In Hardware on Jul 13, 2009 at 00:01

Here’s my new super duper reflow setup:


I decided to use a JeeNode as reflow controller. This is all about hammers, nails, and dog food, after all :)

The grill is turned on and off via an FS20 remote power switch, which is controlled by the on-board RFM12B. No need to get into relays and high-voltage stuff. Total cost including grill, LCD, and remote switch is under € 100.

The whole setup can be battery powered. I used the new plug breadboarding approach for prototyping it all:


The “P” at the lower left indicates it’s currently in the pre-heat phase, now at 110° with a target of 140°.

The port assignments on the JeeNode are:

  • Port 1: serial 4×20 LCD display, based on the Modern Device LCD117
  • Port 2: NTC on AIO with 1 KΩ pull-up to 3.3V, piezo buzzer on DIO
  • Port 3: two leds: green is running, red is grill power on
  • Port 4: two switches: white is run/stop, black is select profile 0..9

I’m not using PID control. Merely did a manual test to determine the overshoot, and am compensating for that by turning off a bit earlier. It turns out that this grill has a huge overshoot: some 65°C ! … so I removed both Teflon-covered metal plates and am now using a 0.5 mm aluminium panel instead. Overshoot is now around 30°.

The system can be configured through FTDI / USB. Once set up, it can be operated without computer. The LCD display is optional but a great convenience.

Here’s the first test board (using some scrap boards from SparkFun):


This is the cool-down phase, i.e. with the grill open. The temperature sensor is taped to a separate board to track things under similar conditions. The tape needs to be heat resistant, of course… I got it from DealExtreme:


And here’s the final result:


Only used a few resistors for this trial, and spread some paste on the TQFP pads to see how that comes out.

If you look closely, you’ll see that the leftmost resistor under header pin 13 is “tomb-stoned” – it’s standing straight up as the capillary pull of one pad took over. I probably put too much paste on one pad and not enough on the other.

But the rest looks good… f a s c i n a t i n g !

I seem to be getting a decent temperature profile. Things heat up nicely, stay in soaking ramp-up mode for a while, and then go up to a few degrees over 220°C. When the grill has to be opened up for cooling down after reflow, the system beeps to draw my attention.

FWIW, the source code for this setup is available here.

There’s still an intermittent problem with the NTC hookup – I’m getting occasional flakey connections on the enamel wires, which are simply wound tightly around each end of the NTC. Can’t use solder there, and I haven’t found another solution for that. For now, the controller just powers off when the NTC reading is bad, but that’s a pretty horrible approach.

Conclusion so far: reflow is absolutely doable for hobbyists (my second try went perfectly!), with the paste being the most tricky bit. Stencils are probably very useful to apply paste quickly and evenly.

See also Stephen Eaton’s recent post, where he describes his latest results with SMD reflow.

Reflow temperature

In Hardware, Software on Jul 12, 2009 at 00:01

For my upcoming reflow experiments, I’d like to use a JeeNode to track the temperature and perhaps also control the grill. As the saying goes… when you have a hammer JeeNode, everything looks like a nail sketch.

So I hooked up a 10 KΩ NTC resistor rated up to 300°C as voltage divider with a 1 KΩ resistor pull-up. Bought a cheap and probably not-so-accurate multi-meter with thermocouple-based temperature readout. And then the fun starts – turned my grill on and watched it heat up and then cool down, while jotting down the “readings” from this “experiment”. I felt like a high-school kid in physics class again :)

The results:

Picture 1.png

Not bad. The readout is a bit coarse at the higher end, but I think it’s accurate enough for reflow soldering in the 170° .. 240° range. The grill goes up to 260°C from what I can tell.

But I didn’t want a lookup table, so I went looking for a polynomial least-squares regression fitter on the web and found this one. Great: copy and paste the 18 heat-up readings, then try out a few polynomes – turns out that a 7th degree polynome fits well, matching the measured data within a few tenths of a degree.

And then I found an even better site called ZunZun – it lets you fit polynomes and graph the results and generate the C++ code. No need to copy lots of coefficients!

