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How to make your own write protect DIY style USB-Drive.

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#1 MedEvil


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Posted 03 October 2014 - 10:15 PM

If you're a technician, you know the value of a USB-Stick with write-protect-switch.
And you will probably also know, that this class of device is disappearing faster than VHS.;)

The crazy part about this is, that every controller chip used, still comes with the wp feature present.

So it's basically just a question of adding a switch at the right contacts.

Depending on the design of your USB-Stick finding a small enough switch or a suitable place to install it, may be the biggest challenge.
This however is completely up to you and not covered here. ;)

But now to the electric part.

At first a disclaimer. Soldering on smd level is required! If you don't know, what you're doing, it is very possible to destroy your device. So exercise good judgement.

Now let's get started.

The first thing we need to know is, what controller resides inside our target device.
To figure this out we can either simply read what's written on the chip or use a software like 'chip genius', if we do not want to open the device yet.

Once we have the information, we need to find the data sheet for the chip.
We simply google it.

From the sheet we need the following data.
- which pin enables write protect (sometimes also labeled wp or w/p)
- if wp is high-enabled or low-enabled
- if it is high-enabled the positive power terminal of the chip

How to properly use the information?

High-enabled simply means, that wp is turned on when the pin is supplied with the correct voltage.
What is the correct voltage?
Well luckily we do not really need to know or care. We simply connect the wp pin via the switch to the positive power terminal of the chip. That has always the exact right voltage.

Low-enabled means, that wp is turned on when the pin is drained of its voltage.
We do this by simply connecting the wp pin via the switch to ground (GND).
Ground are always the wide circuit traces. It does not matter which or where we connect to.

That's it! Not really complicated, but very fiddlig.

And now ....

Last but now least ....

To the really cool thing about this ....

The same goes also for USB-Enclosures!!!

Yep, you've heard right, you can build your very own write protected USB-HDD or even USB-SSD.
Something that's not even on the market, as far as i know.

btw. Another advantage of the enclosures is, that they are pretty big and roomy in comparison. So way easier to find a right sized switch and place it.

Have fun with this little weekend project.

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#2 RoyM


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Posted 04 October 2014 - 04:09 AM

"Thank You MedEvil, Wonderful information."
I will be exploring this little gem of info for all my devices,
and then hopefully contribute to this thread in the near future with my experiences.
Please allow me to expand on some of the electronic stuff.
Sorry MedEvil but you said:
Well luckily we do not really need to know or care. 
We simply connect the wp pin via the switch to the positive power terminal of the chip. That has always the exact right voltage.
So yes, using the "positive power terminal of the chip" or Vss is advised.  (Voltage supply source)
We do need some care here though. 
If you supply an input pin on an IC device a higher voltage than Vss,
"You WILL destroy that input and perhaps the device."
Also, It is not a good idea to connect "any" pin of an IC directly to Vss or Ground,
unless it is, Vss or the Ground input pins, "Usually one of the corner pins of the IC chip, But not always, You "must" check the Data Sheet."
http://www.datasheetarchive.com/   <-- This is one of my bookmarks I use.
The idea is, that you don't want to subject a sensitive input pin on the IC
to all the nasty, dirty things going on at the power supply level.
so, you add a small amount of resistance in-series, to pull the pin low, or hi as needed. 
A 1000 ohm resistor is usually sufficient.  "Check Your Data Sheet"
Below is a simple schematic of how to acheive that.
You Simply solder a 1Kohm resistor to the common terminal of the spst (Single Pole Single Throw) switch.
then the Normally open contact goes to ground, and the Normally closed contact goes to Vss
Pins on the switch are usually marked as such, C= Common  NC= Normally closed  NO= Normally open 
                    1 Kohm Resistor
               .___/\/\/\___ Common --> wp pin
Vss  <--./  .-->  Ground
       NC     NO
To reiterate as MedEvil noted, "PLEASE use good judgement.", 
and further, you may destroy your hardware or PCB traces with a soldering iron if not careful.
"Some helpful hints will follow, from a professional in the industry"
I just recently purchased a Tenma rework station from MCM for <= $60 US.
I have yet to use the station?, but on first inspection it seems like a very solid
device concerning the hot-air side, but it also comes with a very fine tip
on the soldering-station side which I hope to use very soon.
I like to think it will occupy a dedicated spot on my bench very soon.
For those of you just getting into servicing SMDevices, 
Please allow me to suggest: http://www.chipquik.com/store/  <-- This is a wonderful product.
In a nutshell, it is a solder that melts at a lower temp, and then also stays molten longer.
allowing for easier removal and desoldering of smd devices.

#3 MedEvil


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Posted 04 October 2014 - 10:35 AM

Thanks for providing your additional information RoyM and correcting some of my bad unusual english. ;)

But i'm not so sure about your switching diagram. It makes things a bit more complicated and i checked two of my USB-Sticks, which come with a wp-switch, before beginning the conversion. They both had simple open-close switches.

Just some points i like to clarify to any layman reading this thread.

- Adding a small resistor to the circuit is a good safety feature. I might will add it too.

- A rework station is a nice thing to have, but definitely not needed here.
A soldering needle or electronics soldering iron with temperature control are sufficient even with little experience.

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