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Previous Content:

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors

In The Air: Episode 5 - PCB Design

In The Air: Episode 6 - Getting Ready For PCBs

In The Air: Episode 7 - Still Getting Ready for PCBs

In The Air: Episode 8 - SMT Population

In The Air: Episode 9: Pump Control




The original concept for this project was to develop a wireless node capable of determining PM10 and PM2.5 levels. The PM10 and PM2.5 quantities are globally recognized as the standard for measuring air quality, and as such the demand for robust embeddable modules is increasing. A CC3200 launchpad would be coupled with a particle counting sensor and a pump to create the node, and it could be embedded into any unit a user desired for monitoring air quality. I'll take you through the project progression chronologically (First blog post introduced my idea and disclosed my credentials). Below is a picture of the Air Quality Node created from a custom Booster Pack PCB and a CC3200 Launchpad. Note, the sensor and pump are not pictured.


CC3200 Launchpad showing the Particle Counting Booster Pack (click to zoom).


The Beginning

I have experience working with surface mount components, but usually I have access to expensive equipment to use such as microscopes and manual pick-and-place machines. In this case, the budget was $500 and I only had a fine tip soldering iron to solder with. I did my initial pricing research for instruments and tools and came up with a list of items to purchase (second blog post):



I ordered these parts and I revealed to the community this same list. I did this because not everyone would have experience working with surface mount and I thought it would be useful. The items were delivered promptly to me. At this point you might be wondering why the surface mount equipment was even necessary. Texas Instruments was a sponsor and had a list of suggested components to use in the challenge. All of the components were only available in surface mount packages, so it was obvious that some surface mount work was likely to occur.



I started working on the Particle Counter Booster Pack schematic and PCB, and in the meantime I did a primer on using the surface mount equipment I ordered. I made it video based, showing how to remove components and how to reflow using solder paste. WURTH ELEKTRONIK was another sponsor in this competition, and provided some really great design kits for their product line of inductors. In fact, there were so many choices it required a decent amount of investigation to select the appropriate inductor. As such, I did a blog post on the selection of inductors and which I had chosen for my design. I was doing a mixed-signal design (analog and digital) so inductive isolation/coupling was necessary. I created the graphic below to aid in my selection, and I also talked about the different benefits of each inductor type.


WÜRTH ELEKTRONIK Inductor Selection (click to zoom)


As part of the inductor post I also talked about using inductors on PCBs for such things as filtering, ground bridging, supply coupling, and buck regulators. At this point I completed my PCB design. and of course, I blogged about it. This post included text, pictures, and videos. I explained some basic PCB design guidelines and how to practically separate analog and digital portions of the circuit board. I've reproduced a 3D rendering of the board below:


3D Render of the Particle Counter Booster Pack PCB (click to zoom)


Having finished the hardware design, I completed my bill of materials and placed my orders:

  • I placed an order from Newark (Element14) for my components with my remaining budget. This included a lot of the passive devices and one function generator. I'll discuss the generator later. This order was placed on Dec 4, 2014.
  • I placed a sample order all of the ICs from Texas Instruments, but I did it through Element14. Basically, Element14 requests the samples on my behalf. Since Texas Instruments was a sponsor of this competition I used only their ICs in the design. There were a total of 8 different ICs, and I asked for 5 of each. This order was sent out around Nov 29, 2014.
  • I placed an order from Wurth Elektronik for some headers, terminal blocks, and two capacitor design kits. I can't get an exact date for the Wurth request because I did it through the E14 website and the message just says "3 months ago".


I ordered my PCBs from Seeed studio because I had already depleted my budget, and the boards were only going to be $15 USD. Wurth's capacitor design kits showed up very quickly, and the PCBs arrived in a timely manner.



