Introduction: Electro Static Discharge (ESD) Safe Seat Cover
Electro Static Discharge (ESD) is a big problem for electronics. Static electricity can build up on surfaces (you, your table, your chair, your floor, etc) and when it discharges it does so with a bang. When you get a shock touching a door handle or another person after shuffling around on your carpet that shock is the result of electro static discharge between you and the object. While it's only annoying and surprising for you, it's critically bad for your electronics projects. If your workspace isn't ESD safe then you're taking chances with your ESD sensitive components. The most insidious aspect of ESD damage is that it is cumulative. Your components will continue to work but they won't work as well and any failures are likely to be inconsistent and mysterious. You can read more at sites like http://www.esda.org/ (The home page of the Electro Static Discharge Association).
The good news is that it's relatively inexpensive and simple to protect yourself from ESD damage. The key is making sure that all parts of your work bench (including yourself) are at the same electrical potential. You do this by working on a static dissipative mat (http://www.esdmat.com/), wearing an ESD wrist strap (http://en.wikipedia.org/wiki/Antistatic_wrist_strap). Even though you're wearing a wrist strap it's important that you don't do anything to build up charge on your body. Shuffling your feet on the carpet or sitting on a synthetic fiber chair can both build significant static charge on your body.
My electronics work bench is right behind my computer and so it would be convenient to be able to swivel around from computer to work bench on my office chair. The problem for me is that my office chair is upholstered with polyester and builds static charge on my body very quickly. As a result I never use my office chair while working on the electronics bench and switching from computer to project is as a result a pain in the neck. Since I'm often programming an Arduino or other microprocessor I have to switch back and forth quite often.
The solution was to fit my chair with a static dissipative surface. My search for ESD safe chair covers brought up a number of very expensive professional options which were well outside of what I wanted to spend and so I turned my attention to making one from scratch.
I knew the surface needed to be electrically conductive and I first looked into conductive fabrics like this stuff: http://www.sparkfun.com/commerce/product_info.php?products_id=9769. The problem with that stuff is, once again, the price tag. I knew I could do better price-wise. The solution turned out to be space blanket. This stuff is polyester sheet with aluminum vapor deposited on it in a super thin layer. The aluminum layer is conductive and the stuff is super cheap; it was the perfect solution.
Step 1: Ingredients
Stuff you'll need:
1) Space Blanket. You can get this from camping stores or good hardware stores. You can also pick it up cheap online like from Amazon. Expect to pay around $3 (US) for one.
2) 18-gauge stranded copper wire, preferably with green insulation (green is always ground in the US). 6 or 8 feet will be fine.
3) One 500 kilo-ohm resistor. You could just as easily use a 1 mega-ohm or 2 mega-ohm resistor. The purpose of the resistor is to make sure that current doesn't flow from some other part of the project into your seat cover. That wouldn't be nice at all.
4) Heat shrink tubing
5) Two alligator clips.
6) Packing tape (optional)
Step 2: Upholstery 101: Make the Seat Cover.
For the seat:
Unfold your shiny new space blanket and inspect it. One side will be shinier than the other; this is probably the conductive side. Get out your multimeter and test for conductivity. One side should be non-conductive (the polyester only side) and the other will be very conductive (the aluminized side). You'll want the aluminized side to be facing up on the chair so that it is in contact with your body.
Using sharp scissors cut out a strip of space blanket that is a little wider than your chair and long enough to cover both the seat and chair back with some extra for folding over the top of the back and the front of the seat.
Lay the space blanket (aluminum up) on your chair and arrange it such that it won't be stretched when you sit. The extra bit at the top of the back should be folded over the top and taped such that a pocket is formed for the chair back to sit in. Do the same thing to the space blanket on the seat bottom. You should end up with two pockets: one at each end of the space blanket strip.
Step 3: The Conductor.
Your seat cover doesn't do any good if it's not grounded to the grounding system on your work bench. All of your ESD safety equipment needs to be at the same potential. You'll have to build a cable to plug your seat cover into your ground network.
1) Take a small (5 inches, or so) length of flexible green wire and solder an alligator clip to one end. Heat shrink over the connection.
2) On the other end of the small length of wire solder one terminal of the resistor.
3) Cut a longer piece of the wire (6 feet or so) and solder one end of it to the other end of the resistor.
4) Use heat shrink around the resistor to relieve the solder joints of stress.
5) On the other end of the wire solder the second alligator clip. Make sure to put your heat shrink on BEFORE you attach the alligator clip. If you forget you'll have to rework this step and you'll be disappointed in yourself. If your grounding system uses banana plugs then feel free to use a banana plug here instead.
6) TEST your cable using the ohmmeter built into your multi-meter. The cable should have the resistance of the resistor you incorporated into the cable.
Step 4: Plug It All In!
Now that your upholstery job is done and your conductor is tested and working the only thing left to do is to plug it in!
Clip one end of the wire to your seat cover and clip the other end onto a terminal of your ESD protection grounding system. If you measure the resistance from the grounding apparatus to any point on the seat cover you should get something just slightly higher than the resistance of the cable alone. One concern you may have at this point is that the cable hanging loose from the seat cover could eventually tear the space blanket material. In order to avoid this, tape the cable to the blanket to provide strain relief.
Now that your seat is safe you can swivel back and forth from computer to work bench all you want without fear of cataclysmic damage being inflicted on your project's bits. Your shiny space age seat is sure to impress your friends.
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