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What are you working on today/this week?

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I'm working on a reflow oven project. I wanted to keep all the mains voltage safely contained, so the triac/PSU part of it sits inside the oven casing. Whilst testing just this bit I set the oven on and just let it go, partly to see if the temperature inside the oven case (which can reach about 50C) would cause any trouble.

 

Anyway, this is what 280C does to a PCB that the thermocouple was attached to. It's only a little bit over reflow temperature, so I want expecting all the smoke and black gunk dripping out!

 

attachicon.gifIMG_20140714_191224.jpg

 

The PCB was one of the daughter boards that came with RobG's nanopad. Sorry, Rob!

Check your award list. @@Fred

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Whilst I'm here, I had an unusual day today. UK comedian Ross Noble dropped round to my house with a TV crew to ask me about my NFC implant! He did a series a while ago called Freewheeling where he was led to random things by Twitter. Well worth checking out. Anyway, series 2 will be on in December / January. I'll have to wait until then to see if I made the cut. He left my place to go find a taxidermist who stuffs mice into positions doing things like reading newspapers.

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I have been working on "otter," which is a POSIX-C app that interfaces a TTY with a relatively generic packet-based, binary protocol.  Github: jpnorair/otter

 

With the proper setup, otter can act as a client-side shell, in which the text translations occur on the client rather than the server.  The advantage here is that implementing a classic, text-based shell on an embedded device can consume a ton of resources, but the binary protocols used by otter incur only a modest overhead.  I've implemented some of them on MSP430 and Cortex-M devices.

 

otter is my first attempt using pthreads, and it was also an opportunity to try building in XCode.  The Clang/LLVM debugging is really, really nice.  I wish desperately for Clang to get more traction with embedded.

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Finally got done with job in a power plant (replacing piping manifold to turbo on 3000HP 8cyl diesel) so now, trying to debug a Monroe LA5-200 calculator. Don't have a ton  of hope. It was sold as 'working', but smoke came out the moment it was plugged in. Got 75% off due to smoke, so not a big money buy. May need to rewind motor. Eventual goal is to us it as an ALU for something. Mechanize button presses and some means to read the display, counter, and, maybe, internal register. Thinking mag sensor to count teeth as they go by or maybe measure distance as the drums shift sideways.

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Was trying MIT's App Inventor 2 to build an Android app that will talk to a OpenWRT router that in turn connect to an F5529 LP. 

 

appinventor.mit.edu

 

Since the embedded side (5529LP + TMP006 booster pack) is already talking to the TPLink 703N router via ser2net and communicating with exosite in lua, I tried to make a little front end in Android to control the 1602 LCD screen navigation (that used to be accessed only by push button on the LP), as well as a ready made web page showing exosite portal for my own LP. A new lua script in the 703N is coded, and for the Android side? 90% drag and drop in the App Inventor GUI!

 

I'm surprised how easy and convenient it is to compile an App for Android device using App Inventor. No installation is needed, just web browser and download apk. Should my Android is on a home wireless this could be easier (my phone is providing hotspot to my pc and that apparently rendered App Inventor unable to direct download the apk). It's kind of like the mbed environment.

 

For Android app to talk to embedded devices, I used to rely on RFO Basic. Now that with some success with App Inventor there is one more option. It only too about half day to make an app that can feed input from Android phone to the 5529 and control the LCD display, and show on the Android app an Exosite portal page that is constantly updating data from the 5529 through an OpenWRT router.

 

 

App Inventor seems to be able to capture data from the internal sensors of the phone like accelerometer, GPS, NFC, and orientation. These all are nice to me as they are already in the phone and are thus free sensor modules for the LPs :laugh:  

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I've been playing with NRF24L01+ modules and launchpads. I've got 'em to where they can run and send packets on a (&*@#% little solar panel out of a harbor freight solar path light, and make it through the night on a 300mAh NiCd. Next up was improving their range, as the PCB trace antennas aren't especially good (even more so when you have metal walls to punch though, like I do).

 

post-28741-0-66407400-1408982132_thumb.jpg

Cut traces, soldered mini-coax from a dead WiFi router in place.

 

post-28741-0-40060600-1408982140_thumb.jpg

Built a mini-yagi!

 

post-28741-0-74214400-1408982149_thumb.jpg

The test package, a MSP430G2553 launchpad, a JeeLabs AA power board (3.3v boost converter, extremely efficient and low quiescent current), a $1 NRF24 board, and the Yagi.

 

Works great! Over doubled the range I get through the wall, it went from ~60-80' depending on direction to 200-240'.

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You actualle got hold of a real NiCd cell?! Those have been banned for quite some time, haven't they?

Just in the EU I guess. They aren't used nearly as widely here in the US as they used to be, but they're still around.

 

From what I've read they turn out to have some very useful properties for small/cheap solar applications. Primarily that you can charge them at 0.1C indefinitely and they don't explode.