It was at this point that I started waiting for things to be delivered, and while I waited I worked on what code I could for the CC3200 Launchpad. I waited until the new year (2015) before I started inquiring about my orders because there is always a holiday rush to deal with. Then one day a box from E14 showed up! I opened the box, and didn't recognize any of the parts. I e-mailed E14 about it, and it turned out they were parts for Peter Oakes' power supply project. So, my order was resubmitted. In the mean time I contacted Wurth and according to FedEx the headers and terminal blocks were delivered, but I never received them. So, they sent a new batch out, and a few days later I received them. By this time it's late in Janurary 2015, and I'm getting worried that my parts from Newark and the samples from TI won't show up in time. Other competitors were having logistics difficulties as well, and it became apparent, through another competitors digging, that there was a bigger issue at play. This other competitor called Newark directly and found out his order had been cancelled because an item was no longer in stock, but he was never notified. I was curious, so I checked the availability of my items on Newark's website and, sure enough, after I ordered my components an item had gone out of stock. It was a function generator that I needed to simulate a particle sensor. I chose a different generator and sent in my order. A short while later the generator showed up, without any of the passives I ordered.


The TI samples never showed up, so Christian (from E14) ordered them directly from Newark and had them delivered to me. Only ones available from the US distributor could be delivered in a timely manner, which meant I had to sample the others personally. That usually means a lower limit on the sample quantity. It turned out OK except for the DAC7574, which is a higher cost item (about $10 USD). I could only get one of that particular chip. The ICs showed up, but I still was waiting for capacitors and resistors from Newark. I managed to get components from work so that I could populate the board, but a lot of the components are not the correct values or right size footprint.


Populating the Board

By this time it's about February 12, 2015. (almost two and a half months from when placed my orders). I populated the board and made a video about it, which was the topic of my 8th blog post. Populating went well, and while troubleshooting some initial problems I had a few inductors blow up (see below). The board was finally powered without any shorts occurring. And, before you ask, no, I do not own a current controlled power supply, but I guarantee it's the next thing on my list!


Inductor Graveyard and Modified Particle Counter Board (click to zoom)


Getting the Hardware Working

To measure particle counts one needs to flow particles through a sensor. For flow control I opted for a PWM approach, and I blogged about that in my 9th blog post. I took a video of the PWM signal that drives the pump and showed an interrupt based approach to modifying the PWM duty cycle. I needed to make a board modification as well, so I made a video showing how to make a modification without ruining the board as part of that blog post. See above for a picture of the board mods.


I was moving at a decent pace once I had my board populated, but once I started programming the I²C devices on my board I found out my DAC7574 is dead. Well, it's outputs are dead. I'm not sure what happened to it, but it's definitely dead. The device acknowledges every command sequence I send it, but the outputs will just not turn on. The code comes from a known working library, and I've decoded the data on an oscilloscope. The next step will be to replace the chip, but I have to wait for those original samples to show up.



Hopefully you've made it through my wall of text and pictures to this section. I figure it will take about 4 weeks to finish my prototype once I receive my parts. I've got the code in place that creates the wireless node, which is easy since TI practically provides it, and I have also developed all the code to process the particle counting and determine the PM10 and PM2.5 levels. Mainly, I have to replace the DAC and get the hardware working. If you are a sponsor reading this for judging purposes, I apologize for not completing the project in the the specified amount of time, and I wish you the best of luck while reading about everyone's projects.


Until next time ...

Previous Posts:

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors

In The Air: Episode 5 - PCB Design

In The Air: Episode 6 - Getting Ready For PCBs

In The Air: Episode 7 - Still Getting Ready for PCBs

In The Air: Episode 8



Board is complete, and not destroying components ... anymore. I went through 4 inductors troubleshooting this one. I really should get a current limited supply. Here's a picture of the inductor graveyard.


I also lost a buck regulator in the process. In fact, it was the buck regulator that originally caused the first inductor to be destroyed. There was a short from power to ground under the regulator, and that's how the first inductor got destroyed. Then, somehow, the power supply leads got reversed, and that took out another inductor. Finally, four inductors later, the board is working. These inductors are quite handy when prototyping; they act like a fuse of sorts.



