Led used as regulator in calc's
 

I took apart a bunch of old calculators and found that the LED's are actually working as a cheap power regulator. Usually found in solar / dual power calculators to keep the voltage limited to about 1.5-1.6v (that's where a red led begins to operate).

On one solar calculator, I took out the led, and the display would show all 8's! Every display segment lit up when I got close to strong light source or sunlight. Without the led the thing gets overvoltage from the solar panel and goes nuts!

Most solar calculators have a panel with 4 or 5 segments, and each one of those segments gives about 0.5v (with full sunlight), so what the manufacturers do to make these things run even with low light sources, is to put more solar segments than needed

The 5 segment panel can reach up to 2.5v with full sunlight (and the led is there to keep the calculator from getting any more than 1.5-1.6v

I found that putting black tape over half the solar panel would make it work like it still had the led in it. (leaving about 3 segments uncovered, for 1.4-1.5v in bright light)

i paid i believe like $5 at allelectronics.com ( i think) about half a year ago and got like half a pound of leds, i had (and probably still have) about 60 of those styl leds in 4 different colors.