For Halloween 2022, we wanted to top our previous record attempt for “Most Complicated Way to Make Trick-Or-Treaters Suffer in a Haunted Maze of Endless Bureaucracy”, and settled on the perfect idea: the DMV.

With some slight candy-adjacent-holiday themed rebranding, ten yards of cubicle fabric, over $1k in off-theshelf and government-surplus IT equipment, and a sizable band of willing volunteers, the California DTV (Department of Treat Vending) was born.

The DTV

The DTV (Department of Treat Vending) is responsible for issuing Treat Licenses (which look kind of like California driver’s licenses, but with a cat instead of a bear, random memes in the watermark, and no personally identifying information other than first name and favorite color) to trick-or-treaters. Holders of a Treat License may present their card at any participating DTV branch (currently just the one) in order to acquire a full-size candy bar.

Of course, acquiring an ID card is not a simple process. In order to acquire an ID card (and the associated candy bar), trick-or-treaters must arrive at the DTV and wait in line for a Request For Candy (RFC) form. At the end of the RFC form line, each trick-or-treater is provided with a clipboard and an RFC form corresponding to the type of candy bar they wish to receive a Treat License for, as well as an auto-generated ticket number corresponding to their slot in the DTV ticketing system. The number of RFC forms is tied to the number of available candy bars of each kind, so after a few hours of operation, many DTV branches may run out of the more popular variations of the RFC form.

RFC forms are available for a number of candy bar variants. The different types of ID forms are listed below.

• Twix: RFC-53-TW
• M&M’s: RFC-53-MM
• Snickers: RFC-51-SN
• Sour Punch Straws: RFC-68-SP
• Skittles: RFC-67-SK
• KitKat: RFC-53-KK
• Reese’s: RFC-52-RS

Each RFC form has identical fields except for Section 3 (Verification), which varies by form type.

Once a trick-or-treater has received a ticket number and an RFC form, they proceed to the waiting area where they fill out their RFC form with the relevant details and wait for their number to be called by the automatic ticket calling system. Current wait time and ticket numbers being served are displayed on the ticketing system display, and new tickets are automatically verbally announced by the ticketing system when they are called. The ticketing system also displays a delightful collection of advertisements and Public Service Announcements on the screen to keep trick-or-treaters entertained and informed while they wait.

After a trick-or-treaters’ ticket number has been called by the ticketing system, they must proceed to their assigned service counter. Upon arriving at the service counter, the trick-or-treater’s RFC application is reviewed, and may be approved or denied based on the information provided. If the RFC application is denied, the trick-or-treater may be asked to correct the deficiencies in their form and return to the counter with another ticket number, or fix their RFC form at the counter. If the RFC application is approved, the service counter attendant creates a Treat License ID card using the information included on the RFC form, and may take an ID photo of the applicant for use on the card if the applicant consents.

Once a standard Treat License ID has been printed, the trick-or-treater is handed back their application form (stamped with “Approved” and relevant service desk information), along with the ID card and the relevant full-size candy bar.

The DTV proved to be incredibly popular among trick-or-treaters; during its four hours of operation in 2022, it issued 269 unique Treat Licenses (and the same number of corresponding full-size candy bars).

Full walkthrough:

The Crew

The DTV was a very ambitious halloween costume, and it would not have been able to come together successfully without the help of a number of dedicated friends and coworkers. Caitlin and I set up the IT systems behind the DTV, and Kenneth built out the graphics assets, employee ID cards, and treat forms, but many many hours of costume assembly, setup, and operation were made into light work by the excellent crew who showed up to help out.

How It Works

The DTV costume was primarily a custom IT infrastructure project, with some web development, arts and crafts, and graphic design thrown in for good measure.

Ticketing System

Trick-or-treaters arriving at the DTV are served on a first-come-first-served basis. In order to evoke the linoleum-floored-purgatory vibes of the DMV, as well as for the practical purpose of keeping order among the massive blob of children, we knew that we would need to implement an automated ticketing system.

The DTV’s ticketing system is built by Caitlin in Django, with dedicated web pages for creating tickets, as well as consuming tickets from each available service window. Tickets are created by the employees at the RFC counter using the Create Ticket view, at which point they are added to a ticket database. Employees at a DTV service desk window can mark tickets as completed and ask for a new ticket number to be assigned to their window using the Window view. When new tickets are assigned to a window, they are displayed in the Status view, and announced via the text to speech API on the web browser of the machine driving the waiting area television display. Current wait time was estimated by looking at the time elapsed since the oldest still-incomplete ticket was created.

Overall, the ticketing system worked extremely well for the duration of the DTV’s operation. Wait times were estimated remarkably accurately (we topped out at a 23 minute wait time), no tickets were lost by the system, and window assignments were clear and never in conflict. We did encounter some issues with trick-or-treaters losing their ticket numbers, not going to their window when called, and then waiting infinitely while the service window assigned to them sat empty and eventually moved on to a new ticket. That said, these instances were rare and most trick-or-treaters who were assigned a ticket got processed in a timely manner.

ID Card Creation

IT infrastructure for the DTV costume was based on a Django web server running on a laptop at the primary service desk, which was connected to a LAN network being broadcast by a wifi router. Each DTV employee could use their laptop or mobile device to connect to the wireless network and access an employee portal for creating / editing ID cards, printing ID cards, creating or consuming tickets, and viewing live system statistics.

