There is a common interview question that asks the candidate to estimate the amount of manhole covers in North America.
This is a particularly nice interview question, as there are a dozen different ways to approach it, no clear right answer, and no easily searchable result online. Here I present my quick take on it, and some assumptions that could impact results. I will make every effort to avoid searching anything online (during my guess phase), and present purely my guesstimates for all values. To do this, I decided to try out a very nice online tool: getguesstimate.com
To get an easily tweak-able, reasonable value, I decided to start with the population of North America. Taking the percentage of the population that lives in urban (or suburban) centers, I figure out how many people have sewer access. I then work from the other direction, and count the rough amount of sewer mainlines needed per person. Lastly, I make a wild guess (and this is the part people should take a peak at) that there are 1-5 manhole covers per mainline.
This gives me a rough answer of 22 million Manholes in North America, with reasonable lower to upper bounds of 19-46 million.
The result above is very sensitive to final “Manhole Covers per Mainline” step. My reasoning to take a 1 to 5 guess is:
Each mainline should have at least 1 manhole to check for issues/do repairs
Each mainline would stretch at most 5 km, more that this and likely 2 or more mainlines would connect togeter
In the 5km case, a maintenance worker would not be expected to walk 2.5km underground, in full gear.
Ideally, the worker would have to walk a maximum of 1km. Hence, 5 covers for 5km mainline.
The EPA estimates that there are about twenty million manholes in the United States [*]
If we take EPA’s 2012 estimate to be accurate, we are getting roughly 16 people per Manhole. This would give us (if we take US numbers to be fairly average for North America) about 29 million manholes in North America. This means my guess above turned out to be a fairly accurate, a 30% error!
Many of you may have seen the Adafruit PCB ruler (also used by the awesome Voltera).
The design was actually adapted from my own design back in University!
To improve on the original idea, I have created a reference business card.
Here is a computer render of the design:
PS. The dots on the front are the phone number in braille, the dots and dashes are Morse for the website, cause who doesn’t surf the web by Morse?
I wanted to show every common component I could think of, and how they relate. Often when zoomed all the way into a digital design, you can lose track of what 5 mils really means. It is TINY!
Here is what it looks like in real life:
The PCBs were made by Dirt Cheap PCB and overall look pretty good. The silkscreen can be a bit fuzzy or misaligned, but at ~2$ per card, this is great! The slight annoyance is that they plated the AWG holes, even though this was not specified.
Another tip: You can see the maker’s code on the front of the card (white text below the braille). Manufacturers will often put this on the “back” side of your design (bottom layer) so for next time, I will swap the layers to place the mark on the less pretty back.
Feel free to fork the project on the Circuit Maker main site (free login needed)
The FCC has made some good and bad choices in the past. While overall they do their job quite well, sometimes they make a poor decision. Once such decision is the idea to require device makers with WiFi devices to lock down the RF-controlling software on those devices to prevent users from installing the software of their choice.
This means not only routers, but also many phones, tablets, laptops, and any number of new devices that are WiFi capable would now be required to implement a DRM system that prevents users from modifying the operating system and/or firmware on those devices.
Do not allow this FCC regulation suggestion to pass! This will limit innovation in wireless networks, and impact robotics development!
Comment period ends TODAY, please register a comment (from anywhere in the world, US regulations dominate worldwide markets)
There is something lovely about displaying data in well thought out, detailed ways. I love that feeling of potential, that if we just think of one more way, one more infographic, then perhaps people will change the way they behave. It is a semi magical process by which numbers in columns on a spreadsheet turn into meaningful data.
Add to this my love of handwriting postcards, and this just seems like an amazing idea!
Another great, more polished example is feltron, a blog by a prominent ex-Facebook design team member Nicholas Felton.