MK-52 Calculator

The MK-52 is a Russian calculator that is interesting for a number of reasons. Being Russian, it has Russian manuals and Russian keys. This is a bit of a problem for me as I don't speak Russian. I find it difficult to remember the key markings and what they relate to on the keyboard. So, I thought it would be worth having a go at creating some English keys for the MK-52, just to make using it more comfortable.

My first idea was to machine the keys out of plastic and engrave the legends into the top of the key. This could then be filled with a coloured material and there you go. After some research I found that nylon has a high enough melting point that I could maybe machine a set of keys from it and then fill the engraving with polymer clay and then cure that at 105C. Hopefully the nylon will stand up to that treatment and the polymer clay will be robust enough if it is cured and recessed into the key.

Setting all this up is a bit tricky as the parts have features on all sides and work holding and work order is complicated. So, as a trial I had a go at 3D printing the keys. This actually turned out fairly well. I have printed in PLA, so polymer clay may not work, but I have a couple of keys that I've printed and engraved with legends. I then used a pencil to colour the engraving, rather than use polymer clay.

The result is a bilingual MK-52:

The two white keys have English legends. Here's a close up:

The engraving is a bit wonky, that's just the setup I used. this was just a trial and I was more concerned about the engraving. Engraving PLA is not easy, but after a few passes at different depths and RPM I managed to get pretty good 2mm high text. This is about 0.5mm deep which is OK for the pencil colouring, but for polymer clay more depth might be needed so more material is in the keytop.

I tried using the keys after I put them back in the calculator and they seem to work fine. The pin on the back that presses into the membrane mechanism is a bit shorter on the 3D printed part, but that doesn't seem to affect operation at all.

I also had a look at getting some keys 3D printed by Shapeways. I hoped that they'd do full colour printing but they don't, a single colour is all that they do, and that comes out at $6 per key. This makes it too expensive to consider, I think, which is a bit of a shame.

Holding the keys is a problem on the engraver, then I had a thought. The keys were held perfectly well on the 3D printer, why not use the print bed on the engraver?

 On the Fisher the print bed is sort of circular and removable. So, I tried that and it worked:

I managed to engrave a set of 13 keys, albeit a bit off centre.

I've used an amalgam of English wording that I found on the internet. I've had to change the letters so that there's a maximum of three per key, as four doesn't fit with the text size I've chosen.

There's a problem using polymer clay with PLA: it requires too high a temperature to cure. Instead, I tried air-dry clay. I got some white and used it to fill the engraving, so there's a contrast between the letters and the button. I also tried a mould-able glue product (Sugru).

These are some of my attempts, together with an original button on the right.

The air dry clay has worked best I think, as it doesn't colour as much from dust when I sand and polish the top. Maybe if I left the mould-able glue  to cure longer than 24 hours then it wouldn't pick up so much dust. The dust gives the letters an off-white colour that isn't as nice as the whiter finish of the clay.

I polish the tops of the keys which gives them a nice shiny finish. The sides are still the normal 3D printer layer finish. The bottom with the 2mm cylinder that pushes the key is a bit of an untidy mess.
I've decided to print the keys with a hole in the base and fit a length of 2.5mm styrene rod there instead of trying to print the feature. You can see the lower side of one of the keys here:

Printing this part has the advantage that it's flat on the bottom and sticks well, and the keytop is at the top of the part which means it has a nice finish which polished well and also doesn't suffer from the gaps that the first layer of a print seems to have. These gaps filled with white clay and created a pattern of thin lines on the top. You can also see a finished key in the above photo and also several keys that have had clay applied and are drying before I sand and polish the top surface.

I then made a full set of keys (well, the Russian ones, I left the original number and function keys) and fitted them in the MK-52:

These are air dry clay filled except for the R/W and ADR keys, they are filled with paint. I've also used some 2.5mm diameter styrene rod for the part of the keys that pushes on the membrane. I glued the rods in a hole that is printed on the bottom of the keys.

This is very much a trial, so we'll see how well they work.

You can see the keys working in this video:

Here's a short timelapse of the keys being engraved. You can see that one of them popped off as the tool came down to touch it. The fixing method isn't perfect! I could always engrave more keys and get some redundant ones for this sort of mishap. I engraved a block of keys like this several times and that key always fell off. At least it's consistent.

Making a Night Light from an IKEA LED Lamp

After a request for some night lights I remembered that I had some IKEA motion sensitive LED lamps that were bought for use in cupboards. One is still in use, the other (they come in packs of 2) was just lying about. They are easy to fix to walls and battery powered, so no trailing cables. I tried one as a night light months ago, but it was very bright and came on during the day as well, so it was removed from duty.

Thinking about the new request, I wondered if the lamp could be modified to add a 'darkness only' feature. Dismantling the lamp revealed a standard PIR chip inside: the BISS0001. Looking at the datasheet revealed even more good news: the chip has a function that  disabled the output from triggering if it detects light. The example circuit has the components for this feature included.

I ordered the parts (100k resistor and an LDR) after working out the values I needed.

I modified one lamp and it didn't work, this was due to the routing of tracks on the PCB, so I lifted the pin on the IC and added the components directly to the pin. SOIC chips don't have robust pins, so bending it up once is about all it can manage. Any more flexing and the pin falls off.

After adding the components correctly, the lamp now only triggers and turns the LEDs on when it is dark. I've also removed three of the four series resistors that current limit the LEDs, as the amount of light given off was too much for a night light.

You also have the option of changing the time that the lamp stays on for, by changing the value of a resistor (or capacitor). This is detailed in the datasheet.

Lift the pin 9 leg:

Then put a 100k resistor from pin 9 to pin 11:

The LDR (light dependent resistor) looks like this:

I used my CNC router to cut a hole that matched the shape of the LDR and glued it in place:

The legs from the LDR wrap around the PCB quite nicely:

One leg can be soldered to the track at the edge of the PCB, the other requires a wire to run to pin 9:

I have modified one of the lamps to have a longer ON time, the rest are as they came. They've been running for a while now and are pretty useful.

These are the LDRs I used:
https://www.ebay.co.uk/itm/192823191519
The resistor was a standard 1206 100k surface mount resistor.

Chocolate and Orange Hot Cross Buns vs Dualit Toaster

Toasters toast things, but when the things contain chocolate then they also melt things. This is fine if you eat chocolate infused thingies (I don't), but unfortunately when chocolate melts it also tends to escape. This happened in our Dualit toaster recently when some chocolate escaped onto the prongs that lift the toast up and down.

A quick cleaning attempt and I found that it was tricky cleaning those prongs as they are down in the toasting slots. No problem, I thought, I'll take the toaster apart and give it a good clean. Being a Dualit it is made to be repaired so should be easy to dismantle. A bit later and I found that yes, you can take bits off, but not all of them. Irritatingly the bits that need to come off to free the prongs have been pop-riveted into place. This meant that I'd have to modify the toaster to get the prongs out.

So modify it I did.

The pop rivets:

were drilled out and replaced with  bolts:

 This makes it a lot more fiddly putting the bits back together, but at least now I can get the prongs:

 out (and back).

 All cleaned and re-assembled:

I think I'm going to investigate threaded inserts for the two bolts I added (and maybe some others that are a pain to fit) and try fitting them the next time I do a deep clean.