User`s guide

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From: Rubywand
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).