Below is the Booster Pack Header pin out. I made a mistake in my original layout, because I used the I²C lines, P1 - Ref 9 & 10, and I used the PWM available from P4 - Ref 1. I'm using the PWM signal to drive a pump. If you examine the Dev Pin numbers, you'll notice that the I2C_SDA pin is the same as the PWM pin for Ref 1. The * indicates a hardware modification is required to connect the PWM, but you can't use both at the same time. So, I made a hardware change; that is, I cut the trace for the PWM from Dev Pin 2 and soldered in a jumper wire for Dev Pin 64 instead.


HeaderPinOut.pngFigure 1: CC3200 Header Pin Out


Figure 2 shows my modification. It's a yellow wire at the bottom of the figure. If you look closely you can also see where I cut the trace beside the resistor I jumpered to. There is a second modification as well, but that's just a voltage supply mod.



Figure 2: I²C & PWM Conflict Modification. The yellow wire at the bottom of the figure is the jumper.


Pump Control


Having fixed the hardware issue, the software to control PWM was next. The CC3200 offers example code from the SDK for PWM control. I started with that example and modified it to work for Pin 64 instead of the I²C pins. I started with the wlan example and started bringing in all the functions from the PWM example that I required:

  • void InitPWMModules()
  • void DeInitPWMModules()
  • void SetupTimerPWMMode(unsigned long ulBase, unsigned long ulTimer, unsigned long ulConfig, unsigned char ucInvert)
  • void UpdateDutyCycle(unsigned long ulBase, unsigned long ulTimer, unsigned char ucLevel)


You'll likely have to add a few includes and a few defines, but it's relatively simple.


pinmux.c should have these lines in it:

// Enable Peripheral Clocks

// Configure PIN_64 for GPIOOutput
MAP_PinTypeGPIO(PIN_64, PIN_MODE_0, false);


Your main program will have something like this implemented:

UpdateDutyCycle(TIMERA2_BASE, TIMER_B, pwmValue);


I wanted to be able to control the PWM duty cycle from one of the kit's switches, so I used the button_if example from the common directory in the SDK, and this is where all my problems started. I could read the button pin, but not get any interrupt from it. I searched for quite a while on this topic, and I happened upon a forum where a guy said that in order for his interrupts to work he had to use UniFlash to completely clear the ROM on his board. So, I did this and the interrupts now work. Apparently, there can be conflict between using the OSI examples and mixing them with the examples from common. I'm not sure what happens or why, but just be warned.




Here's a video of the interrupt driven PWM control working.



And a video of my modification explanation.



Until next time ...

Previous Posts:

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors

In The Air: Episode 5 - PCB Design

In The Air: Episode 6 - Getting Ready For PCBs

In The Air: Episode 7 - Still Getting Ready for PCBs



I have received some more parts recently, but unfortunately, I am still missing critical components. I'll provide some history of the procurement here so that you can understand why I have not written a blog post in over a month.

  • I placed my first order very quickly after being selected as a competitor. My design was going to include many surface mount components, so I ordered the appropriate tools as part of the budget. Everything arrived very quickly.
  • My circuit board design was complete, and I was ready to order components.. I placed my second order on Dec 4, 2014. Christmas came and went and I hadn't received any parts, but I figured that was typical of the holiday season. I have been in contact with Christian Defoe doctorcdf27 throughout the process. I'd like to thank Christian for doing all that he could up to this point to ensure I get my parts in time.
  • I also ordered some passives from Wurth, and Fed Ex says the parts have arrived, but I did not get them. I ordered some design kits and received those, but the headers and terminal blocks never arrived. Simon at Wurth remedied the situation and the parts are on their way again.
  • I used all TI ICs in my design because they are a sponsor of the challenge. I sampled those parts through E14 from TI, but they never showed up. Christian fixed this by ordering what parts he could from Newark for me, and I sampled the rest from TI under my own name.


Latest Work


I made a video showing the population procedure. I've only placed the components I have thus far.



Here's a photo of the board BEFORE cleaning up the extra solder from the reflow process.



Here are two pictures showing the board after removing excess solder. I've plugged it into the launchpad to show how it will look in the end.




I have not powered my board yet because I was only able to sample a single piece for some components. If I destroy the component, it's game over for me! It's also possible I would damage the launchpad. I lack a current limiting power supply, so I'll have to come up with a solution.



As part of my last order I got an arbitrary function generator (AFG). I thought about it for quite a while, and decided that I indeed should get it. Hooking up a sensor up to an embedded system seems simple, but before anyone should consider doing so, the embedded system needs to be tested with a known signal. Sometimes this is as easy as using a DC voltage source, but for a particle counting system waveforms are required to test the electronics. So, I ordered the AFG because I need to verify that the system works.


I'll hopefully be testing the system soon.... Stay tuned!


Waiting for parts and working with the Launchpad CC3200. No major progress. Merry Christmas (see Figure 1). I modified the year in the picture.


Figure 1: Closed for Christmas (Source: Holiday Closure | Fowler Museum at UCLA)

Previous Posts:

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors

In The Air: Episode 5 - PCB Design

In The Air: Episode 6 - Getting Ready For PCBs



Guess what came in the mail? Two things. First, my boards have arrived, and I've taken a picture to show you (Figure 1). It doesn't matter how many boards I've designed, when they arrive it's always like Christmas.



Figure 1: The Particle Counting PCB.


Secondly, the samples and the Newark order have not arrived yet. It's a pretty busy time of year, so they may not show up for a little while. I'm still working through Shabaz's (@shabaz) tutorials on the CC3200 (part1, part2). When I installed all the software and I plugged in my CC3200 Launch Pad the USB connection did not enumerate as a virtual com port, as it does in the tutorial. I think it may be due to a newer version of the software I installed. Anyway, if you experience this as well, you need to right click the CC3200LP items under the Universal Serial Bus Controllers in Device Manager and select properties. In these properties you need to set the device to use VCP. I'm at work right now, and I don't have the Launch Pad here, so I can't get a screen shot until later. This was pretty straight forward, and I found it by messing around with the settings.

Previous Posts:

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors

In The Air: Episode 5 - PCB Design



This week involved procuring parts. I contacted Wurth for the following parts:



The first two kits are for the 0805 and 1206 capacitors I used on my board. The third item is a 2 position terminal block for a power supply input. Items 4 and 5 are male and female headers for the TI Booster Pack Interface. I used all Texas Instruments ICs on my board, so I contacted Christian (@doctorcdf) and he was able to procure samples from TI for me. For everything else, I used my budget from Newark. So, while I wait for everything to be delivered I began working with the CC3200 Launch Pad. I'm following the tutorials (first, second) created by @shabaz for the CC3200.


This leaves me with not much to talk about for this week's blog post. How about some more about PCB design, and then a little about reading? Figure 1 shows a zoomed portion of my Booster Pack design. U8, down at the bottom left, is a TINY humidity and temperature sensor. I was surprised that no one who read my last post challenged me on the layout decision for that chip.



Figure 1: Zoomed picture of my Booster Pack design.


When placing the sensor directly on my main PCB like this, it should be thermally isolated from the rest of the board. Figure 2 shows how TI suggests doing this in the HDC1000 datasheet. The white portions are slots drilled in the PCB, and this reduces the thermal mass. What's that all about? Well, for our purposes, a low thermal mass allows the sensor to respond quickly to temperature fluctuations. What I'm really getting at here, is that I didn't include any slots in my PCB design.



Figure 2: Isolating the HDC1000 and reducing its thermal mass.


Why, oh why would I have done this? I've got three reasons:

  1. I don't want to measure the ambient air temperature. I want to know the board temperature. The microcontroller might have a temperature sensor built-in, but that's on the launchpad not the Booster Pack.
  2. Slotted holes are always a pain when getting your PCB fabricated. I'm not sure why this is an issue, but every time I've gotten quotes for PCBs the manufacturers were more concerned with how many slotted holes the board contained and not the overall hole count.
  3. I don't even have a good reason to have this chip included, I just thought it would be fun to put it on the board.


I suppose this brings me to the section where I talk about reading. The more I mentor students, the more I reflect on my own education and experience. Inevitably, when I first started with a student we would have a conversation something like this:


     Student: Hey Mike, I can't seem to get XYZ working.

