19 Mar
2018
19 Mar
'18
9:06 a.m.
The circuit board material of the straight 8 front panel is fragile. The
resin
used melts at soldering temperatures. As a consequence my front panel
was munged up before I got it. Foils lifted kind of thing. I decided when
I
next took it apart I would replace with LED's. Around 1995 I had some
super bright white LED's and I worked out an appropriate current limiting
resister value and replaced three bulbs with LEDs. They work but the
color temp was far too blue. Also they activate at far too low of current
and will trigger from cross talk in the wiring when the cpu is running.
They also turn on and off too quickly. The thermal inertia of a filament is
missing.
If you were a bit crazy you could use a CPU with an 8 bit A/D and drive a
color LED to accurately emulate a bulb. Simpler is something like this.
Bulb lead
|
+----+---+
+---+ \ |
|LED| / ---
+---+ \ ---
| / |
+----+---+
\
/
\
/
-----
---
-
Parts list is LED, two resistors, and a cap. The resistor on the bottom is
to limit the brightness. The resistor in parallel with the LED is to
control
the LAMP off brightness and cross talk illumination. The cap is used to
slow down the on/off time and make it look more like it has some thermal
mass.
The minimum needed to make this work is the LED and the series resistor.
With modern super bright LEDs I would sand off the dome making a flat
face which diffuses the output making it unfocused. I am also betting
that 1 ma will be too bright but lets shoot for that. In the case of the
8/e
with an 8 volt lamp driver (assumption) if you want 1 ma of current and
the LED is ~3 volts for the white ones. (8 volts - 3 volts)/0.001 amps
gives
a resistance of 5000 ohms. I have seen white LEDs with a voltage of 2.7
and up to 3.2 and this gives a range of 4.8 k to 5.3 k for a 1 ma current.
Depending on what you get for LED's will determine the value of the
resistors needed.
My 8/e variant is a DECSet 8000 and the front panel has a red filter so it
would look pretty much the same with lamps or period correct red LEDs.
Best Wishes!
--
Doug Ingraham
PDP-8 SN 1175