Cheap DRO Revisited

I thought it was time I revisited the cheap DRO, so I have put the files on github and made a video showing it all working. The files are very much a dump of my code and PCB files, there's no guarantee it will build.

DRO Github

It's a start though, if you want to have a go at making one. The video shows the physical arrangement and also shows the update rate.

I milled the PCBs on my CNC3020, so they were very cheap, if you were to have them made then you'd probably want to reduce the size quite a bit, or the cost could be quite high. They are all single sided.
I have not put it in a case, I have mounted it up high in the workshop so chips don't get to it (I only have a mini-lathe and mill, so they don't throw things about much).

I think the cost I gave of £150 is probably quite a bit on the high side, as I milled the PCBs, which is cheap and used cheap calipers. Some discrete components came from my parts bin, too.

Latest OLED Watches

The latest iterations of the OLED watch are finally approaching wearable. Well, the first of the two is wearable but a bit thick. The second is much more wearable size.

The first iteration is the smallest PCB so far fitted into a 3D printed case and mounted on a cloth strap latched with velcro.

The switches are mounted on the top and the clear front flexes when they are pressed.

The overall thickness is 15mm, which is a bit too thick to wear comfortably. It gets caught on sleeves and catches easily on fabric.


The latest iteration is a one that doesn't use a PCB. This removes 1.6mm of thickness immediately, but makes the construction more manual.

The wiring is manually laid out and not very tidy. the thickness is much less, at 9mm, or 10mm to the tops of the front cover screws. Compared to my main watch this is very similar.

The components are mounted in a 3D printed frame like this earlier version:

The minimum thickness is fixed by the switches that I have used. The wiring adds a millimetre or so to the thickness, that could be reduced by inlaying the wires into the frame. More use could also be made of the 3D space in the frame. At the moment there are no components on top of components.

Using a PLA 3D printed frame is useful as it is possible to heat components and wiring up with a soldering iron and push it into the PLA. This then holds the component firm when the plastic cools.

There may be a better way to wire the components, maybe using a smaller PCB that just has the surface mount components on it.

Main DRO PCB 2.0

I've decided to put the entire Lexpresso card onto the DRO main board as it saves me the hassle of laying out a processor board, and I get the debugger/programmer half of the card as well. I can lay out a processor card later if I want to. The card also has nice 0.1inch holes for mounting on a PCB, which the Arduinos don't I thought about using and Arduino Mega, but mounting it was just too difficult.

New main PCB being milled:

I'm doing two passes, the first with a 0.6mm bit and the second with a 0.2mm bit. The first pass gives me more clearance around the tracks and pads and th eseocnd does th efine isolation. This should make it a bit easier to solder without shorts like I had on the first PCB. Of course, it means two 7 hour milling sessions, not one, but it's probably worth it.

Change of DRO Direction

 I've spent a lot of time recently trying to get the PIC16F18875 working as the main controller on the DRO. It's sort of working but there's problems. Whenever I make a simple code change (something like changing a /1000 to a /10) the code sometimes just fails to run. This would be fine, as it's probably a bug, well maybe, but as the Microchip debugger also doesn't seem to work, it's all a bit of a disaster. Looking on the web it seems this is a fairly common experience. It's a shame as I've used PICs a lot in the past and they were useful little devices. Anyway, I've decided to ditch the PIC and try something else. At the moment he plan is to use an LPC1114 ARM based device. This has the added advantage of more flash (32K), similar clock speed and lots of GPIO. Unfortunately it is a 3V3 device and that means I will have to deal with level shifting and add a regulator.  Anyway that's all OK, just a bit more complexity and if it works I can lay out a mostly 3V3 PCB with the LPC on it.

The other thing about the LPC chips is that they don't come in any DIP packages. Not that are available to buy anyway. So I have tried to mill a QFP48 footprint that is one of those used by the LPC1114. Once I used a 0.1mm cutter and a shallow depth of cut, I managed this:

It's not perfect, there's a bit of copper that hasn't been milled away. I think I may be able to sort that with either a second milling or slightly more depth. Anyway, it looks to be possible, so that's the plan.
Those pads are 0.5mm between centres, by the way.

Spindle Camera Mount

I made a quick little bracket for mounting the spdinle camera on the CNC milling machine while it is operating. I can then see what's happening without having to look at the spindle directly.

