Computing stuff tied to the physical world

Meet the Utility Plug

In Hardware on Aug 23, 2010 at 00:01

To end the current series of new plug announcements, here’s one which is a bit of everything and nothing, called the Utility Plug:

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It comes as a kit:

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It brings out 6 pins to a modular jack, the same kind as used by fixed-line telephone sets around the world.

In its simplest form, you can just put a dab of solder on each of the 4 solder jumpers, and you’ll end up with a connector that brings out the 6 pins of any of the ports on a JeeNode. This plug kit includes a 1-meter length of flat white cable with a connector attached to one end. The other end is … up to you!

Modular plugs and jacks were chosen because they are very easy to get and crimp on (there are cheap plastic tools for it). If you don’t need the PWR and IRQ pins, i.e. if you only need to use AIO, DIO, +3V, and GND, then you can even take a spare telephone cable, cut it in half, and end up with… not one, but two pieces of cable which fit this connector! That’s because most telephone cables only connect to the inner 4 wires, but they still use this same 6-pin connector.

As far as I understand it, the 6-pin variant is called RJ-12, and the 4-pin variant RJ-11. They can be mixed because they have the same physical dimensions.

One reason I’ve wanted this Utility Plug for some time, is simply to have a more solid connector to plug and unplug stuff from, without having to mess with 6-pin headers. Unlike the headers, modular jacks are polarized, so there’s never a chance of connecting stuff the wrong way around – quite important once you’ve built a project and finished it by putting it inside a box, for example.

But that’s just the first half of the story…

As you can see, there’s a bit more going on with this plug. There are some places to add resistors, transistors, and diodes. The reason for this is that you often have to put a resistor in series (for say an LED), or to “pull up” the voltage on a pin, or even a small transistor to handle a little more current than a ATmega pin can by itself. Other uses would be to adjust the level of input signals, or to add reverse-polarity protection to an input pin.

The Utility Plug has 2 identical areas to handle such simple modifications:

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Each of those dotted areas has the following circuit traces laid out for it:

Screen Shot 2010 08 22 at 19.55.15

(this is the DIO side, the AIO side is virtually identical)

This won’t be sufficient to deal with every scenario you might encounter, but my calculations tell me that it will deal with 27.183 ± 0.0002 different cases :)

To make this practical, a few extra components are included in the kit:

  • 2x 100 Ω, 1 kΩ, and 10 kΩ resistors (1/6 watt)
  • 2x 1N4004 diodes, for AC to DC conversion and to drive inductive loads
  • 2x 2N4401 transistors which can switch up to 500 mA @ 40V

One word of advice when using this plug, is that you should really do some back-of-the-envelope calculations, to make sure things will work within the range you’re designing your circuit for. The resistors will let you keep currents down, and the transistor will let you generate a slightly more powerful output signal.

Remember: “E = I x R” is your friend!

Other components than those supplied can also be used, of course. Each section on this Utility Plug is nothing more than a miniature breadboard area with some pre-defined circuit connections.

See the café and shop pages for further info.

Now I’ve got to fire up some projects to use this thing. I’ll keep you posted when I get to that.

  1. great plug. minor nag : you say you deliver 1/6 watt resistors. never heard of those. should that read 1/8 watt ?

    • No, they really are listed as: 0.167W, 1/6W – 0.075″ Dia x 0.134″ L (1.90mm x 3.40mm), by Yageo.

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