4.140/MAS.863

Final Project: Take 2

So this is my final project take 2. I was able to integrate a couple more processes therefore incorporate more skillsets from the class. Some things were successfully integrated, some not to the extent I would have liked. There’s also room to have done a few more concepts, which I’ll point out a bit as they come up. Basically, my idea this go around was centered around my cat ownership. She is ruining my computer because she loves to sleep on a warm surface. Additionally, I have a five drawer dresser. Two for me, two for my sister, and one literally just for the cat to sleep in, because without one to call her own she just ruins the clothes we have in the other ones. So for this second go at a final project, I decided to create a simple little cat house for a kitten to sleep in, and I incorporated a couple of light sensitive heating pads so that she’ll be drawn to the warm area.

Components List:

• 2 sheet of 1/8x24x48 in^3 reasonably cheap wood
• Considerable length of wires
• 9V power adapter
• PCB board
• Electronics components: one 10 kOhm resistor, one 1 kOhm resistor, one comparator, one photoresistor, two heating pads from adafruit, one potentiometer, one NPN transistor
• Variety of epoxies/wood glues

Methodology

Ridiculous Challenges and Setbacks Faced: I think this is an important paragraph to have, because it was a vital part of the machining process. I think it’s most definitely noteworthy to be able to identify key problems and see in exactly what way they delayed my process, and then be able to figure out how to have avoided them or how to work around the issues. It’s an important thing to be able to do I think, troubleshooting and acknowledging imperfections in a process, especially one that is human labor intensive. I learned about budgeting more time than anticipated to complete a job, and to expect to have to do multiple run throughs when things don’t go as expected.

There were some restrictions that came from working on this project post term. First off is that some of the supplies we had from certain weeks were no longer available, so in particular I scraped the idea of doing a composite. Additionally, I did have to beg off a bit of shop time from a few sources around campus that also had limited hours, so working that around a class schedule and not being able to machine at 3 am….little bit limiting, but maybe also healthier for my sleep schedule? Then came the snow days and vacations where show owners were traveling….so shops were closed and I was just cooped up wishing that I had a shop in the basement of Senior Haus (if only MIT were so kind). Parts that I had ordered came in late, and then ultimately some parts were even defective. My last major restrictions came at the best time of course--physical restrictions right when it was time to construct my parts. Wrestling I have learned is a dangerous sport. My first problem was from a concussion I sustained mid January or so…..that put me away from loud sounds/bright noises for almost 3 weeks until I had recovered. The next and still present issue is broken bones in my dominant hand, which I sustained during a match. I’ve learned that I’m not as ambidextrous as I thought that I was, and soldering surface mount components with a wrapped up hand or the off hand is hard. Apparently I lack spatial awareness, and in addition to my splinted up mess of an arm I also managed to burn myself nicely at all of 720 degrees with the iron.

Electronics

Modela Mill: the modela is the first method I attempted to use to cut out my boards. I’m actually very comfortable with electronics production and I did well during that week of class. I’m not at all a bad hand at soldering and the modela is a lot of fun, something I would want to definitely own if I had my pick of little machines. Anyway, that being said, I ran into some unexpected problems. One day the modela just flat out fritzed out on me, and I was having issues running the module. I just threw my hands up, came in the next day, and problems were magically solved. I taped up a PCB board (one of the last big clean ones because the IDC was running low on supplies) and slapped it on the mill, zeroed out and was ready to go. Less than a minute into the operation, there was an ugly scratch dragged across, a nasty snap sound and I paused the job. Took a nice good look at the bit…..and it’s broken. I went to Charles somewhat shamefully to let him know that I had busted the only 1/64 endmill that he had in the shop. He to my surprise produced one from his desk and I went back into the shop. I changed some cut depth settings and also adjusted the z height before starting the job again, and I changed out the board of course. Going back in for round two….it’s cutting strangely deep in some places before the same issue happens once more, ugly scrape, snape and this time the real last 1/64 in the shop was gone. I had Charles inspect everything I had done because I was frustrated and honestly couldn’t figure out where I had gone wrong. Charles looked over my work for several minutes and was similarly stumped, until he saw me pull the most recent board. Holding it up for examination, he determined that the board had warped most likely in the weather, such that the center was raised up. With this problem the modela would find zero at a reasonable location, but thinking it’s dragging above the plane to it’s central location it would have no clue that other areas of the board were not flat, and that’s where I literally ran into problems. So completely out of luck now on the modela with the broken endmills and not a single board that Charles thought was salvageable due to all the warping, I moved on and started looking for alternative methods to make this board.