Here’s what I ended up with, using a “Lowest sum of orthogonal distance (ODR)” fit:

Picture 1.png

Corresponding C++ code:

Picture 4.png

To be able to configure this thing, it would be nice to be able to enter degrees. That’s easy to do by also fitting the inverse function:

Picture 7.png

Corresponding C++ code for the inverse:

Picture 6.png

Great. Onwards!

Programming a 32-TQFP chip

In AVR, Hardware on Jul 11, 2009 at 00:01

With SMT parts, all sorts of things change. For the JeeLink, I’d like to set the fuses and flash the bootstrap code to the ATmega chip before soldering it to the board. Here’s how:


This is a zero-insertion-force socket for 32-TQFP chips. You put the chip on, press down, and the whole thing presses these 32 gold-plated contact fingers onto the chip. There are several alignment edges in there making it relatively easy and quick. It’s no doubt made for machine operation but it works nicely by hand as well.

And here’s the result – an ISP programmer which works with the Arduino IDE:


I’ll start flashing a couple of ATmega168 chips, in eager anticipation of the JeeLink boards…

Documentation area

In News on Jul 10, 2009 at 00:01

There have been some changes to this weblog since it was moved to another server a few days ago. The navigation links at the top of each page now work better, for one.

Another change is that I’ve started setting up some more documentation pages, and am integrating them into this website. Here’s what you get when you click on the “DOCS” link now:

Picture 1.png

I would have loved to get some really nifty sidebar navigation in there, but for now there are simply two mechanisms to help you out: first of all there are quite a few links between pages – this is automatic, and hence very convenient for me: wherever text appears which matches a page in the documentation, that text will be linked. The other navigation feature is the “page index” link at the bottom of each page. It’s not very sophisticated, but hopefully workable for now.

Keep in mind that searching in the weblog will not find anything in the documentation pages – these are internally two completely different systems.

Needless to say perhaps, but these documentation pages are still at an early stage. They will be filled-in, extended, and updated for some time to come.

Mounting a JeeNode with plugs

In Hardware on Jul 9, 2009 at 00:01

As promised: here’s an idea how to combine things. First let me show that plug pinout again - because I can…

Picture 3.png

An idea I’d like to present is to mount the JeeNode facing down. The board would look something like this:


Each of the four port groups has the corresponding pins tied together. With a JeeNode plugged in you get this:


And with some example plugs added:


Note how plugs mount the proper way up, i.e. lying flat on the perf-board, because the upside-down mounting of the JeeNode takes care of mirroring the connector. The only thing to keep in mind is that port numbering now runs clockwise, with port 1 in the top right.

The extra female headers can be used for testing, or as simple breadboard pins for hooking up some LEDs. Some space could be saved by leaving them out, though.

I think there could be several advantages to this setup:

  • plugging stuff in the “right” way becomes quite obvious
  • if you wire up plugs appropriately, you can chain them sideways (i.e. for I2C use)
  • mix and match: put some components directly on the board if you prefer
  • it’s all easily constructed with off-the-shelf “perf board” (with or without copper)
  • it could even be made from a piece of cardboard with a few holes in it
  • my favorite fastening system works: a couple of zip-locks to tie things down
  • for better protection, simply add a cover and leave cables coming out the side
  • the whole sandwich can be solidified with spacers and remains low-profile
  • it’s all laid out in the plane – place a paper design on the board to guide you
  • it’s easily documented: just take a picture of it…

But perhaps best of all, you can still create a completely different setup for permanent use. Just unplug everything and re-assemble it in some other orientation. So while this approach takes up relatively much space for the connectors, it does give you total flexibility to re-arrange things any time later.

I’ll try this out in a couple of projects. But even just with this mockup I noticed that having the plugs flat on a supporting surface makes it all surprisingly stable, and easy to carry around.

Plug pinout

In Hardware on Jul 8, 2009 at 00:01

Here is the standard pinout I will be using for all my JeePlugs and port connections from now on:

Picture 3.png

Do not ask how many different variations and scenarios I’ve been through. You don’t want to know. I’ve got the scars and JeeNode debris lying around here to prove it. I agonize over these choices so you won’t have to …

There are difficult trade-offs whichever way you go, but this is it. The best choice, period.