The shiny red bit is a mount off a cheap magnetic base from ebay. It works well, I may get some more. Unfortunately the base isn't very strong.

Here's a picture of some PCB milling going on:

here's a video of the same.

DRO Display Running

The main display PCB has now got some firmware that does something. The code can talk to all three OLED displays and also to the three LED readouts.

It can also get pixels on the TFT, but the range of co-ordinates seems limited in some way. There's also some spurious pixels on the OLED displays, but it seems deterministic so I don't think it's noise.
I need to get the touch screen code in a fit state to register keypresses.

I've also routed a couple of gauge channel daughter boards:

The bottom one is milled using a 0.2mm bit as I have done with most of my PCBs so far. After the problems with shorts on the display PCB (which turned pout to be soldering problems, as there's no solder resist), I have tried a double stage milling process in the second PCB. The first stage is a mill with a 0.6mm bit, followed by one with a 0.2mm bit to cut away the final detail. This leaves a larger gap around most of the tracks, which I hope will act as a sort of solder resist.

Here's a populated gauge channel PCB:

The socket is for the PIC12F1822 which reads the gauge data stream. The transistors are used for level shifting the data to 5V and also for driving the data and clock lines in order to put some gauges in to fast mode.
There's also a 1V5 supply generated with a simple voltage divider.

PCB Vias

It's a pain to solder vias with wire from one side of a PCB to the other, and if the vias are under an SMD IC then it makes soldering the IC tricky.

Different methods I have found or dreamt up are maybe possible:

1. Put wire through and solder it. This is easy and thevobvious way but has problems outlined above.

2. Use proprietary conductive epoxy paste in the hole. There's a commercial system that does this and it seems to work. It looks a bit messy and I'm not sure of the cost at the moment.

3. Put wire in the hole (wire that is thick enough to not fall out) and then cut it to a small distance from the surfaces. Then use a press to compress the wires and form a small rivet with very flat heads. This should then fit under the ICs. I'm not sure how well the rivet would handle age and oxidantion of the copper. Would it have conduction problems?

4. Use PCB pins. these are tapered pins that you put in the hole and snap off then solder on both sides. Problems are that they protrude above the surface like wire. they also don't seem to be made any more.

5. Use test pins. Similar to 4., but they are still made and sold.

6. Use small 0.8mm copper rivets.

7. Use a 3D printer to fill the via holes by printing conductive copper into the hole. I like this idea as it means that the hole drilling gcode or gerber file can be used to position the 3D printer and all the holes can be filled automatically. Problems include how well the plastic will fill the holes and how well the conduction will work. Conductive plastics for 3D printing are also not that conductive and it looks like a via would end up as around 50-150R, which is maybe usable, but care would be needed when placing them in a circuit.

I'm still investigating...

More PCB Milling

I'm experimenting with milling more detailed and double sided PCBs. So far I have tried a smaller cutter (D bit) that was 0.1mm, but I suspect turned out to be a bit larger than that. The fine tracks have disappeared in a few places so it's not usable, but overall it's encouraging.

The four large holes are my attempt at registration pin holes, which failed as the PCB software auto centres the artwork so when I milled the traces they weren't registered with the pin holes properly. I need to experiment with methods of creating these holes.

The second attempt at the double sided OLED watch PCB was more successful. I mistakenly drilled the via holes from the wrong side, I hadn't remembered that the holes are drilled from the bottom of the board, so I milled the top side tracks then drilled holes. Of course they were wrong and this PCB has an extra set of via holes. It is probably just about usable, but I am going to make a new board more suited to milling and also add some of the circuit changes as well.

This second PCB looks a lot better. I cleaned the copper up using some fine wet and dry paper as there's a lot of rough burrs as it comes off the milling machine.
The blue PCB is the one I had made in China, for comparison.

The registration using pins worked well, the program I wrote to generate the pin holes now uses the board outline, which has to be the correct size.

First Milled PCB

This is something I've wanted to do for a very long time. PCB milling. The first attempt has turned out rather well. It's not the most complicated board in the world, none of these milled PCBs will be, but it's turned out nice and clean.

This was with a 1mm D bit, I've ordered some smaller bits to see if I can get to a finer level of detail. Even at this level it's going to be usable for quite a few circuits.