I 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.
Do 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.
'Alice'
Respond below, or to alice@electronicsweekly.
No email addresses are collected for marketing purposes from responses to this blog.
I will keep it that way for as long as possible.
Comments (3)
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 ?
Or
The diode looks like a germanium type so perhaps battery reverse polarity protection ?
The LED perhaps a a rough 1.8V voltage reference ?
Posted by Mike Meakin | February 1, 2010 2:28 PM
Posted on February 1, 2010 14:28
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!
Steve
Posted by Steve Kurt | February 3, 2010 4:01 AM
Posted on February 3, 2010 04:01
Thanks Steve
That is where I was going too.
I need to find some time to:
A, probe the caculator
B, contact Sharp
'Alice'
Posted by 'Alice' | February 3, 2010 1:31 PM
Posted on February 3, 2010 13:31