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I've been playing with NRF24L01+ modules and launchpads. I've got 'em to where they can run and send packets on a (&*@#% little solar panel out of a harbor freight solar path light, and make it through the night on a 300mAh NiCd. Next up was improving their range, as the PCB trace antennas aren't especially good (even more so when you have metal walls to punch though, like I do).

 

Built a mini-yagi!

 

Works great! Over doubled the range I get through the wall, it went from ~60-80' depending on direction to 200-240'.

Can you give details of your mini (micro?) Yagi antenna?

 

David

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Sure! What sort of details are you after?

 

Design/construction wise:

I used the excellent Yagi Calculator by John Drew / VK5DJ (link) for the basic design, the element lengths and locations and such.

Once I had an output I took my digital calipers and sketched it out on some paper.

 

post-28741-0-31748300-1409071624_thumb.jpg

 

I hot-glued the shell of a rubber duck antenna to the paper along the long axis, then trimmer aluminum electric fence wire to the proper lengths (roughly. Within 0.25mm anyway) and glued them to the antenna shell and paper.

The dipole was made using the mini-coax cable, the yagi calculator gives numbers for both dipole and folded dipole elements, I went with dipole as it's easier.

The center wire of the coax became my driven element, it was already the proper length from its previous duty as the guts of the WiFi antenna (2.4GHz is 2.4GHz, after all). I cut a measured length out of a bit of 22gauge wire stripped from a discarded 8con cable to act as the ground element, and soldered it to the coax shield.

 

Ideally I'd have a blocking collar (there's an official name for it, I forget what it is) around the end of the coax, but that's more effort than I'm willing to put in.

 

The end result you've seen above.

 

 

Now using the antenna is fairly interesting. At close range it's happy to function as a normal old dipole, though reception of signals from behind the reflector element is dubious at best and reception at angles not inside the forward facing cone the antenna is built for is between OKish and not very good. The downside to a directional antenna is that you have to aim it.

Once you get to farther away having the antenna aimed correctly becomes important. Due to how I mounted the stick there is a slight bend in the rubber bit, I had to adjust things slightly to get all the elements line up right.

 

Two especially interesting things are being able to "see" the angle the strongest signal arrives from, once you're on the outer edge of the signal range it becomes very important, a 5

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I hot-glued the shell of a rubber duck antenna to the paper along the long axis, then trimmer aluminum electric fence wire to the proper lengths (roughly. Within 0.25mm anyway) and glued them to the antenna shell and paper.

The dipole was made using the mini-coax cable, the yagi calculator gives numbers for both dipole and folded dipole elements, I went with dipole as it's easier.

The center wire of the coax became my driven element, it was already the proper length from its previous duty as the guts of the WiFi antenna (2.4GHz is 2.4GHz, after all). I cut a measured length out of a bit of 22gauge wire stripped from a discarded 8con cable to act as the ground element, and soldered it to the coax shield.

My antenna design is a bit rusty, but I think folded dipoles allow for a wider frequency band without attenuating too much. So if you work on a single (or a few very closely clustered) frequency, you'd better use a straight dipole. For a wider band (say all 13 channels in the 2.4GHz band, or even 2.1GHz UMTS + 2.4GHz Zigbee/WiFi) you'd better use a folded dipole.

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Note to yagi-uda builders: you can get more directivity by adding more elements to the front.  There are already two in the design above.  Just cut more wires of the same length and space them at lambda/4.

 

 

My antenna design is a bit rusty, but I think folded dipoles allow for a wider frequency band without attenuating too much. So if you work on a single (or a few very closely clustered) frequency, you'd better use a straight dipole. For a wider band (say all 13 channels in the 2.4GHz band, or even 2.1GHz UMTS + 2.4GHz Zigbee/WiFi) you'd better use a folded dipole.

 

A straight dipole or monopole will have no problem covering the entire band 2400-2500.  These antennas are 75 or 37 Ohms impedance respectively, and the RF source of this device is 50 Ohms, so this is easy.  I recommend this approach.  I also recommend experimenting with trimming the antennas and testing RSSI against a reference because the wire you use will have some inductance (and capacitance) and the best resonant length will be less than lambda/4.

 

The bandwidth of a folded-dipole is theoretically the same as on a straight dipole, but it many designs the aperture is made wider, which decreases the efficiency a bit and changes the polarization a bit, but increases the bandwidth.  You can do the same thing with a regular dipole by constructing a "fat" dipole.  A thing called a "bowtie" dipole is a common structure.  Mainly, a folded-dipole has an impedance of ~300 Ohms, which sometimes is preferable.

 

But any-old straight dipole should have no problem covering 10% of center frequency, which is quite fine for 2400-2500 MHz.  If you are trying to build a multi-band antenna the easiest way is to build a multi-element inverted-F monopole or perhaps a spiral antenna with multiple turns.

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