Re: Bi-color red/green LEDs: Disk II read/write?-- update

[Rubywand <rubywand@swbell.net>]

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."
> ....


     Since the last post, there was a chance to do some experimentation.
The circuit below seems to work well ...


NPN transistor Collector to Bi-color LED center
NPN Emitter to GND
NPN Base to two 3300 Ohm resistors
    One resistor goes to GND
    The other resistor goes ULN2003 pin 15 (connect to R2 at point
      closest to ULN2003)

LED Red (lead has right-angle at LED) to 150 Ohm resistor
    Resistor goes to pin 6 (Q output) of a 74121 One-Shot IC

LED Green (lead has slant-angle at LED) to 150 Ohm resistor
    Resistor goes to pin 1 (/Q output) of 74121 IC

74121 IC pin 7 goes to GND
         pin 14 goes to +5V
         pin 3 goes to 74LS125 pin 8 (connect to R16 at point
           farthest from ULN2003)
        (pin 1 goes to resistor to LED Green)
        (pin 6 goes to resistor to LED Red)
         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
           Resistor goes to +5V (can connect to 74121 pin 14)


     The NPN transistor can be any general purpose NPN type (e.g. a
2N2222, etc.).  It's input (Base) is driven by the /Enable after it has
been inverted by a gate of the ULN2003. When /Enable goes low to select
the Drive, the signal at the Base goes high. This connects the LED
common lead to GND through the transistor.

     If the Drive is selected (Enabled) and there is no Write operation,
the Green LED will get a + voltage through its 150 Ohm resistor from the
74121 /Q output. So; the Green LED will light when the Drive is enabled
(e.g. during boots and for any READs).

     If the Drive is selected (Enabled) 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. 120ms pulse at output Q. For the duration of the pulse, +V
goes to the Red LED through its 150 Ohm resistor. The Red LED lights.
(Meanwhile, /Q has gone low so that the Green LED turns OFF.)   

     Probably, the first circuit posted earlier will work, too; but, I'm
still not sure that Write Request is always long enough to be easily
visible. This is why the One-Shot IC circuit is used. (You can make the
pulse longer by using larger values for the capacitor or increasing the
value of the 22000 Ohm resistor.)

     Anyway, the circuit works well for showing Reads and Writes with a
standard bi-color LED. It's the one I installed on our Disk ][ drive.



     If you decide to try the NOR gates circuit posted earlier, the mod
below is a simpler version which should do the same thing
...               

In the circuit below, "[ ]" numbers are Disk ][ Analog board
connector points and "( )" numbers refer to pins on a 7402 Quad
dual-input NOR gate IC.

  [10*]/WR-REQ ------ (2)
                         NOR-1 (1)---- 150 Ohm Resistor to LED Red lead
  [14] /Enable -------(3)           | 
           _________________________|
           |
           |
           |__________(11)
                         NOR-3 (13)--- 150 Ohm Resistor to LED Green
lead
  [14] /Enable -------(12)

  (7)  to GND
  (14) to +5V

*Note: To have the Write LED remain OFF when Write Protect is ON,
connect instead to 74LS125 pin 8 (connect to R16 at point farthest from
ULN2003)

LED middle lead goes to GND



     In both the One-Shot and the NOR gates circuit the IC is driving
LEDs. This is why 74xxx instead of 74LSxxx or CMOS IC's are specified.
Should you use a higher-current bi-color LED, it may be necessary to add
the driver transistors included in the original circuit.

     By the way, it turns out that the WR Data signal is frequently
active (with pulses) even when a Write is not in progress. This pretty
well makes it useless as a way to monitor Writes.


 
Rubywand