ID cards were created using a web form connected to a python script on the server computer. ID card information filled out in the webform would be sent to the python script in order to automatically generate an image of the ID card with text positioned in the correct locations with the correct format, as well as ID images and watermarks on the card.

Once a DTV employee created an ID card, they would click the “Print ID” button on their ID card interface to send a print job from the server computer to the ID card printer located at the primary service desk.

ID Card Printing

ID card printers can be quite expensive, but when sourcing equipment for the costume I found out that many old-generation printers were being sold off as surplus as schools, security offices, and government agencies were switching over to new models of printers. Venerable workhourses like the Zebra P-series were being offered for as low as 1/10 of their original list price, allowing me to snag a Zebra P330i single-sided ID card printer for under $250 (I found a deal, but normal prices are usually under $400 for devices in good condition). The printer was missing a critical component (cleaning cartridge), but with some 3D printed plastic parts (and later a used part sourced from eBay), we were in business.

While bringing up the printer, I was continually plagued by issues with cards printing some or all of their colors with a nasty offset. Looking at the card from left to right, it seemed like there would be a sudden vertical bar and everything to the right of the bar would be a smear of black or other mushed-up colors. Fortunately, this turned out to just be an issue with the color sensor having drifted over time, and running a ribbon sensor calibration with the stock driver was able to get things working consistently.

Each ID card is printed on a standard CR80 PVC card blank using a CMYKO (Cyan Magenta Yellow blacK Overlay) dye-sublimation ribbon. These ribbons are super cool (and very expensive), as they include a long chain of stitched-together color panels which are run across a thermal print head one at a time in order to print the various color layers onto the card. With genuine ink ribbons and the cheapest CR80 card blanks I could find on Amazon, we were averaging around $0.50 in materials per ID card. Expensive, but not bad considering that a full-size candy bar would be around $0.80.

Set Design

In order to capture the aforementioned “linoleum-floored purgatory” vibe of the DMV, we knew we were going to need to incorporate cubicles and a stuffy waiting area into the driveway of my parents’ house. My brother designed a half-cubicle / service counter to be constructed out of thin 1×2 planks and cubicle fabric, and we scrounged around the backyard for any lawn furniture we could find in order to create enough seating for the waiting area.

The primary service counter was constructed out of four identical wood-framed cloth panels framed with 1×2’s (we couldn’t find any non-crappy ones at home depot for a good price, so I ended up ripping down some nice 2×4’s with a table saw). These 1×2 frame pieces were cut to length and joined using pocket screws, and some convincingly cubicle-y fabric was stretched over each frame and stapled down. M6 threaded inserts were installed on each frame piece in order to join them together in a strong but reversible manner (we needed to disassembly the desk to transport it to my parents’ house, and I didn’t want to drive screws in and out of soft wood multiple times). There were some brief issues with getting brackets from Home Depot that would fit our M6 fasteners, but a sketchy hole-embiggening rig made out of a trigger clamp and my drill press made quick work of that. The plastic-lined countertop at the top of the service desk was built from some cut-down melamine shelf material, and iron-on edge banding was used around the edges to complete the appearance. LED lights were mounted to the underside of the counter to provide lighting behind the desk (for service counter employees) and along the front of the desk (to illuminate signage). A power strip was installed under the counter to allow IT equipment at the primary service desk to be powered conveniently.

Signage on set was constructed from coroplast with vinyl lettering cut on my plotter. Many friend-hours were dedicated to carefully lining up and applying lettering with transfer tape.

I’ve always enjoyed cooking, especially Asian food (lots of influence from my mom’s side of the family in Wuhan) and slow-smoked barbecue (idk it’s tasty). One day in the summer of 2021, Caitlin (my Significant Other), Kenneth (my brother), and I stumbled into a strange cocktail of boredom and hubris and decided to start a farmers’ market stand specializing in hot food of the Asian-American fusion variety.

Chapter 1: Haha What If We Ran a Farmers’ Market Stand

In truth, we thought of the name first, Baobeque, and thought it was a fun enough name that it was worth thinking up a menu for a farmers’ market stand with that name, you know, as a joke. And cooking test recipes for it, you know, as a joke. And filing for the Fictitious Business Name. And registering for a resellers’ certificate and building a budget and starting a sales tax account with the CDTFA and getting a federal EIN and shopping around at commissary kitchens and writing a business plan and applying to farmers’ markets. For the joke. Haha so funny what if we actually ran a farmers’ market stand that would be so crazy we’re all engineers we have like no time.

Chapter 2: Why Won’t Anyone Let Us Run a Farmers’ Market Stand

On the outside, Farmers’ Market food stands look like a great idea for a business. They serve simple items, have low capital costs, and absolutely print money. For the cost of a tent and some minimal fees paid to the farmers’ market, you can have access to a vast number of potential customers who are willing to pay relatively high prices for unique and interesting food. So why aren’t there more of them?