Breadboard: I used a breadboard briefly in order to test out my new circuit. They’re handy in that they’re able to quickly test whether or not a circuit is in the correct configuration to perform what you’re asking of it. It’s not a permanent solution by any means, rather a quick prototyping method. I checked my second board schematic with this. The only downside is that it isn’t something for surface mount components, so I wasn’t able to lay out my original design and see if it was going to function well barring the manufacturing process issues.

Acid Etching: Acid etching is a new method that I was forced to learn after the failed attempts with the modela mill. It ultimately seemed like it was the best course of action to provide a board in a timely manner. The way it works is that you take a circuit design (in my case something I’d sketched up on Eagle), and onto glossy photo paper you print a mirror image of the resist. You put the paper onto the board, with toner applied, and shove it into a laminator which uses heat to melt the image onto the board. Once that step is complete, if your image is clear, then you can put it in the acid bath. It has to go about 30-40 minutes, where it’s agitated at room temperature in ferric chloride, which strips away all the copper not covered by the negative of the circuit layout. One lesson learned is to make sure that it’s mirrored properly, otherwise you won’t be able to solder the parts in the correct configuration….i.e., sad board is sad.

Physical Structure

Shopbot (yay CNC!): So the shopbot was the primary tool that I used for the physical structure. I took my Solidworks assembly, turned it into a couple of drawings and arranged them such that they would be able to be cut out of a large 4x8 piece of wood that I purchased from Home Depot. I then exported that file as a dxf and loaded it up onto the shopbot software. The settings I used were the pocket cut with the outside trace to avoid sizing problems due to the kerf of the bit. Took two jobs to cut all of my pieces due to the size and layout.

Electronics

Old Board

This is the first iteration of my design, and ultimately is not the final circuit used. I’m reasonably confident that it would have worked, but a lot of actual physical manufacturing barriers were in the way, which I’ve somewhat outlined in my sections on methodology. The ultimate factor I think in not using it was that I’d fried some parts we were short on and genuinely was having a lot of problems soldering the components with my hand, and then once I’d had what I thought was a working board, something still seemed off. I have attached the eagle board and schematic files, as well as the arduino code that I would have used and that did correctly burn onto one model of the board that I made.

Basically, this board is very simple. Through a voltage divider with the phototransistor, a voltage is obtained and input into the attiny44. that voltage is then converted into a value. the microcontroller reads the value and compares it to an internal designated value in the code. If it is below the threshold it means it’s dark, and outputs logic high to an output pin. That then drives the heating elements through a MOSFET, which is also a transistor. If it’s above a certain threshold it means it’s light, and outputs logic low to an output pin, which correlates to no current through. All the other components are basic requirements to have the Attiny44, as seen in one of the earlier FAB weeks when we had to build the ISP board. We also in this design had inserted an LED which was supposed to illuminate the phototransistor on a constant basis unless otherwise covered.

New Board

My final iteration of bound wound up removing most all of the components of the previous board. After many failed attempts at just assembling the board due to a lot of really unfortunate circumstances, I chose to re-evaluate my approach. I was very limited in my options on a tight timeline working with non surface mount components, so simplification was a necessity. While working on the alternative FAB methods in MITERS, it was pointed out to me that for such a simple input/output method the operation could be performed without the use of a micro-controller. MITERS happened to have comparators in the shop, as well as other basic circuitry components, and after a quick run to Radio Shack I had a packet of photoresistors in hand. I again used acid etching and after a couple of attempts was able to produce something useable. Once the board was finished in the acid bath, it also had several through holes that needed to be drilled on a desk top drill press, at a .016 inch diameter. Anyway, onto the actual board itself. Below is a picture of the schematic, and I’ll explain how the whole thing works.

