How I design an electronic product

I thought I would share the process I go through to design an electronic product.  I have been doing this type of work since the mid 1980s and while the tools and parts have changed, the process has remained much the same.  As an example I will show how I designed my own product which is a Differential Temperature Controller designed for solar heating systems.  

This device controls fluid circulation pumps that move heat from solar collectors to storage tanks.  I sell these products from my ART TEC Solar web site.

I begin by designing the schematic in CAD (Computer Aided Design).  This is the hard part that requires a lot of research to find the right components and then figure out how they interconnect.  In most products these days there is a microcontroller chip that is programmed to perform all the functions.  More on that later.  Selecting the right microcontroller begins by deciding how many things it needs to connect to or control.  These include buttons, LED indicators, LCD (Liquid Crystal) text display, and connections to external parts like pumps, sensors and power source.  The schematic drawing defines how all these parts connect to each other.

Next I design the circuit board (PCB) in CAD.  The layout software imports the signal connections from the schematic making it easier to ensure all the right connections are made.  This PCB is the physical embodiment of the schematic and involves carefully placing the parts such that the interconnections are optimized.  Key parts like the controls and screw terminals need to be placed for ergonomics and easy access.  For me, this is the fun part because it is a lot like creating an artwork and I enjoy making an aesthetically pleasing layout.

The location of every part has to adhere to electrical AND functional rules and the size of the circuit traces has to be scaled to the amount of power it has to carry.  Circuits that carry more current are wider like the green ones along the bottom in the design above. The green represents the conductive traces on the bottom side of the board, and the red are those on the top of the circuit board.  A great deal of thought goes into every minute detail to optimize size and cost.

The parts need to line up to make assembly by robot or hand easier.

Once the design is complete the file is sent to a fabricator that makes the bare circuit board.  This is a fiberglass board with plated copper traces connecting the holes where the components get inserted.  For volume production, they are tiled up into groups to facilitate machine assembly.  

Once an assembly machine has been programmed to insert all the parts, it can just copy the sequence to the rest of the boards on the panel.  For hand assembly, boards are separated and parts are inserted by hand and then hand soldered on the back.  In my design, parts are installed on both sides.

Once a prototype is assembled and tested, the next step is to write computer code for the microcontoller chip.

I write in BASIC language and have been coding in some form of that language for over 40 years.  The code defines the functions of how the device responds to inputs like buttons and sensors.  The heart of the code defines the functions of what the device does.  In this case how it responds to sensor readings and when it decides to activate a pump.  It also displays real-time temperatures on the LCD and all the interactive menu features.  The code is them compiled into machine code and downloaded into the flash memory in the chip in much the same way you save a file to a thumb drive.

Finally, I design the front panel and case.  Often for my clients projects, I work with a product designer who does 3D CAD design.  In this case, I used an off-the-shelf basic box and set up tooling to make cutouts using the woodworking equipment in my workshop.  It would have been prohibitively expensive for me to design a custom enclosure and have molds made for injection molding a low volume product like mine. 

The front panel of this product is a membrane that I designed and had fabricated.  It is flexible so that a light touch can move the membrane enough to actuate the switches behind.  It has clear windows for the LCD screen and a green LED indicator.

The final step is to assemble the whole product.  All the parts get screwed or glued together and the membrane is adhered on to the front.  Here is one that I use in the solar heating system for my workshop.

The process of developing this product took dozens of hours over several weeks.  Finally I have a product that has done relatively well in the niche market of the DIY solar heating world.  I set everything up to be scalable from making them by hand to volume contract assembly.  In boom years I contracted an assembly company to make batches of 100 pieces.  Otherwise I just build them by hand myself and it takes me about 30 minutes to assemble.  I call this my "get rich slow scheme" as orders come in almost every week. Over the years I have sold over 200 of this particular model and have two other models that have sold over 1000 pieces total. 

Many of my clients start out by ordering 1000 products for test marketing, then scale up once they have created a demand.  Most volume manufacturing is done off-shore, but for under 1000 pieces there are companies in the US that can be competitive.

If you have a great idea for an electronic product, visit my Product Design page and contact me.

