I have a question about red LED HPs. One of my sons gave back to me for Christmas, the HP-31E that I had given him 22 years ago for school. I have two other HP calcs with red LEDs: a 25C and a 67. Some observations: The numerals of the 31E are larger and brighter than the other two. Also while the LEDs of the 25C and the 67 are about the same size, it seems to me that the display of the 25C is somewhat brighter than that of the 67.

Any thoughts?




There are many factors that determine the brightness of the LED display. Each display is a different kind: The HP-67 has (as I recall) 3 LED modules with 5 digits each soldered like chips on the circuit board. The HP-25C has a module that fits over pins, and the 31E is similar, but has pins soldered to the module, and it slides into the circuit assembly. All have magnifying lenses.

The light output is dependent on the size of the LED and how it is magnified, the darkness of the red filter lenses, the rate of "scanning" (only one digit is "lit" at a time), the lumen rating of the LED (usually expressed in micro-candle power), and many other factors. The method to adjust some resistors on the HP-67 to change the brightness is posted on old threads on this Forum. I'm sure the same can be done with the HP-25C, though with the way the 31E is made, it would seem difficult to change the display brightness.

My brightest LED calculator by far is an old TI SR-50A. Those LED's are REALLY bright.

Hope this helps.


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