It turns out that Farmers’ Markets are not just a collection of stands competing with each other on one street. The whole Farmers’ Market is carefully curated, competing with other Farmers’ markets in the area to attract the most customers, retain the best vendors, and secure the best venue. Farmers’ Markets are run by market managers who are in charge of admitting vendors to the market, collecting fees for the markets’ parent organization (usually one organization will run a number of different markets spread out throughout a large area across different days of the week, so that vendors and market staff can work multiple days a week). Because space in each farmers’ market is limited, market managers have to pick and choose which vendors are allowed a spot. In order to keep vendors happy (by avoiding too much competition in each product catrgory), and in order to provide market customers with a variety of choices, market managers usually limit hot food stands to one per specific food category (e.g. there can only be one hamburger stand). If the market is especially large, or there is extra demand for a certain category of food (e.g. artisan bread is super popular), market managers may allow a larger number of similar food vendors. If two vendors overlap but not entirely, market managers may artificially restrict the menus of the two vendors in order to resolve the conflict by reducing competition (e.g. if there’s a noodle vendor that also serves dumplings, they may not be allowed to sell dumplings if entering a market with a pre-established dumpling vendor).

All of this is to say, Farmers’ Markets aren’t just a collection of people who showed up with tents. Each vendor in the market is hand-picked to cater to the most likely tastes of the local clientelle, while providing mutual benefits and minimally destructive competition to other vendors in the market. Kind of like some kind of artificially constructed coral reef, if the corals are vendors and the fish are people who get up at the crack of dawn on a Sunday morning looking to purchase expensive produce from a tent outside a Starbucks.

Because of the large number of prospective vendors that want to join a farmers’ market, and the intense selection criteria created by market managers, many applications to join farmers’ markets are denied, and many more are placed on years-long waitlists to stand by in case another vendor drops out. After a number of fruitless applications, Baobeque seemed like it wasn’t going to get into any farmers’ market anytime soon. But, we were convinced that our concept was unique and provided an angle that wasn’t present in our local market, so we pressed on and began to (politely) badger the local market managers about our vendor application whenever we visited their market. After discussing our concept with them and staying engaged in for a few months, we managed to warm them up to the possibility of adding our tent concept to the market, and eventually they asked us for samples of our food (one of the final steps in the application process)! By some miracle, the market managers liked our food, and our application was green-lit and forwarded to upper management for final approval.

Chapter 3: Oh No, We’re Running a Farmers’ Market Stand

Crap crap crap crap quick we gotta get everything ready!

With an approved application, we jumped into the process of acquiring equipment, figuring out branding, building our menu, booking time in a commissary kitchen, and iterating / practicing our recipes. We purchased a used cargo trailer for transporting equipment,

As a dry run for our first farmers’ market, we catered a college student event for free (they paid for the ingredients, and we cooked everything and served it), which was hugely helpful for pointing out many flaws in our process; our prep/setup/everything time was way too slow, our menu was too big, and our recipes weren’t scaling consistently. Fortunately, college students like free food, and didn’t complain–but we knew we would need to step things up before setting up shop at a farmers’ market with real paying customers.

Problems identified during dry run:

• Low efficiency
  • We spent probably 20 hours preparing, catering, and cleaning for an event where we served ~70 people. Definitely not scalable, but a lot of time was wasted in figuring out how equipment worked, loading equipment into separate cars (instead of a single trailer), and generally being extremely slow about everything.
  • Our roles in the tent weren’t well rehearsed, and we kept tripping over each other and wasting a lot of time being blocked by each others’ tasks.
  • We were missing some key equipment for improving our efficiency. For instance, we were julienning onions by hand (badly), when we could be saving a bunch of time by using an inexpensive manual machine to do the same task.
• Unscalable recipes
  • One of the main pain points of the catering event was homemade bao buns, which we were making from scratch and steaming ourselves. These buns took a tremendous amount of time to hand-craft for us (as culinary noobs), and worse, were easy to over-steam and had very little durability for hot-holding. If one of our bao buns was overcooked or hot-held for too long, it would end up looking like a wet version of Emperor Palpatine’s face, and would stick to every available surface inside the steamer. Not good.
  • We had a stir-fried eggplant dish that was 1) extremely slow to cook and 2) not that good. We needed to find a better way to cater to vegetarians.
• Dumb mistakes
  • We made some small mistakes that added up to become large issues. Most of these were related to missing items that we all assumed someone else had remembered to pack for the event. For instance, we forgot to bring honey with us to add to the char-siu pork belly burnt ends glaze, which meant we weren’t glazing the pork belly burnt ends in a delicious sugar crust, but rather frying them in some kind of pink pork oil/water mixture. Still tasty, but not nearly as good as what we had prototyped.

Happy takeaways from dry run:

• Our food was edible
  • People seemed to like what we were serving, and thought it was very unique (in a good way).
• Operations had vast potential for efficiency improvements
  • Most of our issues were related to a few small bottlenecks, and a lot of our production capacity was being wasted. This was great news! If we could eliminate some of these bottlenecks and streamline other parts of our operations, it was obvious that we could greatly improve our overall eficiency.
• This could work
  • Running a farmers’ market hot food stand is going to be a LOT of work.
  • Cooking for a large number of people on a tight schedule is fun, but in a stressful / kinda terrifying way.

Chapter 4: The Real Deal

Baobeque’s first farmers’ market appearance was in early October of 2021.