Replacing the battery in my Kindle Fire HDX 8.9"

I have had my Kindle Fire HDX for over 4 years now and I like it a lot.  It is my go-to device for social media, email and web browsing while at home.  Recently the battery has been lasting only a few hours - previously it would last over a week of normal usage.  So I decided to replace it.  I found a deal on ebay for about $25.00 which is a LOT cheaper than replacing or upgrading to a new tablet!  I found a instructions on ifixit that shows how to open up the Kindle and replace the battery and it did seem a bit daunting, but I'm an engineer dammit!  It turned out to be quite challenging, but I got it done without ruining the tablet.  At one point I did puncture the batteries (there are 2) and smelled a strong solvent odor, but nothing exploded or got hot so I proceeded.  Those batteries are glued in there quite firmly, and it took about 15 minutes to pry them free.  Replacing them and re-assembling the case was relatively easy.

From a sustainability standpoint I'm pleased with the outcome.  I can responsibly recycle the old batteries at Lowe's.  Also I'm not contributing the the consumer culture that drives people to upgrade their devices every year or so.  Plus I have delayed the day that I will have to recycle this great tablet, and I saved a bunch of money.  I do wonder how these things are taken apart and recycled given how much trouble I had.

Here's a brief photo summary of what I did:

 Here is the replacement battery and the relatively useless tools that came with it.
 

The blue spudgers broke and I ended up using a flat blade screw driver and my trusty Swiss Army knife to pry the case open.  There was a tiny specialty screw driver tool that was needed to remove 4 small internal screws, but it was for a smaller screw.  I was able to grind down the tip to make it fit.

Here's the case opened up and the display disconnected and off to the left.

  

You can see that I had to do horrible things to remove the batteries!

 

Here's the back of the device after I got them out. 

I don't recommend this for the faint of heart or those not "tool enabled".

Solar and snow

The first morning after a snow storm is often sunny.  This morning (Feb. 8, 2018) was a good example and I got up early so I can clear all the snow off my solar panels and collectors.  Yesterday's total was about 8" capped with a layer of ice.  I have a snow rake that can reach up about 25 feet with multiple extensions.  Each different type of snow requires a different strategy.  Sometimes I can just whack the panels and it all breaks loose in big chunks.  Other times I have to chip away at it.  Today I worked my way up from the bottom.  Some big areas broke loose and came down hitting me in the legs which is why I wear waterproof slickers.  That stuff is heavy!

Here's a time-lapse of the process:

Obviously the sooner I get everything cleared, the more free electricity and heat I get.  Maine is at the 43rd parallel and we get a lot less sun in the winter so I want to optimize every Watt.  My electric bill is at the minimum connection fee for most of the spring/summer, but jumps up quite a bit in the winter due to my increased use of electricity for heating and the reduced solar.

Repairing the tankless propane boiler for my solar augmented heating system

4 solar collectors shown at right

When I moved to Maine in 2001 I converted the open barn on my property to a well insulated workshop.  I also designed and built a heating system that uses solar and propane to heat the radiant concrete slab on the ground floor and baseboard radiators on the 2nd floor office/lab spaces.

The system relies heavily on the Bosch Aquastar BS124 tankless propane boiler that has performed flawlessly since it was installed with only token maintenance.  (You can see a system diagram and live performance charts on my web site.)  Unfortunately it recently failed dramatically due to a prolonged period of extreme cold. 

In the images above you can see the storage tank that accumulates solar heated water during the sunny hours of the day.  Above left is the boiler that automatically brings the water temperature up to 140F when it is lower than that.  In the dead of winter I am lucky to get 140F in the tank so the propane boiler runs a lot.  To reduce the propane consumption, I use my wood stove.  The drawback to the wood stove is that when lit it pulls air into the building and that comes through the powered exhaust vent above the boiler.  Originally, the boiler vent had a flapper valve on the outside to prevent a back draft, but I had foolishly removed it.  So what happened this winter is we got a prolonged period of very low temperatures with night time temps dropping below zero a lot (see the red line below).

(lost data on Dec 28 for some reason)

So on December 30th when the wood stove was running and the heater was off - the cold back draft got down to -15F and eventually a pipe burst in the top of the heat exchanger:

Heater running AFTER repair - arrow shows location of the burst pipe

The arrow points to the section of pipe that blew open.  I immediately researched replacement parts and found a source for a complete replacement copper heat exchanger and in a panic - ordered one online for $350.  But then after calming down, I realized that the damage really wasn't that bad, and that I probably had the plumbing skills to repair it since I had plumbed the whole system myself!

Here is what the burst pipe looked like when I looked closely at it.  It was fairly simple for me to gently hammer the lips of the burst out section back together after draining the system down below the leak.  Then I carefully sanded the whole area around the cut to expose clean copper.  I wiped on a lot of paste flux and warmed it all up with a blow torch.  By gently applying solder, I was able to fill the crack and build a layer of solder over the whole area.  Problem solved and I canceled my order for a new heat exchanger!

