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09.001 How do I add a write-protect On-Off switch to my Disk ][ drive?


This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.

09.001 How do I add a write-protect On-Off switch to my Disk ][ drive?

    By:  Stephen Buggie

Adding an Auto/Manual Write-Protect (A/MWP) Switch

     Often, as when doing copying, it is useful to be able to guarantee that a
diskette is Write-Protected whether or not the side is notched. Other times, it
is convenient to defeat Write Protection-- for example, when you wish to write
to a diskette side which is not notched. And, naturally, you also want a
setting which permits Normal, notch-controlled, Write-Protect.

     Based upon a circuit suggested by David Wilson (Australia), the A/MWP
enhancement offers full user control of Write Protect. The switch's three
positions are

Normal:      a diskette side must be notched to permit writing.
Protect ON:  Writing is prohibited regardless of notching.
Protect OFF: Writing is allowed regardless of notching.

A/MWP Step-by-Step

Unplug the drive from the disk controller and remove the case.

Drill a mounting hole. This can be at a convenient spot in the back or in the
plastic front panel. On the front, a good spot is at the lower left, above and
to the left of the "in use" LED. Another open spot is at the upper right in the
space just above the diskette slot; but, we're saving this place for Part 2's
enhancement. (You can't use the lower right, of course, because this would mess
up the "Apple" logo!)

Cut three wires (Brown, Black, and White) long enough to run from the mounting
point to the Large Connector plugged onto the back, middle of the Disk ]
[ circuit board.

Solder the leads to a Single-Pole Triple-Throw mini toggle Switch:  Brown to
center, White to one end, Black to other end. Twist the leads or encase them in

Mount the Switch. Normally, the Switch handle will point ...

o- in the Black lead direction for "Normal" (center and White connected)
o- at Center for "Protect ON" (no connection).
o- in the White lead direction for "Protect OFF" (center and Black connected).

Mounting the switch with the Black lead on top is recommended because it is
easy to remember that Up = Normal. Route the leads to the area next to the
Large Connector. (Make sure no wires will get in the way of an inserted

Locate the Brown and Black leads coming from the Notch Detect micro-switch. The
leads are the Brown (bottom) and Black (top) pair near the right end of the
Large Connector (as viewed from the front of the drive).

Cut the Notch Detect micro-switch leads about 1" away from the Large Connector.

Connect the Black Notch Detect, Black Large Connector, and Black Switch leads
(i.e. strip ends, solder, and cover in heat-shrink tubing or tape).

Connect the Brown Notch Detect lead to the White Switch lead.

Connect the Brown Large Connector lead to the Brown Switch lead.

     Viola! Now you're ready to replace the cover, plug in the drive, and try
out your A/MWP enhanced Disk ][.


By:  Ed Eastman

     Most of the time what you want to be able to do with a Write Protect
modification is turn On Write Enable when a disk is not notched. That is what
this mod for does. Details relate to the 5.25" Unidisk but the method will also
work with other Apple II 5.25" drives.

     The way the write protect sensor works on newer drives is that an LED on
one side shines on a phototransistor (the sensor) on the other side. When there
is a notch in the diskette you insert, light passes through the gap and the
sensor 'closes' to complete the Write Enable circuit.

     What we will do is give the Write Enable circuit an alternate enable
option using an On/Off switch. A mini toggle switch is okay; but, I like to use
a small normally-open momentary contact pushbutton switch from Radio Shack.
It's compact, looks nice, and pressing the button for the few seconds usually
required for a file, etc. write is no problem.

     I normally mount the switch in the upper left part of the face plate,
opposite the light to balance the look. Drill a hole a little smaller than
required and use scissors or a larger drill to taper the hold to just where you
can screw in the switch.

     Before soldering on leads and mounting the switch, decide where you want
to make the connections. You can locate the output leads from the sensor and
splice one switch lead to each; or, you can find the place on the circuit board
where the sensor leads go and connect there.

     On a Unidisk you will see a large connector labeled "CN1" near the front.
Pins 9 and 11 of CN1 are the write protect sensor connections. More convenient
connection points are the circuit board edge side of R12 and the wire at J29.

     Solder on leads long enough to reach the connection points and screw in
the switch. On a Unidisk, connect one switch lead to the R12 point near the
edge and one to the wire at J29.

     Now when you need to override write protect on an unnotched disk, you
simply press the button while writing. Go ahead and plug in the drive and give
it a try.

By:  Stephen Buggie


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