Core memories from The First Market:

• We lucked out and happened to debut during the stormiest market of the year. It started dumping rain around an hour after the market opened, and not the light drizzle kind–the raining-horizontally kind.
• As total noobs, we figured everyone weighs down their tent with sandbags at the market, since that seems like a tenty thing to do. It turns out that sandbags are heavy and usually not needed at all, so nobody had sandbags on their tent except for us. Due to the aforementioned wind situation, we watched from inside our tent as other tents were blown over our heads and down the street during the market. I distinctly remember seeing an Indian food tent being blown about 20ft above the street with a propane tank still attached. There wasn’t much food selling happening at this point as everyone was focused on holding things down, trying to keep stuff dry, and helping people dodge flying tents / pick up the mangled pieces.
• Someone placed a cash tip into our open container of green onions, which was unfortunately located near the pickup window. Oops.
• Our total take from the first market was around $600. Impressive for a super rainy day, but not nearly enough to break even. Still, a great start!
• Some very kind friends stopped by to help us clean up after the market. Their help loading up our trailer in the pouring rain felt like a saving grace after a long day of prep / setup / cooking and getting soaked.

Sometime in our first few markets, a local student named Derek came and asked to film a food review. We were happy to say yes, and his well-produced video is still probably the best-documented video snippet we have of the customer experience. Of course, we were still getting our food ironed out, and a lot of the food presented in the video wasn’t yet our best work, but we were very happy to get a shoutout on the internet!

It was clear that we were bringing something unique to the farmers’ market, and customers seemed to really enjoy the dishes we were making. After our initial market debut, we continued to improve our operations and menu. Some improvements that we made:

• Added a stir-fried Tomato-Egg dish as a vegetarian option. Probably our personal favorite dish but never got very popular among customers, for some reason.
• Wrote full checklists and written recipes for ingredient shopping, prep, setup, and teardown.
• Bought a $700 dough sheeter from Italy for improving our consistency and efficiency in preparing our custom green onion pancakes.
• Played around with some specials, including smoked beef back ribs.
• Cut out little blocks from 2×4’s and added some adhesive-backed sandpaper to make levelling blocks for our tables (the street was quite sloped where our tent was set up, and everything kept sliding to the back of the tent).
• Worked out a really efficient system for writing down orders (written shorthand), calling orders within the tent (verbal shorthand / keywords for acknowledging with a callback), and staging orders for minimum wait times (FIFO with early warnings for any potentially blocking shortages).
• Caitlin got really good at estimating wait times from the backlog, and usually was able to provide every customer with a wait time estimate that was accurate within a few minutes. She would write the time that the order was taken at the top of every new ticket, and this would be used for estimating current wait times. If an order was taking longer than the estimated time, everyone in the tent would know about it, and an apology could be made to the waiting customer (usually with a free drink or something similar).
• Kenneth became a bao assembling machine. Most weekends he would assemble 300+ bao for customer orders.

As we gained a following, and as the market began to get busier (due to the return from COVID and adding new vendors), we saw our sales steadily increase until we were in the range of $1400 to $1700 per market. This was enough to break even with ingredients, rent for the commissary kitchen, market fees, and salary (Caitlin and I were working for free). Seeing the same customers come back weekend after weekend was very rewarding, and assauged my fears that we wouldn’t be able to build a steady business.

Chapter 5: Can we make this a Real Business™?

Business was good, but running a farmers’ market stand on top of having full-time engineering jobs was beginning to take a toll. In addition to working 45+ hour weeks at our full-time jobs, we were working an additional 20-ish hours each every weekend to run the stand. The types of work were quite different, which could be refreshing, but the volume of work was getting to be exhausting.

The business was breaking even, but was not in a sustainable place. From the capital side, our numbers were barely working, since we were only operating one farmers’ market, so all of our fixed costs (commissary rent, equipment cost, business fees, etc) were only being amortized across a single day per week of business operation. From the labor side, our business had no redundancy and relied on all three of its employees working at 100% capacity at all times, since any single person calling out sick for even just a few hours of work on a weekend could shut down the whole operation. We were faced with a decision: ride the burnout train into the ground, or put in one more big huff of effort to try professionalizing and expanding the business to a point where it could survive without continued unsustainable effort from our side. As optimists, we chose the second path, and set about making a Real Business™.

For Baobeque to survive without our own continued effort, we realized that it would need to become a self-sustaining business with its own employees and management. We were already doing things above board (paying all relevant permits and sales tax, great documentation for our processes, etc), which was a great starting point. In order to hire Real Employees™ for our Real Business™, we needed to write up an employment contract and employee handbook, figure out a payroll system, pay for employee insurance (in case of injury on the job), and collect (withhold) income tax from employee wages. This turned out to be substantial effort, but with the help of some of the readily available digital tools for small businesses (Square is a lifesaver), the process was completed within a few weeks. So, we put up a “hiring” sign, set the hourly wage as high as we could possibly afford ($18/hr), and waited for someone to ask us about it!

Within a few weeks, we lucked out and were contacted by someone who was the become our first Real Employee™. He had a great attitude and were quick to learn the ropes. Our original intention was to have him come to the market and learn the process for a while, and then have Kenneth take some weekends off. However, hiring our employee corresponded with an increase in sales, and any slack that was left was taken up by new processes we had to introduce to be compliant with labor laws (e.g. minimizing overtime work, mandatory bathroom and meal breaks, etc). We hadn’t had to stick to these processes previously since the only people working in the booth before weren’t paid employees. Adding these provisions increased the cost of doing business, but were obviously necessary. We hadn’t realized how much we’d been burning the candle from both ends just to make things work! And at the end of the day, if a job needs to not allow bathroom breaks to be financially viable, that job is either vastly under-valued or probably isn’t worth doing in the first place.

