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In brief

This page describes how I designed and built my greenbox:
that is, a device that magically disconnects the charger from the mains when not in use.

  • It's a simple and elegant design, introducing basic interaction design concepts.

  • Nice idea for the energy-conscious consumer and DIY's.

Why nobody unplugs

Unused power adapters should be unplugged for a greener planet, everybody knows it. Nonetheless I keep seeing them plugged all the time, everywhere.

From an interaction designer's perspective, the reason is clear: wall warts aren't designed to be unplugged - period.

As a design excercise, I have built a phone charging station that makes unplugging very easy and instinctive (with affordance playing a big role). So easy you don't even notice it unplugs automagically. The design is made from cheap and available materials, and was a great fun to build on a rainy day.

The 3 ways to unplug
(1) switched outlet
(2) manual unplugging
(3) greenbox

ugly and uncomfortable - nobody loves to bend floor-level

does not work

uncomfortable, messy, sockets always in wrong places

does not work

easy to use and cute :-)

it works!


How it works

A greenbox works like a fridge light: it powers the charger only when you open it. Close the box, and its hidden switch disconnects the charger from the mains.


Affordance is the design aspect of an object that suggests how the object should be used (quote). Affordance is a powerful tool in designer's hands: if applied properly, it help making things more natural and easy to use.

A hinged-lid box has strong affordance, as it is clear at first glance how it must be opened or closed.
A heavy, top-opening, hinged lid actually screams to be closed (it almost closes itself by gravity).
Actually, I forgot the fridge open from time to time, but it never happened for my big, top-opening freezer.

When you open a greenbox and see the charger plug inside, usage become obvious. I kept plug wire short, filling the box with soft material, in order to further confirm the idea of leaving the phone in the box for charging. Few users would seal their phone in the box for charging, but just to be sure I shaped the lid in a way that it can't close completely when someting is in the box.

It's a switch in a box

Charger aside, a greenbox boils down to a box and a switch. Although switches recycled from a dismissed fridge should be usable, I used a lever snap switch I had in my parts bin. This kind of switch is cheap and popular and it is found as safety/detect/limit switch on many mains-powered devices.

As for the box, there are no special requisites, except having a hinged lid opening on top.
Box size must accomodate the charger, switch, mains socket and wiring; still, it must be small enough to force leaving lid open when hosting the phone. Lid mass provides the force the operate the switch, so an heavy lid is preferable.



Mains power cord. I recycled mine from a broken desktop lamp.


Miniature lever switch.
They come in many different flavours. I selected one with Faston connections, and a long lever for a smooth, easy action. If needed, the lever can be manually formed to touch exactly box lid.
Ensure the switch is rated for mains voltage and current. I used a Matsushita part rated for 250Vac and 16A, very similar to the popular Omron switches from Digi-Key (p/n SW133-ND)


Power socket, the kind used for extension cords.
Do not use wall-mount types as they don't provide complete insulation.


Small box with hinged lid. I used a leather/cardboard giftbox.
The box must be as small as possible, but large enough to accomodate easily the charger, switch and power socket.
Insulating materials (e.g. plastic) are to be preferred, as metal boxes increase the risk of electrical shocks and need to be grounded.


Faston crimp sockets with insulating sleeves (full sleeve, not just a ring around the wire end).
Fastons need a special crimping tool to use them (don't even think to use pliers), so it's worth to check compatibility with available tools.


Small tubing for the lever actuator. Soft, easy-to cut plastic is best.
I used a straw and a felt-tip pen (more on this later)


I recycled some packing sponge to create an hidden compartment in the box.
Any insulating bulding material in your scrapbox will do as well (plywood, plastic, cardboard...).


Regular phone charger. I didn't touch it, so manufacturer's guarantee remains valid.



Tools and extras

  • Faston crimping tool
  • Hot-melt glue gun a glue sticks
  • Insulating tape
  • Scissors, blade cutter, screvdriver

Wiring (and warning)

Please note: I built my greenbox as a proof-of-concept prototype. Greenbox is not a finished product or design, by no means. Real products must be safe and pass code, while my prototype doesn't, e.g.:

So dont' try this at home: the information provided in this site should not be used as a guide to build one (even more disclaimers here).


Wiring a greenbox looks like adding a switch to a mains extension cord.

At first I thought that choosing a switch with Faston tabs would provide decent insulation. Wrong! Lever switches are designed as components inside machines, possibly with live wires exposed.Bottom tab on my switch is angled (see below), leaving mains-voltage metal exposed despite the insulating sleeves I used on all Faston connections.

I applied plenty of hot melt glue and insulating tape to fix the problem. By the way, I provided insulation also for PIN 3: despite being tagged as "UNUSED", it can get connected to the mains during operation.

To test the wiring before placing it in the box, I plugged a desk lamp in place of the charger, verifying that the light is ON with everything at rest. Pressing the lever must switch the lamp off.


Wiring ready to be placed in the box




I used a blade cutter to pierce a hole on the back side for passing the mains cord. A knot on the mains cord prevents unintentional pullout.

Then I secured the lever switch to a side with a generous amount of hot-melt glue.

An old felt-tip pen provided a short lenght of plastic pipe, just the right size for the straw to slide easily in, making a lever activator.

Besides guiding the straw, the pipe prevents the sponge from blocking straw movement.

I used hot-melt glue for securing the guiding tube right over the switch lever. Then I inserted the straw cutting it about an inch (25 mm) on top of box edges.

Pushing the straw operates the switch.

With ordinary scissors, I trimmed straw lenght for switching exactly on lid closure.

The switch toggles with a distinct click sound, so power is not required for trimming.


I completed the wiring inserting the charger in its mains socket and laying the wires nicely. Everything is fixed in place with insulating tape or hot melt glue.

The last step is placing the sponge. Mine was thick and stiff, so cutting it a little larger than the box (about 1 cm) is sufficient to hold it firmly in place without any additional support.

Only a short lenght of wire remains visible for connecting the phone: this detail enforces the idea to place the phone over the spongy mattress.




It was a success. For testing, I placed the greenbox in the living room, told my wife it was the new phone charger, and let her discover how to use it.

At that point I revealed her the details of my usability experiment, and we celebrated the success with a superb pizza alle melanzane from Pizzeria Centopizze.


Is it worthwhile?

I'll leave the answer to you.   My house has a digital meter, so I measured the total stand-by power. It's 150 watt, yes, 150 watt, with absolutely everything turned off.

That's an ashaming 1314 kWh per year, about 1000 kg of CO2 or an equivalent $320 on my electricity bill!

However, the amount is shared by dozens of devices silently on 24/7 (e.g. microwave oven, air conditioner units, tv antenna amplifier, burglar alarm, heating thermostats, phone answering machine, wifi router, gate opener, VCR, HDR, TV, set-top boxes, PCs, wireless phone, irrigation timer, water purifier, porch light photocell, fridge thermometer, satellite dish LNB, powertool chargers including a cheese grater, dishwasher, doorbell...).

150W  x
24 hours a day  x
365 days a year  /
1000  =

1314 kWh

italian residents pay 0.18 EUR = $ 0.242 per kWh
(as per 04.04.2009 change rate).

1314  x 0.242 = $320

Zeroing-out phone chargers alone lowers the total figure very little. But what if we could switch off more and more devices? Relevant global savings are certainly possible.

My contribution is showing how the way we design can lead to products that are both greener and more usable.
Keep it simple, keep it green, and enjoy!



Jeffrey Matthias developed Sw/tch, an award-winning switching solution based on a sensing plate. It's interesting to compare the designs from an affordance perspective.



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