     Mike: What does the datasheet say about it?

     Student: Ahhh, I dunno.


I think this is a symptom of a larger problem: students can "read" without understanding. I'm guilty of it myself. I can easily recall reading technical documents and stopping only to think, "I haven't understood anything I've read for the last page and a half". Not only did I not understand it, I don't even remember what I read! It's like my "reading" was on autopilot. When I was a teenager, my father always told me RTFQ², which stood for Read the (expletive deleted) question squared, or in this case twice. Although I prefer a cleaner version, I think the principle remains true; reading and understanding is important. I have two techniques I use to help me understand what I am reading. The first is to periodically stop and ponder what I have read. It's amazing how effective this technique can be. The second is to read with a pen and paper handy to make notes about what I've read. I only discovered how helpful this was when I started preparing lectures as a graduate student.


Hope your projects are going well! Til next time.


A Riddle: How many times in three weeks can a 1 year old child catch sick? The correct answer seems to be 3 times ...


Previous Posts

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors



I have finished my Booster Pack design. Figure 1 shows the top and bottom layers. Note, I am an Altium user so the display of the board layers may be unfamiliar to you if you are an Eagle user. If you are familiar with TI's Booster Packs, you'll recognize the characteristic 20 pin headers on opposing sides of the board. In my last post I discussed choosing inductors for filtering/decoupling grounds and power supplies. Accordingly, we will discuss the layout of those inductors and separating grounds for quiet sensor performance in this post.


combo.png Figure 1: Bottom (left) and top (right) layers for my Booster Pack design.


Connecting Grounds

The general approach I have adopted is shown in Figure 3. The signal ground is like an island surrounded by the power ground, and the only way to get to the signal ground is through an inductor. The connector is on the boundary between the two grounds because the sensor requires both power and signal ground. I've created an annotated version of the board layout of Figure 1 and produced it in Figure 4. I have labelled the sensor interface, signal ground, and power ground. The power ground is directly connected to the ground of the C3200 through the Booster Pack header connections, and the signal ground is connected to the power ground through the aforementioned inductor.



Figure 3: Basic idea of the Particle Counter Booster Pack.



Figure 4: Annotated Figure 1 showing the sensor interface, signal ground, and power ground.


In Figure 5 I've zoomed in on the board so the signal ground is more visible, and the ground connecting inductor can be seen in the lower left (L2). You can't see the pads from the component because I used Seeedstudio Fusion for my PCBs, and Seeed Studio's design rules don't use a thermal relief pad connection by default. I'm not sure why, but that's the way it loaded. I didn't personally care for this project because I will be hand building the boards, so I left it alone. If I had loaded the rules before designing the board I may have changed it, but I loaded the design rules after. Disclaimer: especially in mass runs, use thermal reliefs on your ground connections to components as it will prevent tombstoning components during reflow.



Figure 5: Zoomed view of the top layer.


The 3D render of the board is shown in Figure 6. You can clearly see the pads for the inductor L2 in the lower left. I didn't bother using models for any components unless they were supplied by default, because I have no concern for height tolerances with this board. At this point in the design you may be wondering, why did he only use a single inductor, and not tie the grounds together at multiple points? The short answer is you'd be creating a ground loop, which can be a terrible problem to troubleshoot. You can look up the analysis for ground loops on PCBs, but if you only connect the grounds at one point you won't have to worry about it. You may have other issues, but a ground loop will not be one of them.



Figure 6: 3D Render of Particle Counter Booster Pack.


Power Supply Decoupling

In my last blog I spoke of decoupling power supplies as well as grounds. I've reproduced the schematic showing these decouplings in Figure 7.

You can see L2 is connecting the grounds, but what about L1; how is that implemented? In the same way. The inductor pads must straddle the gap between the power and signal ground. If you look back at Figure 5, you can clearly see L1 near the upper left straddling the ground boundary.

Supply Decoupling.png

Figure 7: Decoupling inductors


Still Confused?

How about a video explaining it then!