In order to pay for our higher costs that resulted from converting to a Real Business™, we raised our prices to the highest level that we thought would be fair to our customers (we gave our regulars some discounts during the cost increase period to soften the blow). Due to a number of factors, our revenue increased slightly and reached a more-or-less sustainable threshold. We were cash-flow positive as a Real Business™, but only by a hair. And, we didn’t have enough revenue to pay supervisors or owners more than the base employee wages, which would make promotions difficult.

Some things influencing our profits during the Real Business™ transition:

• (+ revenue ) We raised our prices. For instance, a 3-bao meal with a salad went from its original price of ~$10 when we first opened up to a final price of $18! We tried to keep some “deep value” items on our menu with large portions and low costs to keep hungry customers satisfied (we sold jam-packed containers of slow-smoked pulled pork fried rice for just $12, usually enough to feed two people).
• (- revenue) A really tasty dim-sum tent opened next to our booth in the market, which reduced demand for our product slightly by helping fill out the Asian food niche (but also resulted in some excellent food trades where we got to go home with some delicious shu mai and har gow).
• (- profit) Pork prices went way up, something like 30%.
• (+ revenue) Farmers’ market foot traffic increased as summer approached.
• (+revenue) We introduced a new vegetarian dish to replace our less-popular tomato egg. We began serving hot sesame noodles, a spin on the Wuhanese dish Re Gan Mian, but with slightly different ingredients. This dish was easy to make in volume and had great profit margins.

Having completed our Real Business™ transition, with two employees (Kenneth converted to a full time employee during the transition), two co-owners, and a fully insured and above-board business, we had a toehold on a sustainable business plan, but residual burnout was creeping back in. Caitlin and I wanted some time on the weekends back, and Kenneth was about to start a new full-time job. At work, I was getting promoted to a management position on top of my existing engineering responsibilities, and I knew that I would need to find a way to not do 20 hours of farmers’ market work every weekend (along with additional hours of miscellaneous tasks outside of work hours during the week) in order to maintain my sanity and put my best food forward at work. We would need to find a way for the business to operate without us involved in day-to-day operations if it was going to be sustainable long term.

Chapter 6: The Impossible Chicken Egg Problem

To make the business run truly hands-off, we would need to hire someone who could be an experienced supervisor, and train them to train more people. To pay for an experienced supervisor, we would need to join more farmers’ markets to increase our revenue, better amortize our operating costs, and increase our economies of scale. But, since we already had full time jobs and were operating at 100% capacity, we had no bandwidth to join additional markets, or even really start a hiring search for a supervisor or new employees in earnest. If we wanted to expand the business to a sustainable and scalable point, we would need to invest a substantial amount more resources, into the business in the short term both in capital and in our own time.

Caitlin and I are both very passionate about our jobs (we’re engineering nerds and getting paid to build stuff with technology is actually the best), and neither of us was down to leave our engineering career to work on the farmers’ market business. We spent some brief time searching for a buyer, but didn’t find any initial takers and weren’t willing to run the business indefinitely while searching for someone to hand it off to. Our employee expressed some interest in acquiring the business, and we were excited about the prospect of selling it to him (at a very good price), but ultimately he wasn’t able to secure the funding and support he would need to run it sustainably. In the end, we made the decision to close the business as gracefully as possible while taking care of our employees, friends, and customers as best we could.

Baobeque’s last day of operation was on June 12th, 2022. We notified our customers in the weeks leading up to our closure, and many of our regulars came by to place one last order (or many last orders). We also printed up recipe cards for our most popular dishes, and handed them out to anyone who wanted them, so that anyone who had grown to love the taste of our food could try replicating it themselves.

While closing the business, we gifted our employees a generous severance package, and began selling off our major pieces of equipment piece by piece. Many of the friends we had made at the farmers’ market and the commissary kitchen were more than happy to buy our used equipment, and we were happy to provide them great prices as a token of our friendship. On June 19th, 2022, the Baobeque crew enjoyed a wonderful Sunday morning of sleeping in for the first time in a long time.

Electronics Kit Now Available!

Update 2023-09-05: I finally got around to redesigning the electronics for this kit, and got some PCBs made for an improved 7-segment display as well as a new control board based on a Pi Pico! I also rewrote the firmware in MicroPython to make it easier to play around with. The full kit is available at my webstore, pantsforbirds.com, but of course the updated design and code files are still open source and available on the github repository.

Use coupon code SPEEDSIGN50 for $50 off kits in the first batch! This is mostly an incentive for dealing with any hiccups we’ll encounter while ironing out the order process, and will be removed when the first batch is sold out.

Project Writeup

For Halloween 2020, I decided to build a replica of a radar speed sign using an Arduino and an HB100 doppler radar module. This writeup is gonna be a bit fuzzy, since as usual, I decided to document the project a tasteful 8+ months after it was complete. Have fun reading my semi-coherent image and schematic dump!

