There was a recent opportunity to do the two-color LED mod to some
Disk II drives-- i.e. A bi-color LED is swapped in for the old red "in
Use" LED. It shows Green for Reads and Red for Writes.
Additional testing during the recent work led to an improved
circuit and the discovery of an error in my description of the earlier
First the correction. My earlier post says ...
Quote:> pin 11 goes to + end of mini 10uF capacitor
> pin 12 goes to - end of the mini 10uF capacitor; and
> pin 12 goes to a 22000 Ohm resistor
There is no connection to pin 12 of the 74121. The above should have
Quote:> pin 10 goes to + end of mini 10uF capacitor
> pin 11 goes to - end of the mini 10uF capacitor; and
> pin 11 goes to a 22000 Ohm resistor
The original circuit works fairly well (if you figure out that the
directions contain an error); but, breadboard tests indicate that the
74121 can not adequately source many LEDs which might be employed.
The new circuit uses transistors to drive the LED sections. By
getting LED power from the +12V line switched by power transistor Q1 in
response to /Drive Enable, the transistor used for this purpose in the
old circuit is eliminated. So, the new circuit needs only two
transistors-- just one more than before-- and drives the Green/Red LED
at higher brightness.
As in the original design, the new circuit shows Green for Drive
Enabled + no Write and Red for Drive Enabled + 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.
IC- 74121 or 74LS121 1-Shot multi-*
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 to secure LED in place
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.