Final Remarks

I ordered my board. It was only $10 USD from Seeed Studio for 10 pieces. No one else could compete with that price, so I had to choose them. I've got my bill of materials ready to go, I'm just waiting to hear back about the possibility of samples from TI. Every IC on this board is made by TI, which was a bit of a challenge for me. Hopefully everything can arrive in time to put it together and write some software.



Previous Posts

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings


I'm designing my circuit board as a booster pack for the CC3200 Launchpad. It's taking a long time because I'm trying to use TI parts for every component, and the plague (read flu) just passed through my household. I'll be finishing the board and ordering it and my parts soon. While waiting for the parts, I will work on the software.




Hope on over to Newark or Digikey and do a parametric search for inductors. Ever notice voltage isn't usually a parameter to be refined? It’s always seems to be current for an inductor, voltage for a capacitor, and power for a resistor. This isn't always true, but it is in most instances. Why, do you suppose, are inductors usually current rated and not voltage? I’m going to leave this question unanswered so as to spark a conversation about it.


We were sent four kits as part of the design challenge:


  1. WE-SPC SMD Shielded Power Inductor
  2. WE-MAPI – Metal Alloy Power Inductor
  3. WE-MAPI - Metal Allow Power Inductor
  4. WE-LMHI – SMD Low Profile High Current Molded Inductor


I hope you didn’t throw the boxes away, because there is a pair of plastic tweezers embedded into the wall of each cardboard box, just in case you didn’t have a pair. Note, they’re anti-static plastic tweezers:


I’m not sure how much heat they can take, so I may have to sacrifice a pair to find out. Kudos to WÜRTH ELEKTRONIK. These kits are great. I’m really impressed with the presentation. Wurth definitely met their motto of “more than you expect”.


The Inductors We Have

Let’s have a look at the packages of the inductors we’ve been provided:


WE Inductor Selection.png

Figure 1: WÜRTH ELEKTRONIK Inductor Selection


This image is my attempt at showing the relative size of each inductor package with respect to each other. The packages are associated with a metric number like 4818. The first two digits refer to the square dimensions of the package (width and length) and the second two digits are the height. For example the 4818 refers to a 4.8 mm square package with a height of 1.8 mm. In Fig 1, notice the first three types (left to right) have the term Power Inductor directly in their names. In fact, all the inductors we have been supplied are power inductors. This is because they are typically used in applications involving power supply design. They have other applications, but by far the most common is likely the design of switching DC to DC voltage converters (think Buck or Boost regulators). The WE-SPC is a coiled power inductor (as far as I can tell) and is shielded. Depending upon your efficiency requirement the presence of a shield can be quite important. It essentially isolates the magnetic field of the inductor from the rest of your circuit. Having said that, it also stops EMI from affecting the rated inductance value. The WE-MAPI inductors are shielded as well and made from a metal alloy, likely proprietary and patented.


If you consider all the MAPI packages they cover the same inductance span as the SPC, but their package is significantly smaller. Let's take a 47 uH SPC and a 47 uH MAPI for comparison.


Parameteter74408941470 (SPC 47 uH)74438335470 (MAPI 47 uH)
Rated Current (A)0.40.4
Saturation Current (A)0.751.2
RDC typical (mΩ)9602090
RDC [max?] (A)11332300
Resonant Frequency (MHz)148


The first thing that I noticed was, for 47 uH, the MAPI inductor has over twice the DC resistance of the SPC inductors. That's perfectly fine if your application can withstand the I²R losses. I'd go with the MAPI if possible because it is so much smaller and it's 75% the cost of the SPC inductor. Note, in this instance the MAPI is 3.0 mm x 3.0 mm x 1.5 mm and SPC is 4.8 mm x 4.8 mm x 1.8 mm. Personally, I lean more towards price than anything else, so that is what makes the MAPI more attractive to me. The final inductor in our list, the LHMI, is a low profile, high current inductor. It can handle the same or more current than the SPC package, and it's very low profile.The LHMI and SPC cost about the same, so if you need the space you might choose the LHMI. The specifications of the LHMI claim "no acoustic noise and no leakage field", so maybe they are targeted at quiet power supplies for audio equipment? I've heard it mentioned to steer clear of switching regulators for audio applications, but that was back when the switching frequencies were in the tens of kilohertz. I think I will be using the MAPI inductors, because they offer the best performance for the price.


