An Engineer in Wonderland – Incongruous components lurk in my calculator

calculator-out-tiny2.jpgI had to take a calculator apart this weekend, and inside I found a glass-package diode and an led with no light path to the outside world.

Guessing, I could vaguely imagine a germanium diode helping the solar cell operate, but an led?

Although a recollection is that certain types of green leds make good low-voltage Zener replacements.

calculator-in.jpgDo excuse the awful photo. I need to buy a tripod.

Both are clearly have to be there, as there are plenty of surface-mount components scattered around, and the pcb has been cut away to accommodate the diode.

Anyone know why these to are inside.

By the way, the calculator is a Sharp EL-240S which has an unusual feature for small calculators – as well as displaying normally, like a desk calculator it can truncate to two decimal places, or no decimal places.

I realise this is mostly useless to engineers, and the mode-change is amazingly both touchy to initiate and can happen accidentally, but it is another quirk that endears me to this gadget. 


Respond below, or to alice@electronicsweekly.

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I will keep it that way for as long as possible.

Tags: Wonderland

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  1. 'Alice'
    February 03, 2010 13:31

    Thanks Steve
    That is where I was going too.
    I need to find some time to:
    A, probe the caculator
    B, contact Sharp

  2. Steve Kurt
    February 03, 2010 04:01

    how about this hypothesis: the diode and LED are related to the solar panel and battery charging. The diode is to prevent the battery from discharging through the solar panel, and the LED is a shunt regulator to prevent overcharging the battery.
    The only problem I have with this idea is that these parts would have a lot of leakage current (I think) and would drain the battery fairly quickly.
    I think this calls for poking around with a meter and figuring out what’s going on!

  3. Mike Meakin
    February 01, 2010 14:28

    I surmise they might be used as a temperature compensation circuit for the LCD display.
    Are they connected to each other – the picture doesn’t show quite enough tracks ?
    The diode looks like a germanium type so perhaps battery reverse polarity protection ?
    The LED perhaps a a rough 1.8V voltage reference ?

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