micro:bit edge connector expansion board retainer clip

The problem

I use PCB expansion boards to plug my micro:bit into for a recent project. The micro:bit has connection pins printed along the edge of the board. This is called an 'edge connector'. Have a look here if you are not familiar with the micro:bit board. This edge connector plugs into an edge connector socket on the expansion board. The two components can be easily separated.

As I give the finished devices to other people to use, I need a way to stop the micro:bit from being easily removed from the expansion board. The micro:bit connector is not keyed, so it could be replaced upside down. Then Terrible Things will Happen.

The solution

I designed and 3D printed a clip that hooks over the back of the expansion board's edge connector socket and plugs into the 4mm holes near the micro:bit's edge connector. This clip prevents the micro:bit from being easily removed from the expansion board.

Please find a screenshot of the widget, taken from the 3D design software OpenSCAD used to design it, below.

micro:bit edge connector retainer, OpenSCAD screenshot.

The photo below shows the clip attached over the top of an expansion board edge connector socket, connecting the micro:bit board to it. You can see three of the five 4mm holes that are next to the micro:bit's edge connector half exposed, sticking out of the black edge connector socket of the expansion board. The 3D printed clip slots into the two outer 4mm holes.

Front view of micro:bit edge connector retainer.

A side view of the retainer is shown below:

Side view of micro:bit edge connector retainer.

The retainer shown in the photographs was printed at the 'low quality' setting of my printer as I am still refining the design. The expansion board that the micro:bit is plugged into shown in the photographs is the Sparkfun micro:bit breakout board.


For my flex sensor project I need to add some functionality to the BBC micro:bit. To achieve this, I plug the micro:bit into a small expansion board. The expansion board is designed to allow a few components to be soldered onto it which connect with the micro:bit through the edge connector socket on the expansion board.


I use OpenSCAD, which allows the design to be entered through programming. There are many 3D CAD packages available. I use OpenSCAD as the method of programming in a design goes down my 'brain-hole' more easily than the different methods of design entry used by the other packages I have tried.

Use the design software that suits you

3D printing

I used my Creality CR10 mini printer which I first wrote about here to 3D print the clips. On the 'low quality' setting the print takes about 10 minutes. This print time is a little misleading though. You need to allow for the print bed and nozzle to warm up. Then it is best to leave the finished print to cool down so it comes off the print bed easily. Hacking away with the flat bladed paint remover tool to remove a still-warm print from the print bed risks damaging the print and marking the print bed.

How you can replicate this product

I put the OpenSCAD design file for the retainer in the project GitHub repository here.

The end

The retainer clip will be incorporated as part of the casing for the device. Currently, I have a 3D printed case over the expansion board and a silicone cover over the micro:bit. I will add the retainer clip to the case that goes over the expansion board.

Comments and improvement suggestions are welcome.

3D printing face shields on a Creality CR10-Mini

Like a lot of folk right now (mid Covid-19 lockdown), I'm using a 3D printer to produce face shields. I started from scratch and with little experience of using a 3D printer. This post will detail some of the issues I had and my solutions, in the off chance that this information could help somebody else.

Please see my HSE manager testing one of the completed face shields in the picture below so you know what I am talking about. These face shields are requested by front line workers as there is a scarcity of off-the-shelf personal protective equipment (PPE) . I print the yellow clip at the top. The face shield is an A4 clear plastic sheet bought from an online stationery store. The holes are punched using a four hole A4 punch. The design comes from the national 3D printing society.

Careful testing of the final product.

I work through a couple in Ulverston who set the scheme up at https://www.facebook.com/F3Dprinting. They received some funding through donations and kindly sent me some filament and the pre-punched sheets. Requests for the face shields go straight to this couple. I try to fulfill some of the orders that they receive.

I bought a Creality CR10-Mini 3D printer the same day I read of their project. Why this printer? There's quote "There are two types of fool, one says 'This is old and therefore good'. The second says 'This is new and therefore better' ". I went for the 'old and therefore good' option. The CR10-Mini is a printer that came out a few years ago, so there are many videos on YouTube to show me how to set up the printer and trouble-shoot inevitable problems. The CR10-Mini gets good reviews for the beginner, which I most definitely am. Maybe a more recent printer would give me more for my money, but I needed to get into production as soon as possible. Some of the reviews of more recently produced or cheaper machines showed that simple bodges were necessary to get them running. I can't nip down the local hardware store to get some washers or a spring at the moment. This YouTube video , comparing the Creality CR10-Mini with the Creality Ender 3, convinced me to spend the extra to get a printer that lookes easier to get into production than a cheaper model that may take some extra time and experience to set up. However, if you don't have the extra money, the Ender 3 looks to be a good printer.