Using the Inductors


Let's assume, like me, you are designing a PCB for your project. This PCB is going to supply power to all your sensors and allow the sensor outputs to be digitized by one of your launch pads. I need to drive a pump in my design, so I used TI's WEBENCH software (on their website) to design a 12 Volt to 3.3 Volt regulator using the TPS62175. If you've never tried the WEBENCH Designer, it's pretty neat. On TI's website if you are looking at the "main page" for a part on the right side will be a link to open the WEBENCH designer for that part. Figure 2 shows the resulting regulator design.


TPS62175 Circuit.png

Figure 2: TPS62175 12V to 3.3 Volt Regulator Design from Web Bench.


You'll likely notice the inductor L3, which I chose as a 10 uH MAPI inducutor (P/N: 74438336100). You might also be wondering why there is a pin 11. That's the thermal pad on the underside of the chip. There are other ways to include that pin in the design, I just prefer to add an extra pin. That covers the regulator. Next we'll talk about an extremely important concept with regard to mixed-signal design, that is designs including both an analog and digital ground. Supply and ground decoupling. To emphasize why, let's look at Figure 3.


Supply Decoupling.png

Figure 3: Supply Decoupling (SGND is signal or analog ground)


Figure 3 shows the launch pad designators on the left (GND and LP_+5V) and my decoupled versions on the right (SGND and VCCS). I've decoupled the 5 Volts and ground from the launch pad header connector. Digital circuits are noisy; that's just the way things are. Digital by nature has higher noise immunity, so it stands to reason that the circuitry is likely noisier than its analog counterpart. I don't want any of that noise created by the digital circuitry to show up in my analog circuits (sensors). The easiest way for this noise to get to the analog parts is via the power supply or the ground connection. Given that, we separate them using inductors because our DC signal can pass unaltered and the AC signals (noise) will be presented with a high impedance. Remember the reactance of an inductor is proportional to frequency (X = jωL). This is where the term choke comes from, since AC signals will be attenuated. There are caveats here, especially with the ground decoupling, but unless you are dealing with high return currents it's a pretty safe practice. The issue that could arise is ground lifting. If the return current from the signal or analog ground is high enough the I²R loss of the inductor will lift the analog ground above digital ground. This will likely cause an offset error in your digitized sensor reading.


That's it for now, hope it helps. If you find a mistake, let me know in the comments so I can correct it.

Previous Posts


In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work



I've been combining some "Working With Surface" mount material for this weeks post. The target audience will be anyone who has soldered previously, but has not done surface mount and anyone who could use a few more tricks to add to their repertoire. We'll be covering surface mount soldering using an iron and using paste. If you're not a fan of watching videos, you won't like this post, as I have used videos to show the soldering process.


The Demonstration


This video demonstrates:

  • The setup for surface mount work
  • Removing components with hot air
  • Soldering surface mount components onto a board without using hot air
  • Soldering surface mount components onto a board using hot air
  • Using a solder sucker



The Magnifier Shows


My lamp magnifier showed up after I created the video above, so I made another short video demonstrating the use of the unit.




That's it for now. I hope it's useful when it comes time to solder your project. Good Luck!



Many of the parts we'll be dealing with in this challenge are surface mount. Therefore, it seems wise to me to blog about preparing for the surface mount board build. I have chosen a few items as part of the budget to prepare for this. Note, I am not endorsing these parts, as I have not used them yet. I will be purchasing them and I will comment on how I like them in a later blog post. Why talk about dealing with surface mount? Suppose you are working on this challenge and you've decided to use the TI HDC1000 temperature and humidity sensor, which looks like a good sensor at a great price. Sensirion modules are at least twice the price for the same resolution. I've shown the image of the HDC1000 from TI's website below in Figure 1.


Low Power, High Accuracy Humidity and Temperature Digital Sensor - HDC1000

Figure 1: Texas Instruments HDC1000.