How the Radar Do

The project uses an HB100 doppler radar module and a pre-amplifier circuit in order to convert movement of objects in the HB100’s field of view into a square wave that gets fed into the Arduino Nano. Radar makes everything sound fancy, but in this case it’s no more complicated than what you would find in one of those supermarket automatic doors that triggers when some leaves blow by. The HB100 transmits a 10.525GHz carrier signal, which is reflected off of a moving target (in the case of this project, a person walking across a room at close range). The return signal from the moving target is doppler-shifted, and is thus a higher (or lower) frequency than the carrier, depending on whether the target is moving towards or away from the transmitter. By mixing the transmitted 10.525GHz signal with the reflected signal, the HB100 produces a signal that is the difference of the two frequencies. Fortunately, for objects moving at non-relativistic speeds, this difference frequency is usually very chewable for even the dumbest of microcontrollers, as it’s usually in the range of a few Hertz.

In order to get the difference frequency into a format that can be processed by a microcontroller, the signal passes through a two-stage preamp circuit which amplifies it from a lil squiggle into a rail-to-rail square wave. The pre-amp circuit isn’t that interesting, and is ripped straight from the datasheet. There are some capacitors in parallel with the feedback path in order to generate some poles that roll off the frequency response around 72Hz, stopping the pre-amp circuit from amplifying high-frequency noise that is beyond the range of interest. Also note the chunky (4.7uF) AC coupling caps between stages. These coupling caps are yuge because they need to have a low impedance at the frequencies of interest (<100Hz). If we look at the 12k resistor used for the IF load, and assume that the HB100 IF circuit has a characteristic impedance in the range of 12kOhms, we can see that a 4.7uF capacitor provides an impedance lower than the characteristic impedance all the way down to around 2.8Hz. Neat!

Once the preamp circuit was working, it was pretty simple to stick the output straight onto one of the Arduino Nano’s digital inputs. Input interrupts were used to time the pulses and estimate the frequency of the signal, which can be converted into velocity using the doppler radar equation.

Bigass 7 Segment Display

From the beginning, the real pizazz for this costume was always going to be the giant LED 7-segment display. I’d been wanting to make one for a long time, for no particular reason other than whee LEDs shiny, and more LEDs = more shiny. There have been plenty of 7-segment display implementations with all kinds of hardware, from the WS2812B chips controlling segments with individual addressing to cool multiplexing schemes. However, my favorite approach has always been the tried and true SPI shift register configuration. Using two SPI shift registers (the good ol’ 74HC595), each digit is controlled by a single shift register, with a single segment responding to a single bit output from each shift register. In the past, I’ve used the 8th bit output to control a decimal point, but that wasn’t necessary for this particular project.

In order to make things easy and reasonably professional-looking, I designed a custom modular PCB containing 8 high-power amber LEDs. These hexagonal PCBs could interlock to form a 7-segment display digit, and contained the necessary current-limiting resistors, and switching transistors for turning the digit on and off. Solder pads at the interlocking edges of each PCB were used to share +12V and GND to the entire digit from any single powered segment. Additional solder pads on the edge of each segment connect to the gate of the switching MOSFETs, allowing a low-current enable signal from the shift register to control the segment of high-power LEDs. I should have added a series resistor to this gate network to protect the MOSFETs from ESD…in my late-night haste to assemble the costume, I managed to zap through the oxide layer on several MOSFETs with ESD, killing them. That caused a good amount of debugging headache! If I were to sell these segments or something, I would probably change the drive MOSFET to an NPN BJT with a base resistor for added robustness.

Putting Things Together!

Links

Github Repository

Here’s a list of some of the parts that I used, in case people are looking! These are Amazon affiliate links (gotta make some of them Bezos Bucks).

• HB100 Doppler Radar Module
• DC Converter Module (8-40V to 12V 6A)
• DC Stepdown Converter (1.25V-30V 3A)
• 74HC595 Shift Register
• LM358 Precision Op-Amp
• 4mm x 6mm Proto Boards
• Ceramic Capacitor Kit

This bird is so dumb that it laid an egg on one (1) stick. We built it a tupperware out of wood, so watch it have kids now I guess.

Why did you make this cursed rectangle?

It’s a pretty iconic vignette of 21st century living: you’re checking out of the grocery store, and the cashier lines are long. You glance down the row of checkout stands, and the unoccupied self-checkout kiosk beckons from across the store, willing you to try it out just one more time. You have second thoughts, but are quickly entranced by the circa 2003 3D animations and a strip of paper that billows from that secondary receipt printer whose sole purpose is to print expired coupons for discount plumbing services.

You scan your first item, and then your second item. All seems well, but then the dance begins.

Beep.

Place item in the bagging area.

You gently place your dozen eggs in on the bagging tray.

Unexpected item in the bagging area. Please remove the item before continuing.

You remove the eggs.

Please place item in the bagging area.

Hesitantly, you place the eggs in the bagging tray.

Unexpected item in the bagging area. Please remove the item before continuing.

Two minutes and thirty seconds later, you notice the coupon hunter who was behind you in line at the cashier’s stand leaving the store with her two dozen gallons of milk that she got paid $5.23 to take home.

Unexpected item in the bagging area. Please remove the item before continuing.

Someone is trying to help you now. It looks like they maybe they work here.

Please remove the item before continuing.

