Red LED displays

[everettr]

(02-12-2014 04:29 PM)Jim Horn Wrote:  LED technology has come a very long way since the days of HP's LED calculators in the 1970s. So, since my June 1975 HP-25 electronics has died, I considered re-engineering it to use higher efficiency LEDs and CMOS circuitry to get more performance and many days of use on a battery charge.

Have you looked at something like this?
http://www.adafruit.com/products/399#Description

This module is probably too large for the HP 25, since it is 71 mm long. There may be 1 or 2 mm that could be trimmed off the ends of the PCB?
It does have an RGB backlight, so you could get the traditional wavelengths coming out. I have not tried this one yet, but have heard that the contrast on these negative LCDs is low compared to LEDs or OLEDs. Of course, you would need modern electronics to talk SPI or I2C to the module.
I am curious as to the environment you would be using for development. I have thought about trying to take a lazy approach, using something like an Arduino Mega or maybe an Mbed, and dropping it into a repurposed case with a keyboard (HP 45, TI 55?). Scanning the keyboard matrix ought to be easy, and hooking up a standard serial LCD or OLED also.