From that, several consequences follow. The most important two being that the JeeNode v3 and the JeeLink will be supplied with female headers for the ports, and that these headers should be soldered on pointing up.

If you have the v2 JeeNode, then my advice is to make a couple of conversion cables or plugs, so that you can set up all future connections using new standard. That’s what I did anyway – I don’t even have a v3 JeeNode here yet, and I’m already wiring up new plugs using the above convention. The pain of converting now is nothing compared to having a confusing mix of boards and plugs and cables later.

Depending on the power source you use, plugging in things the wrong way can definitely damage circuits. I suspect that a 5V power source such as the fused one from USB on the JeeLink is relatively safe, but a 12V PWR line hooked up the wrong way is likely to damage the ATmega. So will shorting pins 1 and 2, i.e. PWR and DIO.

If you need to work with such “high” voltages, I would suggest using 4-pin plugs where possible. In other words, don’t even bring that PWR line out to the peripherals / plugs unless needed.

Back to the pinout and orientation of plugs. With female headers on the JeeNode / JeeLink, you can see that plugs will be oriented with the components sticking out.

For plugs requiring two ports, you can mount the male headers pointing down on the JeePlugs and use them as tiny shields. Plugging such a shield in the wrong way, i.e . turned 180°, has no serious consequences. The ports 1 & 4 or 2 & 3 will simply be exchanged. Your sketch won’t work, but at least it won’t cause any electrical shorts or power-line mixups. Plugs used as tiny shields are quite robust in that sense.

That’s it for using plugs (or your own perf-board) and hooking up stuff to each port. By sticking to these conventions, it will be easy to re-use components in different configurations later.

The choice for non-polarized pin headers was a very deliberate one. They are very low cost, small, and widely available. This matters to me because I intend to hook up tons of different things over time. It just means we have to carefully pick some conventions and stick to them. Done.

Tomorrow I’ll describe another variant compatible with this which helps avoid inadvertent reversed connections and which supports chaining with a port as I2C bus. It will also make it easy to construct a more permanent mechanical “fixture” when hooking up multiple sensors, indicators, actuators, etc.

IO Expander

In AVR, Hardware, Software on Jul 7, 2009 at 00:01

Here’s a way to expand the number of digital I/O lines on a port:

3686066291_f856efcb63_o This is a JeePlug filled to the rim with tiny components and connectors. It’s based on a PCA8574A I2C 8-bit I/O expander. Each of 8 pins can be used either as inputs or as “mostly” open collector outputs. See the datasheet for details.

Here’s the bottom side of the plug:


Crowded indeed!

And here’s a demo sketch called “expander” which blinks the eight LEDs connected between PWR and the respective I/O pins:

Picture 3.png

As with yesterday’s example, this is running the I2C at maximum bit-banging speed, and seems to work fine. It’s used to make 8 leds blink:


The connector block on this plug has 8 sets of three connections: GND, PWR, I/O. Note that PWR is connected, not +3.3V. The reason for this is that I’d like to try driving a bunch of servos from this plug one day – it’ll load the JeeNode down a bit but it should be feasible.

The “expander” sketch has been added to the Ports library and is available as ZIP file and in subversion.

External memory

In AVR, Hardware, Software on Jul 6, 2009 at 00:01

For one of the projects in the Jee Labs, I needed a bit more “permanent” memory than the ATmega’s EEPROM or even flash could provide. Here’s what I came up with:


That’s a 64 Kbyte AT24C512B serial EEPROM memory which talks via I2C.

It takes only 4 wires to hook this thing up:


And this is the mess you get in when you don’t have the right connectors and need to deal with older boards:


That’s the “regret plug” mentioned a few days back, to turn the downward pointing male header into an upward pointing female header, plus a “cross-over plug” to let me wire this thing up for the new JeeNode v3 pinout, yet use it on a (minimally populated) v2 board. Yuck!

Lesson: think about headers before deciding how to solder them on …

All future JeeNodes will be supplied with 6-pin female headers for the ports. Soldering them on pointing up is probably most convenient: it lets you stick little LEDs and wires in there, and JeePlugs can be placed pointing upright – such as this memory.