You’re in the parking lot now. You have no eggs. They were terribly sorry, but there was simply nothing they could do. Helpfully, your car is filled with milk and you have five copies of the same coupon to AJ’s Plumbing n’ Animal Control Inc, valid till last February.

Ok, that wasn’t helpful at all. Fine. HOW did you make this cursed rectangle?

Oh, sure! Here’s the parts list:

• 1x Epson TM-T88V USB Thermal Receipt Printer (used)
• 1x Raspberry Pi 3B+
• 1x 7in Touchscreen + 3D printed case
• 1x USB Barcode Scanner
• 1x Anker Soundcore Bluetooth Speaker
• 1x 3-level LED Stack Light
• 1x 80W DC-DC Converter (12V 3A)
• 1x 15W DC-DC Converter (5V 3A)
• 1x Automotive Fuse + Fuse Holder (this was actually a REALLY good idea, because of the next thing)
• 1x 4S 4000mAh Lipo Battery
• 1 sheet of Plexiglass from Home Depot
• Some one-way mirror window film (also from Home Depot)
• Buncha 3D printed parts*
• Buncha Coroplast + 3/16in Aluminum Pop Rivets*

*Buncha refers to the metric Buncha (not to be confused with the standard Bumcha which was deprecated in 1993)

Look! Pictures!

We are weird nerds or something and we want to know more! Tell us!

Ok!

First, the simple mechanical and electrical stuff.

The entire machine runs on a 3S 4000mAh battery from some quadcopter builds I did a while back. This battery feeds into a fuse (important, I debated putting this in at all and it totally saved the whole costume from catching fire when a circuit board came unglued and shorted out the main power distribution board), and then into a power distribution board and a 40W DC-DC converter. The DC-DC converter is used to power a 12V rail that runs the stack lights, the receipt printer, and a smaller 5V DC-DC converter that runs the Raspberry Pi and USB peripherals. The stack lights are switched using a low-side drive circuit I put together using some 2N7000 small-signal MOSFETs. I had to buy an extra power supply brick for the receipt printer just so that I could cut the plug off of it, since the fine folks at Epson like their strange proprietary DC jack shapes.

The Raspberry Pi is interfaced with the 7in touchscreen over HDMI and USB, and the Anker Soundcore speaker is powered by its internal battery, and is connected to the Raspberry Pi using an aux cord. Interestingly (annoyingly), the speaker is “smart” and will stop listening to the signal on the aux cord and shut down if no sound is being transmitted. When a sound byte is sent to the speaker after some silence, it takes some time to “wake up” and can often clip the first half-second or so of the sound byte. This is very annoying if the speaker is being used for sound effects or voice lines! I managed to get around this issue by continuously playing a looping track of very, very quiet white noise from the Raspberry Pi, just loudly enough that the speaker would always stay “awake”.

The costume consists of two boxes shaped from Coroplast: a “checkout stand” block and a “bagging area” block. The bagging area is a simple box with an LED illuminated “Self Checkout” sign on top, and some plastic bags with corresponding plastic bag hangers. Above the plastic bag hangers, there is a plexiglass window with one-way mirror tint allowing the wearer to see outside the costume. The checkout stand has the meat of the system inside it, with the Raspberry Pi, electronics, touchscreen, receipt printer, barcode scanner, speaker, and stack light attached to its inside, front, and top. There are two one-way mirror plexiglass panels on the self-checkout stand, allowing the wearer to see out the costume and the USB barcode scanner to read items that are swiped over it.

The checkout stand can be interacted with from the touchscreen or by scanning barcodes on the scanner. Additional controls are available to the wearer of the costume or a nearby operator via a wireless USB keyboard. Keys on the keyboard are bound to various voice lines and actions, allowing scans to be faked and costume actions / voice lines to be commanded.

Now for the software!

The system is hacked together around a Raspberry Pi, since I figured that was the easiest way to run a complex multithreaded state machine that would also deal with USB devices and audio files. Everything is written in Python, with different threads that take care of flashing the lights, running the USB scanner and printer, drawing the GUI on the screen, and running the checkout stand’s internal state machine that allows users to checkout and print their receipt.

The GUI is made in Pygame, allowing easy capture of user touchscreen events, playing of sound clips, and drawing of on-screen text and buttons. The blissful part of this project was making every part of the GUI aggressively terrible, just like the real thing. I swear that once when I was in Safeway, I had to have an attendant enter their passcode to unjam the uncooperative kiosk I was using, and the kiosk cheerfully stated every number of their passcode out loud, just like it was a quantity of produce or a price. I happily included this helpful feature, among others, in my own rendition of the self checkout nightmare.

The sound files for the text-to-speech were painstakingly recorded and clipped from an online text-to-speech website that I found which had a voice that was pretty similar to what I had heard in grocery stores. I typed out every necessary number and digit combination, as well as the primary voice lines (“unexpected item in bagging area”), recorded them on my browser, then spent hours cutting them down to size using Audacity. With a bit of tweaking, they turned out all right (or at least, as terrible as the real thing)!