There is no running water in the building so in order to re-fill and pressurise the system I had to drag a garden hose across the yard from the house.  This was challenging with 8" of snow on the ground and day time temperature peaking at 10F!  I had to pre-warm the 2 - 50ft lengths of hose in front of the wood stove to loosen them up first!  But it all worked out and it has been holding 50psi pressure for over a week now.  And of-course I made a new back draft flapper for the vent to prevent frigid air from being drawn in again.

Problem solved!  And I saved $350 in replacement parts!  Incidentally the whole building only uses about $200 gallons of propane a year at a cost of about $400 in a good year.  This is very inexpensive and due to the great insulation, interior storm windows and about 1 cord of firewood that I cut for free on my property.

Simple fix for a defective solar light

 Last June (2016) I wrote about these great solar yard lights with built in motion sensors.
 

I have them deployed all around my home and it is delightful to walk around and see them turn on from up to 20 feet away to light my way.  Each successive generation of these lights seems to have more LEDs and are brighter.

I recently ordered a 4 more from a different manufacturer and 1 of them did not work.  I had read in Amazon reviews that this had been an occasional problem.  Some manufacturers are friendly about replacing defective products, and some are not.  I decided to take matters into my own hands ad see what the issue was.  It was easy enough to remove the 4 screws from the back and here's what I found:

The battery clip had come out of its housing.  It was simple to slide it back in place and all was well!

I share this experience in the context of sustainability because many people would consider disposing the defective item in the trash since the expense of returning it would be more than the cost of the unit.  But these contain Lithium Polymer batteries that don't belong in the landfill.  So even if the electronics were defective, the battery should be recycled.  Lithium is a toxin that must be recycled safely and most hardware stores have a collection box for rechargeable batteries.  

It is important to note that these particular batteries are removable and replaceable, potentially extending the life of this product for many years!  The LEDs are typically rated for 50000 hours continuous use (about 5.7 YEARS!).  Since these are used only a minute or 2 a day, they should last indefinitely.  Same goes for solar panels that should last over 20 years.  These particular solar cells are embedded in plastic resin that eventually clouds over, but this does not seem to affect performance.

Partial solar eclipse and solar output

Maine photographer Mike Leonard drove to Illinois to capture the image above of the total eclipse today (August 21, 2017).  Here along the Maine coast we saw a partial (50-60%) eclipse.  It definitely got noticeably darker and cooler and impacted my solar power output.

The chart above came from my Weather Underground personal web page, there's a clear dip in solar Watts.

Looking at the past 5 days of the power output of my solar power system, you clearly see the correlation as the eclipse occurred today.

Zooming in it becomes even clearer.  Utilities that rely on large solar farms need to plan ahead for an eclipse by having their peaking generators on standby.  These are typically natural gas or hydro generators that are nimble enough to be able to ramp up their output rapidly.

I have a single 245W solar panel on the south wall of my house that also shows the power drop.

It was also interesting to feel the temperature drop.  The change was palpable when I was outside.

So that's how a solar energy geek experiences a solar eclipse!  The next full eclipse in the US will be on April 8, 2024 and we should get a nearly full eclipse in Maine.

Maine blueberries and making jam

August in Maine is the time when my fruit trees and bushes produce a bounty.  I have 4 high bush blueberry bushes that are nearly 20 years old and well cared for.  I actively prune them in the spring and feed them coffee grounds and plenty of water in the summer.  This year I have been picking over a half gallon of berries almost daily.  Total crop will be well over 6 gallons I think.  They ripen so fast that I recruit friends and neighbors to come and help themselves.

I store them in the fridge and make small batches of 4-6 jars of jam every few nights.  So far I have made over 35 jars.  I am selling the jam to my airbnb guests and giving it to friends and neighbors.  It is really good!

My cherry tree produced a bumper crop this year, but I could only reach the proverbial "low hanging fruit" which was enough to give batches to 2 of my neighbors with baking skills.  They each made a great cherry pie!

The next crop will be from my large crabapple tree.  Last year I harvested over 7 gallons and made over 30 jars of crabapple butter and jelly.  This whole process is much more labor intensive and I'll have a friend help me.  First I spread a large tarp under the tree and then I shake the tree one branch at at a time using a long hooked stick.  We then sit on the tarp and sort the good fruit into 5 gallon buckets.  In the heat of the summer this can be really tiring.