This chip measures 2.04 mm x 1.67 mm. Grab a ruler and look at it, this chip is TINY. So, to deal with tiny stuff we will need the appropriate tools, which is why I started this blog series on surface mount soldering.



So, here are the basic parts I would choose considering we have a $500 budget. I prefer to work under a microscope, but we'll see how things turn out. C'est la vie.


A Preheater - TENMA 21-10135 MINI SMD PREHEATER, 100-350 DEG C TENMA 21-10135 MINI SMD PREHEATER, 100-350 DEG C



Solder Paste - CHIP QUIK  SMD291AX10  SOLDER PASTE, 63/37 Sn/Pb, 35G CHIP QUIK  SMD291AX10  SOLDER PASTE, 63/37 Sn/Pb, 35G




If your budget is running tight, you could exclude the preheater. The preheater just makes it easier to solder because it holds the board for you and provides heat from below while you use the hot air station above. If you have a stand that can take the heat, you could solder the board with just the hot air station. If your eyes are really good at focusing on small things you could exclude the lamp as well.


I've gathered the photos of the products from around the internet, but mostly Newark. I've also added a blurb about each piece and for which task it is used. Unless indicated these pictures come from the Newark website.


The Hot Air Station

This is the main tool. You definitely need this one or a heat gun. Something has to be used to heat up that solder paste for the QFN (Quad Flat No-Leads) and BGA (Ball Grid Array) packages. We could get away with an iron for SOIC (Small Outline Integrated Circuit) or QFP (Quad Flat Packs) but the QFN would be really, really tricky to do without hot air. You could use a toaster oven for the initial soldering, but you need the hot-air station to rework the board.


The LED Lamp (Picture from MCM Electronics, part supplied by Newark)

As I get older, my ability to see fine items gets worse. If you are young enough that this is not a problem, a simple desk lamp may suffice. Be wary though, some of these parts are quite small. When I do surface mount components, I use a microscope. These magnifiers are typically 2.5X magnification, so it's a bit of a gamble. At $26, it's not very risky though.

Desk Top LED Magnifier Lamp with Flexible Gooseneck- 60 LEDS - MCM Part #: 21-10255


Solder Paste

You'll find two basic containers for solder paste. The syringe and the jar. If you are having a stencil made, you can get a jar. Try to get one that comes with a spatula or squeegee for applying solder. Most likely, since you're prototyping, you'll want the syringe. This allows you to deposit solder directly onto the pads of the PCB.


Dental Picks (Picture from MCM Electronics, part supplied by Newark)

These tools are for positioning the lCs and for scraping away any solder balls that do not liquify in the heating process. They're just really handy for surface mount work.




These are for placing the components onto the PCB and sometimes holding components while you hand solder something. You can use these for positioning as well if you don't want to use the dental picks.


The Preheater

You can see the board holder on the top of the preheater in the picture. The preheater heats the board from the bottom. You set the temperature just below the melting point and use the hot air to finish the job from the top. You can get away without the preheater, but you might not like the look of your board when you are done. Components like Tantalum capacitors and some ICs will "bake" without using the preheater.


Precision Knife (A.K.A. X-acto knife)

Who couldn't use a knife like this when prototyping?


Final Thoughts


That's it for this blog post. I'll be designing a board in this challenge, so the next blog posts will likely be about surface mount soldering techniques. Sometimes it's difficult to learn without doing. I've been working with RF and surface mount components for about eight years now and I'm still learning. If you're designing a board for this challenge, just consider how small the components are before deciding on a part.

I've been selected as a finalist for the In The Air Design Challenge. Firstly, I must provide this disclaimer:


HalTechLogo.bmpI am the Senior Electrical Engineer for Hal Technology. I design hand-held and portable gas meters and particle counters. My idea for this project is definitely not a novel concept, but with the improvement in technology it is more viable. Essentially, as is stated in the project descriptions, I will create a Particulate Matter meter to be embedded in Heating, Ventilation, and Air Conditioning units. The result will be a smart home based sensor that can be used on a WiFi network to monitor a household's air quality.


Good luck to all the competitors.