Classic LED displays


Hi, can anyone tell me what is the normal forward voltage and current for a segment in the classic LED displays ?



Hello John,

The average current in a segment typically ranges from 1 to 3 mA. However, the Classic display is multiplexed, so the peak current is approximately 15 times this value. You'll find a detailed description of the Classic display multiplex mechanism in one of the HP-35 patents, available on Eric's website:

I hope this helps!

Joel Setton


Thanks Joel, I'll check out the patent. I just wanted the information for constructing a tester for the classic displays. I won't be multi-plexing, just driving each segment with it's own current limiting resistor.



Good question John.

You put the 3.6 V DC '+' polarity on pin 14 of a Classic LED cluster (Anode a) through a 100 ohms resistor (here I took a 82 ohms handy) and the negative polarity on pin 7 (cathode 1) ; you light segment 'a' on digit 1.
If you use a 300-400 ohms resistor the light will be dimmer.
These LED voltage drop is 1.7V .

A cheap Classic Led tester!

See the Led Cluster pin out on my site here (to test other segments and other cathodes):

HP-35 Display system

Edited: 10 Oct 2009, 4:44 p.m. after one or more responses were posted


Red LEDs forward drop is 1.6 Volt (Plank's constant and the electron charge are related to such voltage drop and the wavelenght of the red light obtained; but that is another story), so you can select your resistor as per

R = [[VCC - 1.6 V]*1000] / I


R is the series resistor, in ohms

VCC is the power supply voltage, in volts

I is the desired current, in milliamperes

For VCC = 5 Volt, and I = 10 mA, a 330 Ohm resistor is adequate. Please check with the display spec sheet about a safe current value before starting your tests. Good luck!!

All this assumes a permanent condition, no multiplexing or pulsing of any kind.


Hi John,

As I was sorting old stuff I found the datasheet for the Classic displays (5082-7405). If you'd like a copy, please let me know.

Joel Setton

Possibly Related Threads...
Thread Author Replies Views Last Post
  A Wonderful Classic HP Calculator BShoring 2 1,143 11-22-2013, 05:25 PM
Last Post: BShoring
  Classic battery polarity Kiyoshi Akima 2 1,328 11-08-2013, 01:31 AM
Last Post: Kiyoshi Akima
  CLASSIC/SPICE KEY LEGENDS Mike T. 2 1,184 08-21-2013, 04:53 PM
Last Post: Mike T.
  Woodstock, Classic & Spices for iPhone Matt Agajanian 8 2,386 07-10-2013, 06:10 PM
Last Post: BShoring
  Safe charging LED HP calcs Anoop Sahal 6 1,878 03-29-2013, 09:08 AM
Last Post: Anoop Sahal
  Classic calculator display chip type number? Juergen Hofstaedter 3 1,313 01-22-2013, 12:47 PM
Last Post: Eric Smith
  Classic Series Enter Key Dan Lewis 15 3,547 12-19-2012, 09:10 AM
Last Post: Thomas Radtke
  Classic (HP-67) charger issues revisited Matt Agajanian 6 1,927 05-20-2012, 10:50 PM
Last Post: Matt Agajanian
  Classic battery contacts james summers 2 1,275 04-08-2012, 01:45 PM
Last Post: james summers
  That's definitely a Classic! Matt Agajanian 3 1,163 04-02-2012, 02:13 PM
Last Post: Luiz C. Vieira (Brazil)

Forum Jump: