LED in HP 82104A



Do you know some things about the funktion of the LED in the HP 41 card reader?

Best regards.




I have one working card reader and I have seen at least 15 of them. I have never seen a LED in any.

I'm somewhat curious; does it work alright? What is the possibility it's not a card reader inside? I read in this forum that a sort of MLDL was built inside a card-reader's box.

Anybody know anything about it? Is it possible that you have an MLDL (machine-language development l...(?)) instead of a card-reader?

If it is indeed a card reader AND if you, one day, open it, please, take pictures so we can figure out what is this LED for./




I´m sure that it is a LED, but i have never seen it lighting (IR ?). I have an normaly card reader (I hope).

Best regards



I don't know exactly, but I think it's some kind of light barrier ('Lichtschranke')




I think it a IR LED that, with a receptor, "sense" the presence or the card insted or a dip switch activate with the card


I think, that the "sense" is a electrical contact. I don´t see a photodiode or something else. Only a stand alone LED (no colour).



It is an IR led used to detect when the card edge has passed by. I've never had to figure out if it is for start-of-card, end-of-card, or write-protect. There is a tiny photodiode mounted on the little circuit board that forms the switch contacts. The card passes between the LED and the photodiode.


There is a photodiode (or maybe a phototransistor) on the switch contact PCB on top of the reader mechanism. This LED (which is an IR one, so you won't see it glow) forms a light barrier with that detector to
sense the clipped corner of the card fro wrtie protect. Somewhat suprisingly, the card-inserted and card-at-head functions are still performed by mechanical switches.
The LED is driven by the 1826-0322 sense amplifier chip -- the part of that chip that was used for motor control in (e.g.) the HP67.


Hi, Gunnar.

I understood the LED was outside the card reader, as you did not mention your card reader was openned. I gave my thoughts wings and they have gone far, far away...

Indeed, the card reader has an inner tinny light-sensytive device.

Sorry for the bad clues...


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