• Far left pads are for the 9V supply.
• The heating pads are in parallel to each other so that current is going through and not a voltage drop as it would be in series. Those are hooked up to an NPN transistor. The transistor works such that when voltage is on, the base port is high, which means current flows from emitter to collector.
• On the inputs to the comparator there are two voltage dividers. the top one has a potentiometer, which is the large silver knob looking deal. the way that works is that it alters the top and bottom voltage divider which affects the voltage out at the center. This is used to calibrate. We can change the level of darkness at which it becomes sensitive.
• The bottom divider has a photo resistor on the bottom. What’s important to know here is the formula for resistance/voltage for a divider which is [Vout=Vin*R2/(R1+R2)]....Also you should know that a photoresistor works such that an increase in light means an decrease in resistance, which means less voltage out. When it’s black, there’s an increase in resistance, which means more voltage out at the divider point.
• This brings us to the actual comparator. Basically, the output is the supply rail (9V) if the + input is greater than the - input. If the - is greater than the +, then the output voltage is zero. This is the crucial central component of the circuit. The evaluator that eliminates the need for a microcontroller, which would have done literally the same exact operation.
• Also what should be explained is the pull up resistor. The LM339 comparator used has an open collector output, which means when the output is low it goes to ground, when it’s high it’s called ‘floating’ which means to consistently keep it at 9V we have to insert this resistor.
• Some general specs…..The pull up resistor is 1kohm. resistor on the bottom is 10k. The photoresistor was a bit difficult to tell...it came from a completely unlabeled 5 pack of CdS resistors from Radio Shack that provided no tech specs (probably why they’re going out of business), but ultimately it wasn’t important once I found something that would work in my circuit, and then was able to use the potentiometer to adjust the threshold. the comparator is an LM339, a quad comparitor but we used only one of those modules. The transistor is an NPN transistor I found in the MITERS supplies. The heating pads were ordered from Adafruit. Honestly I think these heating pads are somewhat complete garbage. If I could go back and do it all again, I would make my own heating pads. They’re really nothing more than giant resistors with some heating element cloth wrapped around them, however they’re also encased in a particular plastic that I was having some difficulty finding, and I didn’t really fancy the idea of catching a lab on fire.

Physical Structure

For the physical structure there's a bit less to explain here, it was really just aesthetics choice. I had a bird house in mind somewhat and kept it simple according to that....my paternal grandfather passed away when I was 4 years old. One of the few memories I have of him is him taking me to Home Depot with an older cousin and they built a bird house that was later kept in his backyard, and I got to "help". The birds loved it and we'd go out and see it quite often...my grandmother kept it still to this day and it's in good condition. Every once in awhile I give it a new paint job just to liven it up a bit. So when I was constructing for the cat I was admittedly a bit nostalgic but also kept it a nice traditional shape that she wouldn't be too foolish to get lost inside. The drawers are a daily struggle for her.

The computer screenshots are in Solidworks of the full assembly. I chose to make each part individually and then bring them all together at the end. Maybe not always the best engineering practice but I use Solidworks quite often (which is why I avoided it in my initial final project run. It just hadn't struck me as a skill demonstration since I do it quite often for course 2) and it's just my preference for building. The wood I used was 1/8 inch from Home Depot and rather sturdy at first sight.

The less good issues I had with the physical structure didn't really have much anything to do with me personally. Two major issues that really ruined my day though: No matter how high and out of the way you store something, or how obnoxiously you label it, sometimes people like to steal your parts. Awesome, which I found out incredibly last minute and it caused me a lot of heart ache. The second issue that caused about an equal amount of grief was that I honestly did not make the best choice in material. Wood is cheap yes, but it warped as well in this horrible weather and it's destroying the structure.

Future Iterations, Room for some Tweaking

For future iterations of this project there are several things I'd do on both ends. First with the electronics end is that I'd really like to get the microcontroller circuit running just for fun, so when the resources are on hand again, that's most certainly top of the list. I actually was so short for materials originally that I had to desolder components from my ISP board to have what I needed, and I was forced to borrow an ISP with cable from someone in MITERS instead of use my own to attempt to program my board. That's just a little bit disappointing to do, especially after the grief of the physical board making process and stuffing of the board. I'd then also be able to get rid of the ridiculously oversized potentiometer I have right now....again, scrounging around MITERS left me with some limited options. Last on the electronics side of the house would simply be bigger/more heating pads....I hadn't really anticipated quite how small they'd be but I'd like to have a whole floor's worth of them and maybe sew them into the top part of a thin pillow to maximize comfort and warmth.

For the physical structure in the future I would absolutely want to make it out of acrylic instead. Through some other projects I'm really starting to love acrylic, I don't care how brittle people claim it is. It's a sturdy material that isn't going to warp, although it is going to be much more expensive. In any case, I did say I would point out places where I had the opportunity to use other class skillsets that I did not use. The acrylic would easily be cut on the laser cutter. I designed this structure to be cut on the shop bot. The only reason it didn't go on the laser cutter is because the bed size just can't accommodate my parts. In the future I'd just tweak the sizing a bit or break things up into smaller pieces, and then on the Epilog would be a nonissue. There also could have been some composites work here, which I honestly just didn't have the supplies for. Wireless is also an option, and one that I almost incoporated this round, but I had the modela issues so early on that using the bluetooth shields or anything along those lines what somewhat ditched in favor of getting the rest of the circuit up and running.

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