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09.019 I would like to add a Read/Write indicator to my Disk ][ drive. How can I do this with a bi-color LED?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
09.019 I would like to add a Read/Write indicator to my Disk ][ drive. How can I do this with a bi-color LED?
The circuit described below works well. It shows Red for Drive Enabled +
Write and Green for Drive Enabled + not Write. That is, a Read is assumed if
the drive is ON and no Write is occurring. The advantage of this approach is
that you always have a lighted LED 'drive ON' indicator and, so, there is no
need to drill a new hole in your drive panel. The new LED can use the hole
occupied by the old "in Use" indicator LED.
Parts
IC- 74121 or 74LS121 1-Shot multi-vibrator
Qgreen- 2N2222A gen purpose NPN transistor
Qred- 2N2222A gen purpose NPN transistor
Rcath- 120 Ohm 5% 1/4 watt resistor
Rgreen- 3.3k 5% 1/8 watt resistor
Rred- 3.3k 5% 1/8 watt resistor
Rp- 20k 5% 1/8 watt resistor
Cp- 10uF/10v 10% "dipped tantalum" capacitor
LED- 2.2V 20ma 3-lead (common cathode) Green/Red bi-color LED
Building the Circuit
IC- locate pin-1 and mark it on bottom side with white-out.
'Dead-bug' mount the IC using epoxy in the open area near
top-middle of board with pin-1 end pointing to the right.
connect Rcath to solder pad at - (minus) end of C2
connect #20 wire from IC pin-7 to solder pat at - end of C2
connect Rp from IC pin-11 to IC pin-14
connect Cp + to IC pin-10 and Cp - to IC pin-11
connect #20 wire from IC pin-14 to solder pat at + end of C4
connect a wire from IC pin-3 to end of R16 closest to 74LS125
connect Rgreen to IC pin-1 (74121 "/Q" output)
connect Rred to IC pin-6 (74121 "Q" output)
solder transistor Qred Collector to Q1 power transistor Collector
(solder pad area at left front of circuit board just to left of
the power transistor). Position toward left edge of pad.
solder transistor Qgreen Collector to Q1 power transistor Collector
solder pad to the right of Qred.
LED- connect a 3-wire 11" cable to the LED (black to center, red to
to lead with right-angle bend, green to lead with slant bend).
Old "in Use" LED- pop off the retainer ring and push through the
LED. Leave the LED mount in the hole. Cut off the LED, spread
cable ends and tab over with cellophane tape. (Put old LED and
ring into parts box.
Spread apart retainer 'leaves' of LED mount (at back side of panel)
to permit easier insertion of new 3-lead LED.
Run LED + cable over back of board and along bottom toward the
hole in the front panel. Push through hole. (If hole is too small,
use a Dremel tool and steel 'bulb bit' to slightly enlarge front
part of hole.)
connect LED cable black to free end of Rcath
connect LED cable green to Emitter of Qgreen (right transistor)
connect LED cable red to Emitter of Qred.
position old LED cable beneath new LED cable.
connect a wire from the free end or Rgreen (on IC pin-1) to the
Base lead of transistor Qgreen.
connect a wire from the free end or Rred (on IC pin-6) to the
Base lead of transistor Qred.
Add epoxy to the back of the LED and holder to secure the LED.
How it Works
When /Enable goes low to select the Drive, the power transistor (Q1)
switches ON and supplies +12V at its Collector. If there is no Write, 74121
output /Q is high, Qgreen conducts, and the LED shows Green. That is: the Green
LED will light during boots and for any READs.
If the Drive is selected (/Enabled is low) and there is a Write operation
and Write Protect is not ON, then, the /Write Request signal at 74LS125 pin 8
will go low. This triggers the 74121 One-Shot (at pin 3) producing an aprox.
160ms pulse at 74121 output Q. For the duration of the pulse, /Q is low and Q
is high. Qred conducts, and the LED shows RED to indicate WRITE.
By: Dan DeMaggio
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