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Bench Talk for Design Engineers

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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Walking Ham Chapter 2: To Catch A Zombie Mike Parks



If you missed Chapter 1, please find it here: A Day in the Life of a Walking Dead Amateur Radio Operator


A few weeks ago I was able to talk with a guy in Mansfield, Texas, using Morse code. I nearly broke out in tears to be able to talk with another human being. He’s holed up in the Mouser warehouse, an ex-IT employee who has a little electronics experience but managed to hack together a Morse code set up. Turns out Boy Scouts really are prepared for everything. It took a while but over the course of a few days Tony was able to help me locate a shipment of electronics that had been shipped from Mouser to a home nearby, so I thought I would check it out.  


It’s been a few days since I found what turned out to be a massive stash of electronics. Finding that collection of cardboard boxes emblazoned with a beautiful blue logo was literally an “M” for “miracle.” Every single one was from Mouser, and the variety was astonishing. That was a good day, and I don’t get many of those anymore. It’s hard to believe that there was a time when I could have logged in to mouser.com and simply ordered anything I needed, getting it delivered to my doorstep. Now this stash of components, likely a huge backlog of projects, is mine. Over the course of a week I took them all to my place.


151900Z AUG 20


As I rummage through the boxes, I find my mind racing with exciting possibilities. How can I transform these parts into something that could help me stay alive? I have to be careful with my daydreaming though, it only takes a second for one of those skin eaters to sneak up on you.

My stomach growls. Nothing like hunger pains to snap me back to this new reality.


I peer out of one of the various hidden peepholes in my makeshift fortress. Well, if we are being honest, then calling it a makeshift shack is generous. However, my imagining it to be a sturdier structure is a mental boost that is very much needed at the moment. I can see rustling in the bushes and overgrown weeds that now strangle the landscape. Is it a biter? Or is it a potential meal? Maybe even another survivor concealing themselves in the thickets trying to determine if I am friend or foe. Or it could be just the wind. It’s hard to tell from my vantage point, and frankly I have no desire to race outside every time something makes the grass sway.


I cover up the peephole and take a seat at my homebrewed electronics workbench, glad of the single skylight, even though it seems to just add more heat in the summer. I reach into one of the Mouser boxes, scoop up an armful of bags, boxes, and copious bubble wrap and drop them on the table. Spreading them out in a haphazard fashion, I look down at the components and begin to open some of the packages, hoping that a flash of brilliance will come over me, but only the hot silence in the stagnant air inside my shack accompanies my habitual sense of desperation.




So I open another box. Then another, and still another. The frustration of being unable to come up with an idea mounts and my blood boils. In a rage I fling one of the boxes into the corner and parts go flying everywhere. Sweat drips profusely down my forehead and stings my eyes as I yank my my shirt collar open in an attempt to get some air. I look down at the mess on the floor, and then it hits me.


Heat. Temperature. Infrared.


I drop to my knees and eagerly grab all of the passive infrared, or PIR, sensors I can find. These little white, half-domed devices might just be what I need to build some sort of early detection warning system for anything that might approach my hiding spot.


Scrabbling around, I find my most important design tool, my project notebook, and begin to sketch out ideas, and eventually the system I intend to build. How many PIR sensors will I need? This going to take good bit of wire. I figure that I can give myself full 360-degree coverage as far away as a few dozen feet from the front door. It might be a huge security perimeter, but every extra second to respond is definitely worth the effort.


I wire up all the sensors with three wires each. Two wires for power and ground and one for the output signal. Since this is all low voltage I determine that my thermoelectric generator (TEG) will provide more than enough electrons to power my alert system.


Next is notification. How do I want to be informed of a possible inbound creeper? At first I consider a buzzer, but that would give away my position and might attract more trouble than it’s worth. It might scare away a possible dinner as well. Then I come across the idea of using LEDs and placing them in a circle that directly mirrors the placement of the PIR sensors outside. That way if something does set off one of the sensors I will know where it’s coming from. I will have to place a dozen PIR sensors for full coverage, so I position the LEDs in a circle with a roughly 30-degree separation in between each one.


While the LED signal will work fine while I am awake, it will be useless when I am asleep. I find a few TTL-based OR gates and figure that I can tie all the PIR outputs together and should anyone of them detect movement, the OR gate would also drive high and I could have a small vibrating motor switch on under my pillow. Not that I sleep too often or too deeply these days.


011900Z SEP 20


It took me a while to build and bench-test the system. Once built, it was time to venture out and install the PIR sensors. There were a few in the boxes, but I settled on the Panasonic PIR sensors. My man Tony found the datasheet on the Mouser servers he fired up and was able to answer a question or two for me, which is amazing. They have up to 92 detection zones and a sleep mode at 6uA which is perfect for coupling with photovoltaic cells and rechargeable batteries for powering these babies. Time to set them up. You just have to take a deep breath and move out.


While it’s not necessarily a difficult task, the fact that I was going to be exposed left an uneasy feeling in my gut. Or perhaps that was the hunger. I gathered my weapon and a collection of PIR sensors. I attempted to conceal the sensors in various camouflaged enclosures. A mailbox, a bird feeder, and a dead piece of wood were among the various odds and ends I used to disguise the PIR sensors.


Eventually I have every sensor hidden and positioned in roughly 30-degree intervals. While adjoining sensors might overlap given the 120-degree field of view of the PIR sensors, I figure it is better to be safe than sorry. With sensors installed and the cabling back, making the shack obscured as well, I impatiently slip back into the safe confines of my shack. Inside, I make the final connections to power and ground. I saved one sensor for the inside to conduct testing. I wire it into the breadboard amongst the jumble of wires. Now for the moment of truth. I aim the sensor toward the fire that is powering my TEG.


The LED glows bright.  Success.


That night I lay my head down on my pillow and deftly place the vibrating motor underneath. I close my eyes. Even with the added security my mind races. Will the system work? I reason that while the creepers are dead and don’t have a core body temperature of 98.6F, that they should still emit enough heat to trigger the sensors. Shouldn’t they? Honestly, I just don’t remember if they are warm-bodied or not. There were more important things on my mind, like trying to get away, but I make a mental note to find a thermometer in case I ever get the opportunity.


A week goes by and nothing has triggered the system other than my test sensor. I assume it is working correctly. Still, I have found that having this early alert system gives me some small amount of respite from the constant worry of not knowing what is outside the four walls of my shelter. That night I conduct my now nightly ritual of getting into bed and placing the motor under my pillow, just as a child might place a tooth in hopes of summoning the Tooth Fairy.


Buzz. Buzz. Buzz.


I abruptly wake from my slumber.


Okay, keep calm, I tell myself. In all likelihood it’s just a grazing deer or some other animal. I rub my eyes and turn my gaze towards the LEDs. Two LEDs are lit. They indicate movement has been detected to the northeast. I move to the peephole that faces that direction. But before I do, five more LEDs light up. Now there is motion detected to the north and northwest as well. What the heck? A few more seconds pass as I stand stock still, listening, and then the LEDs representing the south facing portion of the sensor network also light up. If it were Christmas, it would look like a red wreath.




Whatever it is, I’m surrounded.


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Michael Parks, P.E. is the co-founder of Green Shoe Garage, a custom electronics design studio and embedded security research firm located in Western Maryland. He produces the Gears of Resistance Podcast to help raise public awareness of technical and scientific matters. Michael is also a licensed Professional Engineer in the state of Maryland and holds a Master’s degree in systems engineering from Johns Hopkins University.

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