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From: david@uow.edu.au (David Wilson)
Newsgroups: comp.sys.apple2
Subject: Re: Bi-color red/green LEDs: Disk II read/write????
Date: 17 Mar 98 23:17:47 GMT
Organization: University of Wollongong, NSW, Australia.
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buggie@capella.unm.edu (stephen e buggie) writes:


>A store is clearing out bi-color LEDs red/green.   These LEDs have three
>wires:  black (negative), red (red +), green (green +).


>I want to add the bi-color LED to my Disk II drives, so that it will glow
>GREEN for READ, and RED for WRITE.   I already have an AE 3.5" 800K drive
>that does this, and it looks nice.   This will be a third enhancement to
>Disk II --- "Super Bugg-Drive."

>First, what value of resistor should be added?  The ad says these are 2
>volt LEDs, and that 20 milliamps should flow through the LED.

>Will the 20 milliamps affect the read/write performance of the drive in
>any way?

>Not sure where in the analog board to connect the LEDs.  Could I attach
>the black lead to a ground point, then touch various analog card points
>while the disk is reading vs. writing?

>Will other components be needed besides the bi-color LED and its padding
>resistors?

>Would it be simpler to just reconnect the red LED to a point on the analog
>board, and then to install a second separate LED to denote WRITE?

>Stephen Bugge			buggie@unm.edu

I have been looking through my copy of Apple's DOS manual with the circuit
diagram of the Disk ][ analog card and Jim Sather's Inside the Apple II and
think it can be done.

The existing LED circuit is:

			   560ohm	Red LED
	+12v switched ----/\/\/\/----B --|>|-- 2---Gnd	current = 

As a Red/Green LED can do 3 colours (Green, Orange and Red) but we cannot
do only Green or Red (as one LED will be run from the existing Enable while
the 2nd will illuminate when writing is also occurring), I would recommend
the following:

			   560ohm
	+12v switched ----/\/\/\/----B  --|>|---+	Green LED
						|
				Gnd--2  --------+
						|
	ULN2003 (D4-10) --/\/\/\/---------|>|---+	Red LED
			   180ohm

This way you get Green for reading and Orange for writing. This immediately
tells you that the drive has been modified and the contrast between green
and orange is higher than between red and orange.

I do not know if the write signal is actually long enough to see (formatting
a disk should be no problem but normal writes may not keep the red LED on
long enough to see). If this is the case, additional circuitry would be needed
to stretch the write pulse to the LED.

Note, this has not been tried, I make no guarantee that this will work.
Try it on scratch disks first.

To calculate LED currents, use the following formula:

	Iled = (Vsupply - Vled) / R

so for the original circuit:

	I = (12 - 2) / 560
	  = 10/560
	  = 17.86mA

The output of pin 10 of the ULN2003 is clamped to a maximum of 5 volts by
a diode so:

	I = (5 - 2) / 180
	  = 3/180
	  = 16.67mA
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia  david@uow.edu.au