I've been looking for an SD Card extension for a while now. There aren't many available and the ones that are for sale are quite expensive.
So I'm going to make my own!
I'm an electronics workshop bench engineer by day and one of my specialties is Hand Held Terminals. Most of the devices have slots for SD Cards. I can take an SD Card slot from a scrapped HHT's PCB and use it as part of an extension.
I'll modify an old SD Card and connect it to the SD Card slot with a length of re-purposed ribbon cable.
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Step 1: Reclaimed Parts
I'm using an old HDD IDE ribbon cable, an SD Card holder from some old electronics, an unloved 2gb SD Card, and the fins from an old P4 heat sink.
Firstly I need to remove the SD holder from the old PCB. To do this I used a soldering iron, some solder braid, and a pair of tweezers to ease apart the joints.
The SD Card was split down the plastic weld lines. You'll have to be very careful and use a sharp knife to get a good separation of the two halves. I find that older SD Cards are easier to split than newer ones.
I was hoping that I could use the internals of the SD Card and simply cut the end off the PCB where the contacts are, but that wasn't the case. In the end I ditched the PCB and decided on making my own contacts.
The fins on the heat sink were pretty lose to begin with, a sharp tug and they came off. I'll be making replacement contacts to go inside the SD Card case from the copper fins.
The only new components are some Dupont connectors.
Step 2: Making the Contacts
I began with pin 9 on the chamfered corner. Details about the SD pin-outs are on Wikipedia.
I cut a strip from the copper fin and matched it to fit over the aperture on the SD Card.
The plastic dividers between each contact aperture look no wider than 1mm. I have to rest the edges of the contacts on them, and make sure that they don't touch each other!
For the moment I have cut rough shapes, I'll do the fine adjustment when I come to permanently install the contacts.
The two parts of the SD case fit together pretty closely, because I'll need to run some wire to the contacts I've scored the plastic and added channels for the wires to sit in.
I have cut a ~4mm strip off the bottom of the card. I'll need the space to attach the ribbon cable at a later stage.
Step 3: Installing the Contacts.
- Clean surfaces, all the contacts were cleaned with ISO cleaner to remove any grease.
- Heat, a hot soldering iron for use with lead-free solder.
- Flux. flux stops oxidizing and helps ensure a flawless joint.
It also helps to keep the objects being soldered from moving around. I used a pair of clamping tweezers.
Each contacts has a single wire thread soldered at the rear of the contact.
Before a wire is attached each contact is adjusted to fit inside the SD case using a needle file.
The contacts are cleaned again and then super glued to the SD casing, be careful not to get the glue onto the areas of the contacts which will be used to make electrical connections.
Between each contact I have placed strips of L shaped paper. The strips are about 1mm wide and 10mm long. The strips are super glued to the back of the contacts with one side of the L going between the contacts.
The last pin, pin 8, doesn't have much space for the wire to run. I put some Kapton tape over the back of pin 7 and ran the wire from pin 8, at a ninety degree angle, over the tape.
The wires have been stuck down with thin strips of Kapton Tape. I've put a paper spacer on the back of the contacts this should help keep them from breaking lose.
I cut down the length of the lower half of the SD case. I'll use that space to solder on the ribbon cable.
Comparing the contacts I have fitted with those of a production SD Card it looks like I have made a perfect match. Unfortunately I won't know until we have more of the assembly complete.
If you have a multimeter it is worth checking for shorts between the contacts during various stages of the assembly.
Step 4: Closing the Card
Behind the contacts, once the back has been fitted, will be a gap if it isn't filled in. I cut a shape of paper to match the area behind the contacts and using super glue again, it was stuck in place.
I made a similar matching part for the back half of the card. A final piece is glued to the space in the other half of the SD Card.
I put the SD Card Lock slider into in track before the two halves were glued together.
It is important to make sure that the halves line-up correctly with each other. If you make a mistake when glueing it will only be a matter of seconds before the glue sets.
The SD Card is pressed together for around 5 minutes while the glue sets.
Once the glue is dry, the excess wire can be trimmed off.
Step 5: Connecting the Cables
There are nine pins on the SD Card.
From the ribbon cable I have separated 9 wires as a single strip.
I cut this strip into two lengths.
Approximately 2cm from each end on both cables I have put some heat-shrink. The heat-shrink should stop the cables splitting apart further than I want them to.
To keep the cables flat when the heatshrink sets I used the thick base from the P4 heat-sink and pressed it onto the heat-shrink while it set.
The ends of the cables are split into separate wires, and the ends of the wires have 2mm of wire exposed before being tinned with solder.
At one end of the longer cable I have placed 5mm lengths of 2mm heat-shrink tube over the wires. To these wires is attached a strip of 9 2.54mm spaced pins. When all pins are connected the heat-shrink is moved up to cover the connections and then heat is applied to shrink the tubes.
At the other of the longer cable I will attach the SD Card.
To make it easier to see the wires I have slipped a bit of scrap paper behind the wires from the SD Card.
This is a very tricky part. Don't rush when soldering onto the SD Card. Make sure the card is held firm, and check the alignment a few times before committing to solder the joint.
Once all the connections are made I have weaved a thin strip of Kapton tape between the wires. This should help with any short-circuits.
Over the connections I have placed a thin foam strip.
Next is a section of heat-shrink. When the heat-shrink shrinks it will compress down on the foam strip and help to keep the connections secure. It helps to press down lightly while the heat-shrink sets.
The longer cable is now complete.
To one end of the short cable attach female Dupont connectors to each wire.
There are nine wires so I have used three 3-pin plastic casings to enclose the bare connectors.
At the opposite end of the connectors will go the SD Card slot previously removed from the scrap PCB.
Before soldering any wire make sure to check that the connections are made in the right order.
You should now have both the completed lengths.
Join the two together to finish the extension!
I have put a joint in my extension as I have a specific use for it. You don't need to have the cable split so you could make it as a single part.
Step 6: Installing the SD Card Extension
I've built the extension to go into the FishPi Base-Station.
I had a series of SD Card corruptions and it was becoming a pain to have to open up the electronics just to take the card out and re-image it.
I'll be putting the SD Card slot into the plywood inspection cover in the keyboard recess in the Base-Station. The split in the extension cable is so that I can move the cover out the way without having to take the SD Card out. All I have to do is disconnect the cable instead.
I cut a hole in the cover slightly larger than the SD Card slot that will be going in it. A backing board is screwed onto the cover from the back. Double-sided tape secures the slot on the backing board. I did think of screwing the slot to the backing board, but there isn't much space for a screw to fit. Nor could I find one small enough. If the tape proves inadequate I'll have to look for an alternative solution.