Back to the memory expansion. Here’s a test sketch:

Picture 5.png

Sample output:

Picture 4.png

As you can see, the count did not start with 255, as it would have if the EEPROM had still been empty. The JeeNode was powered off in between to demonstrate data retention.

This example code runs at full bit-banging speed – seems to work just fine.

The “eemem” demo sketch has been added to the Ports library and is available in this ZIP file as well as here.

There you go … a whopping half million extra bits of permanent storage :)

Ligthy power save

In AVR, Software on Jul 5, 2009 at 00:01

Getting the power consumption down of yesterday’s “Lighty” example turns out to be quite a challenge.

One thing to do is to separate out the logic for enabling the different sensors, and extending it to also support disabling all of them:

Picture 3.png

Powering down completely works best when all internal peripherals are also turned off, as implemented in the following code:

Picture 4.png

Now the trick is to enable some interrupt source to take us out of this deep sleep phase again. This could be the ATmega watchdog, but the radio watchdog uses even less power, so here’s how to stay off for about 4 seconds:

Picture 5.png

So far so good. This disables all power consuming sensors and internal circuits, preps the radio to wake us up in 4 seconds, powers off, and then reverses the whole process.

Bug there is a bug in all this – somewhere… From some earlier experiments, I would have expected to see a power draw of a few microAmps with this code. But for some reason, it never drops below 2.7 mA, i.e. still “burning” 1/10th of full power!

I haven’t been able to figure out yet where these milliamps are going :(

For the sake of argument, let’s assume this works properly. Then the next problem will come up, which is that measuring and sending packets every 4 seconds drains more power than I’d like to. It takes several milliseconds to measure all readings and send out a packet. But who needs readings every 4 seconds?

So the solution to this is to just sleep a bit longer, using the 4-sec wakeups to quickly read-out some sensors, and calculate their averages. Here’s is the final loop of the power-saving “LightySave” sketch:

Picture 8.png

This will integrate readings for 75x 4 seconds, i.e. 5 minutes, and then send out a single packet. Note how the power-hungry radio module is only enabled at the very last moment. All we have to do is make sure it’s ready to send, then send one packet, then wait again until the send is complete.

Then the loop restarts, sleeping in low-power mode, etc.

Only issue is to find out where the 2.6 mA are going! I’ll try to figure this out, and will post here once fixed …


In AVR, Software on Jul 4, 2009 at 00:01

Let’s call yesterday’s mystery JeeNode “Lighty” from now on. As promised, here is the first sketch:

Picture 3.png

LightySerial reads out several sensor values each second and dumps the results to the serial port. Plain and simple – using the Ports library for most pins and Arduino’s analogRead() for reading out voltage levels on A4 and A5.

Sample output:

Picture 4.png

FYI, all the Lighty sketches are available for download as a ZIP file.

Now let’s turn that into a wireless version using two JeeNodes. The first step is to separate the measurement and reporting sides of things.

Here is the main code of LightyTransmit, causing it to send out its readings once a second:

Picture 8.png

It puts all values into a structure defined as follows:

Picture 6.png

… and then sends that off to the broadcast address, i.e. whoever is listening. No acknowledgements are used. If the packet arrives, great, if not then the next one will.

The receiver then takes it from there, here is the entire LightyReceive sketch:

Picture 7.png

Note that the Ports library is not needed here since the receiving JeeNode does not use ports, it merely picks up packets and reports them on the serial line.

That’s basically it. Each of these sketches compiles to some 3 .. 4 Kb code.

We’ve got ourselves a Wireless Sensor Network!

LightyTransmit uses about 24 mA. Tomorrow, I’ll go into reducing the power drain, because Lighty’s tiny 20 mAh LiPo battery won’t even last an hour this way.

Mystery JeeNode

In AVR, Hardware on Jul 3, 2009 at 00:01

Some more pictures from yesterday’s puzzle…


Added a second LDR on the left and a DS18B20 1-wire temp sensor. A 4.5V @ 35mA solar cell has also been soldered in.

