HP-41C Card Reader


I have a question on something I see after removal of the drive assembly from my HP-41C card reader. I can send some photos to anyone, showing a red arrow pointing to the component in question here. I don't know how to attach the image to this posting. It looks like a clear cased LED. In my reader one of the two gold leads to the clear "LED" case has broken from the "LED" head. Is this really an LED and if so what does it do and can anyone give me the specifications for it if I need to replace it?

Also, if it does need to be replaced, how do I get the circuit board out that it is soldered to? Does that whole assembly just lift out of the case, circuit boards and all? It appears that it is just stuck in with some adhesive. Perhaps just cutting the two gold leads and soldering the new component to the old leads.



if the image is somewhere on the internet, you can include it via:
'open square bracket' img:http://www.hpmuseum.org/gto.gif 'close square bracket'

see the advanced formatting features


Thanks Peter, but the image is a JPG image from my camera. However I have finally found that this must indeed be an LED that shines through a small hole to a detector above it on the small circuit board on top of the slot assembly. I suppose it is to detect the card insertion and start the motor since the card interrupts the light from hitting the detector when it is inserted. Now, all I need is to find what kind of LED it is so I can replace it. It looks like a small 3mm diameter LED but I don't know the voltage and current rating of it.



The entire assembly does come out, but first you need to remove the top cover of the piece that plugs into the HP41. It is a little tricky, but it does come off. It is easier to snap it back into position than it is to get if off in the first place. However, once the cover is off, the entire circuit board assembly can be lifted out enmasse. This will allow you to unsolder the old LED leads and replace it with a new one. I believe the LED is a standard IR diode, the same type used in TV remote controls. Should be able to get one from Radio Shack. Make sure you get the polarity right. Voltage and current ratings should be compatible.
John B


Great info John. Thanks a bunch.



The LED is part of the write protection electronics, there's a photo transistor on the switch pcb (the little golden "nipple" pointing downward), driving the write protect switch signal

If this transistor receives IR, it drives WPS active and the card is protected. Unclipped cards block the IR from the transistor and the protection is off. So your reader should already work without the LED, at the cost of missing write protection.

Good luck!


Thanks Rainer.

Which one is the IR emitter and which one is the detector? The clear LED is on the bottom PCB below the card slot assembly. The small photo transistor (?) is on the top of the card slot on the switch PCB as you described. I went to look at some parts this morning and the IR emitter units were in a dark package and the detectors were in a clear package. The part that is broken is the clear LED (?) on the bottom PCB and it looks to be a 3mm size. A larger size will not fit up beside the transport wheel to see the small hole that looks up toward the photo transistor(?)on the switch PCB. I hope this is not too confusing a description.

If all else fails I will just go ahead and use it without the write protection.


John, regarding the polarity of the IR LED, it is usually shown by having one lead shorter than the other or by the LED lens having a flat shoulder on one side. Off hand do you know which of the two leads is the cathode? Looking at the reader from the front, the side next to the calculator, is the cathode the left lead, toward the five colored wire leads or the right pad, toward the motor? The LED lens is shattered here and of course the leads were trimmed when it was installed.



If you have a brand new IR led without already clipped leads, the longer lead is the anode, the shorter one is the cathode.

The anode has to be installed into the card reader, that the anode points towards the five colored wires, the cathode has to point towards the motor side.

The IR led is the light transmitter, the photo transistor is the receiver, if it detects IR radiation, it will switch write protection on, preventing the card reader from overwriting a magnetic card. Please note, that the IR led is pulsed for a very short time just in the right moment when the card edge passes by. So you cannot measure a permanent voltage applied to the led.

Bend the top of the led in a 30-40° angle against the leads towards the calculator side of the reader, the highest point of the led lens should be about 0.8 inches (some 20mm) above the pcb, before you solder it.

Don't forget to insulate the lead wires with a strip of Scotch tape towards pins of the integrated circuit in front of it. Otherwise the led my be bended towards the ic pins during assembly, causing a short circuit and killing the whole thing.

A short test with a clipped card should be performed before completely assembling the card reader.

Don't worry, sounds much more complicated than it actually is.


Hey, thanks Rainer! That's exactly what I needed. I have the IR LED, actually two of them, one for a spare. They are 3mm devices, a little bit smaller than the original but they should fit nicely. I was able to get the bottom PBC board assembly out all OK. I had identified the leads OK on the new LEDs but I could not on the old one since the case was broken. Now I can put it all back together with your very good information.

Thanks again,


Well, it is all back together again. I replaced the sticky wheel with two #60 O-rings, (1/8"ID and 1/4"OD). They work fine. The only problem is that it will not detect a write protected card. It writes on it anyway. I guess the small IR LED is not going to do the job. I installed it with the anode (long lead) toward the five wires and the cathode (short lead) toward the motor. Radio shack has IR LEDs but they are 5mm packages and are too large to fit the placement for the LED. It isn't really a problem though. I'll just be careful to not to insert a program card while in the program mode.



you have mail.



I have my card reader repaired now. I repaired the clutch without using super glue, at least for now. We shall see if it stays repaired. If not I can add the glue. I also found the plastic tubing for the replacement of the gooey roller at a hobby shop. I had temporarily repaired it with two O rings but the tubing is better I think. The LED, DS1, is not an IR LED. I tried replacing it with an IR LED and the write protect didn't work. I have now replaced it with a 3mm clear white light LED and it works fine. I can see the LED flash at the beginning of each card read and it protects clipped cards until Flag 14 is set, after which it will write OK on the clipped card. My thanks to all who have responded.


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