There is a random number generator buried in the state machine, and every additional item that a user scans in to their cart carries a small chance of entering the terrible “unexpected item in bagging area” loop. This feature really seemed to be most enjoyed by the parents we encountered on Halloween night. There were some other easter eggs that we enjoyed using to startle and delight various children throughout the night as well…

And here’s the rest of it! Was a very fun costume to build and play around with on Halloween with friends. 12/10 would recommend. All of the code is fully open source, and if anyone else would ever like to build a similar abomination, I would be more than happy to assist with design files and advice.

Github repo with code, etc.

After learning about the PIC12F752 in a mechatronics class, I was tempted to use the cheap microcontroller to play music. The device has 1024 Bytes of program memory, but only 64 Bytes of flash, making storing a song in data memory very difficult. Instead, I hacked together a code generation program in Python that would read MIDI files and output the notes as a C program, which could reside entirely in program memory.

Even while storing notes in program memory, each PIC could only hold around 40 notes. I took this as an opportunity to create a chainable series of boards, each of which could only play a set portion of the song. Each board has input pins for power and the master reset pin on the microcontroller, and has output pins for the same functions. Powering one board in the stack provides power to all of the boards, and the first board in the stack boots up and asserts a LOW on its master reset output, thus disabling the subsequent boards. Once the first board is done playing, it asserts a HI on master reset, allowing the next board in the chain to play. When the next board is done, it asserts a HI on its master reset output, and the chain continues to play.

Early prototypes of the PIC beeper board were made on a CNC in my dorm room using 1-sided FR4 copper-clad boards.

I later moved to commercially produced PCBs for the final product (2-sided boards). In order to reduce the form factor of the board, I switched to a QFN package for the 12F752 and added a custom POGO programming header to interface with the PiCKit programmer. At some point in this process I also realized that the CR2032 battery didn’t have enough current to power some of the louder / lower resistance beepers I wanted to use, so I switched to a design with two AA batteries powering all of the beepers in a stack.

I purchased the final boards as a small panel with v-scoring, and used a solder paste stencil and my Controleo3 reflow oven for assembly. I used water wash solder paste, and covered the buzzers with Kapton tape to protect them during the water wash / ultrasonic cleaning process.

Once the boards were fully assembled, I fired up my code generator and programmed them in sequence with their corresponding portions of the song.

The assembled chain of beepers worked as intended! Some of the notes are a little bit scratchy or weirdly loud, but I think that’s attributed to them being close to the resonant frequency of the beeper or the PCB. The notes are synthesized by varying the period of a 50% duty cycle square wave, so there are lots of higher frequency harmonics, but it works in a pinch!

I decided to build a bird feeder with a live-streaming camera for my significant other’s birthday. It seems to work pretty well! The system is based on a Raspberry Pi Zero W running the Motion firmware, with an image mask and motion trigger set to activate when birds land on the feeder. Every night, a shell script runs the imagemagick utility to convert all images with the same minute time stamp into fun little combined gifs. This project is a work in progress–automatic image tweeting and imgur uploads should be coming soon! In the meantime, if anyone needs an absolutely massive number of tiny bird gifs, I’m happy to share!

Birdcam Gifs

Parts List

• Raspberry Pi Zero W
• Raspberry Pi Camera v2 (don’t forget the adapter cable for the Pi Zero W! :/)
• Window Bird Feeder
• TP-Link Wifi Router with access to a DNS service

Code

Overview

After building my 3D printer enclosure, I ran into the issue of my printer enclosure getting too warm. Temperatures during extended prints were climbing above 130 degrees Fahrenheit, which was causing my printer electronics to overheat, leading to extrusion issues and missed steps. I remedied this by propping the door open with a rag, but that removed the noise damping benefit of the enclosure. An overcomplicated but fun solution in the form of a PID fan controller was devised. This controller uses and Arduino nano to PWM control a logic-level N-channel MOSFET that feeds into a low-pass filter in order to control a 12V radial blower fan with a varying analog voltage. Bit-shifting in the Arduino Nano code takes advantage of the built-in timers in the Atmel Atmega 328p processor in order to generate PWM frequencies at 25kHz, which is outside the range of human hearing and thus less annoying. Initial attempts to control the fan using straight PWM without a low-pass filter proved to be unreliable, but the current setup with the low-pass filter has been running strong for over 6 months as of the writing of this post.

Code

Parts List

• Arduino Nano Clone
• Rotary Encoder Board
• 12V Radial Blower Fan
• DHT22 Temperature and Humidity Sensor

Pictures

Overview

This is my second iteration of my LED trumpet.  The first iteration used an Arduino Nano, but I upgraded to a Teensy this time around for more processing power (gotta get all them FFT’s).  The system is powered by Lipo batteries (from my rc aircraft projects) feeding a Castle 10A SBEC that provides 5V to the Teensy and SK6812 individually addressable LEDs.  There’s an Adafruit microphone module buried inside the 3D printed electronics housing, allowing the LEDs to run an FFT pattern and volume-activated effects.  Three buttons on the side of the electronics housing allow the user to change modes (or play snake, in one of the modes), and a power switch allows the user to easily flick the system on or off with their thumb.  The top of the electronics housing has an LED voltmeter built in to allow the user to monitor the battery voltage of the trumpet.

Code

Parts List

• Teensy 3.2
  • NEW ALTERNATIVE PART: Teensy 4.0
• SK6812 IP67 LED Strip
• Castle 10A BEC
• Adafruit Electret Microphone Amplifier with Automatic Gain Control