The sensor is a TSL230R light sensor, as Milarepa correctly commented:


It has one frequency pulse output, with two other port pins used to control sensitivity (1x/100x) and frequency divider (2x/50x). The fourth I/O pin is used as power supply for the chip, so that it can be turned off completely in sleep mode.

Here’s the whole assembly from the side:


And here the complete JeeNode (using a different solar panel which turned out to be too weak):


So what does it do? And what is it for?

Well, two things really – but I’ll be the first to admit that this isn’t truly general purpose and unlikely to be very meaningful for anyone else.

The first use is to help me take better pictures. The photos on this weblog are all taken using nothing but a white sheet of paper and plain old-fashioned daylight. Benefits are that it’s free and abundant, it needs no space for a light-box setup, and it tends to produce beautiful images.

The problem which you may have seen on this weblog, is that daylight ≠ daylight. Sometimes it’s too dark outside, and sometimes the light is too harsh. Worse still, many of the photos end up with a blueish tint. I’m having a hard time predicting this, so it seemed like a nice idea to just throw a bunch of light sensors together and try to correlate this to picture quality over a few weeks time.

The other purpose of this unit is to act as test-bed for long-term solar-powered use. That means getting the power drain down to very low levels of course. But I also added the ability to read out the voltage of the solar cell and of the battery, as well as temperature (solar cells are sensitive to that). And LDRs facing opposite directions, to try and detect sun vs. cloud cover weather info.

This seemed like an excellent project for JeeNodes and JeePlugs. Once it’s working well enough – auto-ranging the light sensitivity and such – I intend to put this up on the roof and just let it send out packets every 5 minutes or so, day and night. This will make an excellent yet non-critical test setup as well as allow me to track solar intensity over the entire year. Think solar panels…

Tomorrow I’ll post several sketches: one showing how to read out everything and report it over the serial FTDI connection, then two more to show how to turn this into a send/receive solution with two JeeNodes, and the last one showing how to get power consumption down.

Stay tuned!

Vacation time

In News on Jul 2, 2009 at 00:01

For your information: I’ll be off-line from July 15th through August 15th. It’s that time of year for me to let go and re-charge again, far away from gadgets, internets, and computer screens. Because everything has its place, including the real world.

The intention is to have the new JeeNode v3 kits and boards available before then, but it really depends on whether the latest design works properly. I hope to find out in roughly a week.

The shop will stay on-line, but please be aware that if you order anything there will be no packages sent out during that period. Right now, the shop is still in basic start-up mode. No more, no less. With a bit of luck, I can still send out some new kits before my leave.

Ok, back to some fun stuff. Here’s a mystery JeeNode


See if you can guess what the different pieces are and what all this is for. Two hints: the chip on the right is transparent (it hasn’t been hooked up yet), and there’s a solar cell still waiting to be connected to the top and bottom leftmost two JeePlug pins.

The mystery will be solved in tomorrow’s post.

Simple plugs

In Hardware on Jul 1, 2009 at 00:01

Here are some basic uses for JeePlugs…

First the v2-to-v3 pinout converter:


It flips the +3V and AIO pins. Here’s the bottom view:


Not pretty, but robust, and it works. Here’s a v2 JeeNode which will allow me to use plugs designed for the new v3 pinout:


The shrink-tube yellow bands are a reminder that these are “crossover” plugs. Better avoid major confusion later, once there are more plugs all over the place…

Here’s a weird one, I’ll call it the “regret plug” because it lets me use JeeNodes with a header orientation I’m now regretting:


It lets you turn a JeeNode with downward facing male pin headers into onto one which has female headers pointing up, or male headers pointing sideways. Like so:


This is an odd contraption, but I’m sure it will come in handy one of these days. Note that there are no wires across the plug. Each side is independent and simply has 3 headers with all the corresponding pins soldered together.

The outer female headers have been soldered on at an angle to allow attaching to it – otherwise the JeeNode board edges would obscure the connector.

Note – Apologies for the confusing posting/re-posting today. I knew I had a post ready, but it got back-dated to June 1st, instead of July. Should all be fixed now, but some RSS readers might cache the previous post.