Micro/Pocket PC

This is a 5″ display and the keyboard I just ordered. I think the size is about correct. The trouble is more finding a TFT display that hide the HDMI on the back (inside) rather than demanding cables extending to the sides. I actually need a workable console for Raspberry PI that include both keyboard and display and don’t occupy to much space – interesting…

Pocket PC Keyboard

Not the most interesting topi, but this keyboard fit into my hand and accidentally have a layout that seems to be good for a developer. I ordered one to try out – actually could be an idea to make plug the keyboard into Raspberry PI and make a Pocket PC – it just need a 7″ display on top and a RPI Zero W + a battery.

STM32F105RC

I have written about STM32F105RB and why I chose this before. Sample prices on AliExpress do variate, but I noticed some batches of STM32F105RC for 1.8 USD each so I purchased 10 of them – they arrived today and I will stick one of them on a breakout board later. ST’s own price for this is 3.- USD, so I am not sure why I get these for almost half the price.

The sellers on the RB version advertise these as the 32Kb Versions, but I am not aware of a similar scheme on the RC. Frankly I don’t care either as long as they work as this is for prototype’s. The RC version have 256Kb Flash and 64Kb SRAM.

PLC Rack/Box

This aluminium box cost ca 16.- USD P&P included and is accidentally the correct size to mount 2 of my backbone PCB’s on the inside. I need to do a more exact scaling, but the concept is illustrated below.

To do this I need to make some PCB Holders. The ones illustrated below should be easy to pring with a 3D printer.

The Box have sufficient depth to hide a PSU behind the backbone to deal with mains input. Just an idea

Programmable Lab PSU

Starting at left with 230VAC input we need a classic rectifier and filter that will produce high voltage DC out. This is input to a PWM driver that will use HEXFET’s to generate a MCU controlled PWM into the isolation transformer. The duty of the PWM decides the output voltage. If we have 400VDC and a duty cycle of 10% we will have 40VDC in average out. The 2nd rectifier ensure we got all positive voltage out and the end Filter (illustrated below) convert this to a stable DC out. To Control this we use fast ADC’s to read output voltage and current into the MCU that control PWM duty.

The core of any PWM driven PSU is the prinsiple that we provide a PWM in where an output filter consisting of a coil and capacitor force this back to a stable DC. Voltage out is a simple average calculation, meaning that we control voltage with setting PWM duty.

Building a 0-50V/0-20A Lab PSU

I ordered a DPS5020 (50v/20A) Lab PSU for 45.- USD with USB connector. I intend to hack that serial port and extend the HMI with my own interface, but my primary challenge is finding an affordable AC/DC that will deliver 60V/20A as input. I did find some interesting alternatives:

This is 36V/5A and well suited as input stage for a 0-30A/0-5A Lab PSU. It is also quite small. I ordered one of these because it only cost ca 11.- USD P&P included. A 20A PSU is a monster needed for motors, but a 0-30V/0-5A is a “bread & butter” PSU that you need several off.

The best priced single 60V/20A unit I found is this that actually is 25A at 124.- USD. You have to be aware that both voltage and current will drive prices a bit, but as I know that higher currents are the worst to deal with I looked for 60V/10A units and found the one below.

This cost ca 30.- USD if you order 10 units so I need to think about that. You can also buy them single units for ca 36.- USD. To achieve 20A I would need 2 of these that require some size, but the advantage is that I make 20A wires myself which makes each driver PSU cheaper. This gives me a cost of a driver stage around 70.- ++ USD which is decent.

Getting a box for the largest 50V/20A Lab PSU will cost around 100.- USD sadly. Adding 70.- for a driver stage and 45.- for the module + an estimated 30.- for extra bits I will land on ca 245.- USD for a 0-50V/0-20A Lab PSU.

The “bread & butter” 0-30V/0-5A Lab PSU is easier. We can pack 2 modules into the 16.- USD Blue box I showed you before and with 2 x 30.- + 2×10 and some 20.- in extra parts we should have a small, dual PSU for ca 116.- USD (of ca 58.- USD each).

Both those are decent options taken into account what the spec of these PSU’s will be – that said I want to receive the units and do some testing before I invest more time and Money on this.

Servo Module

The next module I want to make is the Servo Module. This is handy since the 16 channels also can be used as digital/analogue IO signals.

I will use 16 x 3xRight Angle Headers and some jumpers to select between voltage. I will only support 5V & 12V and max 10A in total. Higher voltage or effects will need to get their power directly.

Supporting 16 channels this way is a bit much as the total current usage can be quite high so I will add current sensors to monitor usage. I also need an inductor to prevent pulses from going back to the backplane. I need to review the 5V here because we currently only have a single 5V supply that also is used for the MCU. I probably need to add a 2nd 5V on the backbone to allow for separate PSU’s for modules connected to actuator/servos.

In this case we also need to feed the 3.3V MCU from the 5V/12V used for the servo’s as we otherwise might not have a ground to our signal. Communication with backbone is RS-485 based on differential siganaling not depending on anything but those two wires, so this will work fine. It also means that we will no connect to the 5V MCU/Ground at all on this module. This actually raises a question if I should switch to isolated RS-485 towards the backplane here.