The CR10-Mini was straight forwards to set up. Most of the online reviews are made by people who have substantial experience of 3D printers and may have forgotten what it is like to know almost nothing about them. Even so, bolting the printer together while following this YouTube video was not difficult. An experienced 3D printer guy would laugh, shake the parts a little and have a machine ready. I didn't find the manual too helpful, so stuck with the video guide.


Watch this video on levelling the printer, download and edit the G code as suggested by the presenter. Then run the code to check your levelling and nozzle clearance. I check this levelling about once a day now.

Problems encountered and solutions

Prints not sticking to the glass plate

Check the plate levelling and the clearance between the extruder nozzle and the plate. Get a good tight clearance as suggested in the video linked in the Tips section.

Use decent filament - not the sample that came with the printer.

Decreased first layer print speed to 20 mm/s. This gives the plastic more time to melt and stick to the glass plate.

Increased glass plate temperature from the suggested 60C to 70C. This worked well. I stepped the temperature back down to 60C as my experience and confidence increased. A higher bed temperature increases the chance of the plastic melting and bonding to the plate against using a lower temperature. The higher bed temperature increases the start up time, as the bed has to get to the higher temperature and uses more electricity.

Smeared the glass with a film of UHU glue stick. I was shown this trick by a friend some years ago when having an induction on the lab 3D printer, which I then did not touch again.

After about 100 prints, I was getting the hang of things. So I cleaned off all the UHU glue stick from the plate and found that the prints stick straight to the glass. I guess that I got my nozzle clearance good and tight so that the base layer of plastic squishes down and sticks. Maybe smearing the glue on for a while then cleaning all of this off removed any residual grease on the plate. I will never know.

Knocking sound and a decrease in print quality.

This happened after about 100 hours of printing. Please see the photo below. The layers of filament are not as tight as they should be on the part shown on the top of the photo. The part on the bottom of the photo was printed after I fixed the problem. Note that the prints were both made at low resolution and high speed. The machine is capable of a smoother finish if you give it the time.

Poor quality print (top) from slipping filament. Better quality print below.

I found that that the little gear wheel-shaped filament feeder was slipping on the filament. So less filament was fed to the extruder than intended, leading to material missing on the final print.

I fixed this by cleaning the teeth on the wheel. Please see the photo below where I indicate the component that needed cleaning. In the photo it is shiny and clean though.

CR10-Mini filament feeding mechanism showing the component that needed cleaning to prevent filament slippage.

The feeder wheel is toothed and metal, so inevitably the teeth gum up with bits of plastic from the filament, allowing the filament to slip. I should also look at how my filament reel is mounted, in case there is excess drag on the filament entering the printer, leading to it slipping in the feed mechanism.

To clean the wheel, I first preheated the nozzle to PLA temp, then pulled out the filament. I unbolted the plastic feed tube. I cut a short length of filament and fed it through the feed mechanism. Then I pulled this filament rod back and forwards while holding a toothbrush against the gear wheel teeth.

I reassembled the printer and tried again. Instant fix! A video showing how to do this in detail can be found on this YouTube video.

Printer not levelling correctly

One of the wires connecting the printer to the control unit snagged above the contact switch on the Z-axis controller. This switch is used to help the machine reset its home position. When the printer was resetting, this wire pressed onto the contact, triggering a false home position. The incorrect wire position is shown in the photo below.

Wire on top of contact switch, causing incorrect print bed levelling.


I print two nested head-pieces at a time as shown below. I tried some stacked prints, but they didn't come out as well as the single-layer prints. My lack of experience and the need to maintain production stop me from trying more. A year from now I'll laugh at my not printing stacks, but right now, I'm running with what I know works. As always 'Experience is something you get after you needed it'.

Creality CR-10 Mini printing two face shield clips in a tasteful shade of yellow.

I printed off about 145 clips for local care home and hospice orders and sent a batch of prints to the NHS scheme, for a grand total of 200 prints. Then I was asked to return to my paid work offshore, so my PPE manufacturing came to an end. Until the next pandemic...