Making Elektronika MK-52 VFD scanner

One of my Elektronika MK-52 (Электроника МК-52) Soviet era micro calculators decided to show its age by turning off a couple of VFD positions. In this case the ‘minus’ sign and the first digit are almost invisible, so instead of ‘-123’ you can see only ‘  23’. I have a few spare VFDs and could just replace the faulty one, but that would be a bit boring. Plus, all spare ones I have are from Soviet era as well and there is no guarantee that they will last.

So I’ve decided to replace original VFD with something else, preferably some ready to use parts. Having a few STM32F103C8T6 boards laying around it was the obvious choice – use STM32 to scan VFD pins to read the values being displayed and send them to anything connected to the STM (to be yet decided).

MK-52 drives its VFD using -27V, so direct connection with STM is not possible. But simple resistive voltage divider works just fine. There is how I’ve connected VFD to the STM:

Note that only of VFD grid controls is connected, below I’ll explain why.

This is how VFD digits multiplexing works:

Making XY-LPWM display serial port friendly

Popular PWM generator XY-LPWM has a small, but heavily packed LCD display:

driven by Holtek HT1621 LCD Controller, driven by Nuvoton N76E003 - 8051 based MCU.

This MCU runs on 16 MHz internal RC oscillator and provides 18 KB Flash ROM, 1 KB SRAM, 18 I/O pins supporting two UARTs, SPI , I2C, 6 channel PWM and high resolution 12-bit 8 channel ADC. Sounds very interesting to hack into.

Unfortunately out of box only Keil and IAR supported as development tools. But with Small Device C Compiler (SDCC) happily compiling for 8051 MCUs for years it can be fixed quite easily.

So here it is: Nuvoton N76E003 SDCC BSP.

With this BSP it is quite easy to make XY-LPWM display serial port friendly, just compile and upload xy-lpwm-lcd example, connect the board to UART at 38400 baud and type

 > lcd @0GoodByte

Making Ruideng DPH5005 power supply case fan silent

I use Ruideng DPH5005 as a bench power supply and it is quite good for what I need, the only problem I have with it - 5V high speed fan of the original power supply case which runs always, regardless of load/temperature, making a lot of noise.

DPH5005 board has it's own small fan which kicks in only when power supply is overloaded, so probably I could disconnect the case fan completely or connect it to the heatsink fan, but I find no fun in doing so.

There are a couple of suggestions on the Internet how to deal with the noise of the fan - connect it in series of diodes to lover running voltage or to use Normally Open bi-metal thermal switch to turn on the fan at about +45 C.

I liked the latter, but decided to combine it with the first one to limit the driving voltage with a diode. And I had a nice and proven project for that: Madis Kaal tiny_pwm for ATtimy85 microcontroller.

Using pure PWM did not work with this fan - to run the fan stable at the lower speed I had to use quite low PWM frequency with clock divider 128, which was creating extra noise, diminishing the purpose of the project.

As power supply provides 5V, we can use bump-on-the-wire approach to control the fan. Modified schematics looks like this:

Making simple power bank

Making Bose QC 25 USB friendly

I've been using Bose QuietComfort 25 for a couple of years now. They are great for watching movies on long transatlantic flights, but their miniature on-the-wire microphone and 3mm audio jack makes them unusable for long transatlantic Skype calls. Adding USB capability sounded like a good idea, and it wasn't hard at all with my new Prusa i3 MK3 printer.

For Skype I've been using Plantronics 626 for almost 8 years and finally ear pads started to disintegrate. Instead of soursing a new ear pards for this discontinued headset I decided to used Bose QC 25 as a replacement :)

First make a model in blender3d:

 Then print:

Making black visible on black

Yesterday I tried to print using black PLA on Prusa i3 MK3 and found that black PLA on black PEI is very hard to see :) But just a week ago I got my UV LED flashlight and it did the trick - turned black PEI heatbed coating to greenish and black PLA to blueish colour:

So no problems to control the first layer :)

Making Darkroom FIlm Hanging Clips

I do not have a darkroom, so I dry my 135 film in the shower hanging it to the shower frame using a few pieces of wire and paper clips. Or at least I used to until now. All these wires and paper clips are history now, thanks to my new shiny 3D printer.