Fix it - don't toss it

 I have had this handy dual power (solar/battery) Radio Shack calculator for decades.  Recently it started producing weird results - reminding me of my series of artworks that deliberately satirize our implicit trust in electronically represented numbers.  Here's an example of a recent one:

"Digital Numeric Relevator Mk XXXIV - Hexadecimal Blues"

My "Relevator" series all look like Sharper Image products from the Twilight Zone and they just show senseless numbers that dance, fade, or de-construct in interesting ways.

It is disconcerting when a calculator shows wrong results!  So I opened it up to see what I could learn/do.  All it took was a miniature Phillips screwdriver:

... and there in the corner was a replaceable watch battery!  

It turned out I happened to have a spare battery laying around and "Presto!" it worked fine and the display was even more readable than it had been.

Being a geek, I always test a calculator by entering 22 / 7 since pi is a good exercise for the electronics and I happen to have memorized the first 5 digits.  So it's working fine now.

The point of my sharing this simple repair story is that it is often much simpler than you think to repair something.  So I saved this little item from the landfill and expect it to last a few more decades.  This is the essence of living sustainably.

Previous posts in this series showed:

How I repaired a broken torchiere lamp base

How I repaired my microwave oven

How I repaired my trash can lid

These repairs were simple and easy to do repairs, requiring minimal tools and basic ingenuity.  You can often find detailed help for repairing just about anything on the web and YouTube.

Mason Bees and bee houses

bee house

As many readers may be aware, bee colony collapse is a serious concern and the honey bee population is declining rapidly.  While there is still debate about the cause, the result is the loss of important pollinators.  I recently learned that honey bees are not native to the US and that solitary mason bees are.  These small bees are the hidden gems that help to pollinate our gardens and fruit trees.  They are not aggressive and their sting is mild - more like a mosquito bite.

mason bee

I read up on how to encourage and support them and found that you can easily make or buy simple "bee houses".  The simplest house is made from any block of wood.  You drill a bunch of 5/16" diameter holes 5" to 6" deep and the bees will lay their eggs in these holes along with some pollen. The bees then seal the hole with mud (hence the name Mason Bee).  The eggs hatch into larva and then then dig their way out as mature bees in the spring.  Click the image below for more info.

bee larva and cocoons

I built several bee houses from scrap 4X4" lumber with a small roof made from asphalt roofing shingle.  The roof is needed to keep the rain off and away from the sealed chambers and should protrude about 2".  

The houses need to be securely mounted so they face southeast or due east to catch the morning sun, this helps the bees warm up and get going in the spring. A good location for the bee house is under an overhang and out of direct weather, but they can also be mounted to trees.  Bee houses should also be located close to a source of mud, so near open water or very damp ground is ideal.  Solitary bees have a relatively small range of only 100ft or so, so they also need to be near blooms.  They are very active pollinators for fruit trees and only a dozen are needed to pollinate a fruit tree, whereas the same tree might need over 100 honey bees to do the same job.

Commercial bee houses often use paper tubes that can also be replaced.  There are many designs, here is one example that uses bamboo tubes:

While bamboo may work, I have learned that a 5/16" diameter hole is the size that Mason bees prefer.  If the hole is too small, they cant use it.  Too big and they expend a lot more work to fill the walls with mud to get their preferred size.

Ready-made bee houses are affordable at around $20 and can often be found at garden centers and big box hardware stores or on-line.  I don't recommend commercial plastic bee houses, apparently bees don't like them.  Stick with basic wood houses.

You can also purchase cardboard tubes separately in bulk and make your own house.  Holes or tubes can be lined with parchment paper to make them re-usable. 

It has been my personal experience that Mason bees don't like the cardboard tubes here in Maine and the houses I built by drilling holes in wood have been much more successful at attracting them. 

Three weeks after installing my first bee house in June I found 3 holes plugged up and a bee working on another:

And in early September 2017 one House has many holes filled in, and more being filled:

This is a simple and inexpensive way to support bees.  I have several fruit trees that I value and hope to encourage and expand my local bee population.

Some useful links:

https://insteading.com/blog/insect-hotel/

http://www.masonbeecentral.com/100-tubes-and-liners

http://www.foxleas.com/make-a-bee-hotel.asp

http://www.gardeners.com/how-to/instructions-mason-bee-house/8085.html

Also a search YouTube shows many videos about keeping Mason Bees.