Redesign Questions

[RetroLoom]

Hello! Thank you for all of your awesome contributions.

I am in the process of making some redesigns of the MhzDisplayBlaster to fit a particular baby AT case that I have. It is my first time working with KiCad with this extent and I figured it would be a good project to become more familiar with how to use it. I was hoping I could get some insight about a few things.

Here is a photo of the example display I am trying to redesign.

Everything is front based except for a 3 pin connector on the back that interfaces with the case for power and connection to the turbo button.

It seems the pinout is as follows
( IN ) -> Black Ground
( + ) -> Red 5v
( - ) -> White Turbo Button

This is what I have on the schematic so far.

I plan on reducing the 7 pin to a 4 pin and adding the 3 pin to the back similar to the original display. I am just unsure where to best connect the turbo button ( - ) wire for proper function.

I still have a long ways to go on the PCB design and I need to double check all of the position and spacing of things, but this is what I have put together so far. I am a little worried that the trace routing might be a little too busy with this layout so please let me know if you have any suggestions.

I appreciate all the help in advance!

[scrapcomputing]

Thanks for reaching out, it is really cool that you are building a reproduction of this display!

I don't own a case with such a display. Is this a logic chip on the left hand side?

The easiest way would be to use the MHzDisplayBlaster schematic and just use a new PCB layout that matches the dimensions and hole locations of your display. This would make sure that the circuit is correct and you would just have to worry about the routing of the wires.

But if you have reverse engineered the circuit of the display it would be nice to build an exact replica too :D

[RetroLoom]

Actually yes! This schematic and PCB is just a redesign of the MHzDisplayBlaster, though I have really just started. I thought it would be much easier than trying to reverse engineer the original one. I was also curious about the logic chip on the original as well and thought it was a pretty complicated design for what it is. One of the segments of the first digit is broken on the original so I tried to see if I could fix it. There are a group of resistors underneath the display that you can only get to by removing the display entirely. After I removed it, I also learned that the display traces do not connect to the traces on the back on the PCB, and instead are connected to the pads next to the resistors underneath the display. So you have to melt solder down the pins and through the through-holes and hope that they make the right contact. Very strange design so I thought I would give a shot at rearranging the MHzDisplayBlaster instead.

I already have made an original MHzDisplayBlaster as well, but found that it wouldn't quite work with my case. But I have been super excited that you have created this as an option in general.

So in regards to my question above, the snapshot of the schematic is from your original 7 pin connector. I am going to make it into a 4 pin connector and separate power and ground to the 3 pin connector to the back to fit in the case like the original. But in the schematic you have TurboSwOn, TurboSwCM, and TurboSwOff. It might not be clear from my description or photos, but I am trying to figure out which on of those my turbo switch should connect to. I am sort of thinking TurboSwOff might be the most appropriate, but I thought I would try to get some more perspective of how those three pins relate to each other.

Thanks!

edit - here is a photo of the back side with the logic chip if you are curious

[scrapcomputing]

So in your case there are only 3 cables coming in to the display: 5V, Ground and Turbo.

The turbo switch in the MHzDisplayBlaster's schematic is a 3-pin 2-way switch connecting TurboSwCM to either TurboSwON or TurboSwOff. To get the same functionality with a simple 2-pin on/off switch you would need a different schematic. There is a way to get it to work using a chip like the 7406 (as on your display) and one additional resistor, but I would need to know where the other pin of the Turbo switch is connected to. Is it 5V so when the turbo switch is pressed you get 5V on the white cable, and if it's not pressed you are getting 0V ?

[RetroLoom]

Ah ok this makes a lot more sense and why I was a bit confused. I opened the case up and did a little probing. It seems that when the turbo button is not engaged, it is sending 1.62v through the white Turbo wire. When the Turbo button is engaged, the white wire is reading 0v.

I have also created a fork and committed my initial changes to branch Gecco if you ever feel like to taking a look.

[scrapcomputing]

1.62V is a strange value.

(i) Is this the voltage when the display is connected ? Could you measure it with the display disconnected?
(ii) Are the LEDs red? Because 1.6V is suspiciously close to the voltage drop of red LEDs if I am not mistaken.

Nice, I will take a look at your branch.

[RetroLoom]

1.62V is a strange value.

(i) Is this the voltage when the display is connected ? Could you measure it with the display disconnected? (ii) Are the LEDs red? Because 1.6V is suspiciously close to the voltage drop of red LEDs if I am not mistaken.

Nice, I will take a look at your branch.

You totally nailed it. This is the voltage with the display connected and the LEDs are red. When measuring the white cable while it is disconnected it reads 0v with the Turbo button pressed and depressed.

On the topic of red LEDs I was thinking of using this display instead of the green.
https://www.mouser.com/ProductDetail/859-LTD-6940HR
It seems to match in general, but will this red display work with the MHzDisplayBlaster in it's current design or will it require some modification?

[scrapcomputing]

Hmm I still don't understand how the white wire works. So the white wire should be connected to one pin of the power turbo button. Is the other pin of the turbo button connected to ground ? But if this is the case there should be some resistor in between otherwise you wouldn't be getting 1.62V.

Regarding the LTD-6940 display, I think it is compatible. It is a common cathode 0.56" display and the pinout looks identical to the LTD-6440G that I used.

[RetroLoom]

Thanks for your patience while I figure this out. I pulled the LED / Button panel off of the case to see if I get a better look at what is going on.

Here is a photo of the button from the inside.

So I pulled the multimeter out to check continuity and this is what I figured out.

White <--> Black -> Display Board
White <--> Blue -> Turbo Sw on MB (which is actually not present)

When the button is pressed it is bridging White to Black / Ground and would simultaneously be bridging the Turbo Sw pins on the MB if it were present.

How this relates to the function of the display I am not really sure because the display is configured as it was when I picked it up from a craigslist ad. It should have had a 386 / 486 in it originally, but the original owners had upgraded to a socket 7 Pentium at some point. Pressing the turbo switch currently does not change anything on the display. It would however be nice to have it functioning and at some point I will probably use it with your throttle blaster design.

[scrapcomputing]

OK so white and blue connect to the motherboard to notify it that it should be in turbo mode and get shorted when you press the turbo button.

Same for white and black, but these connect to the display. And you have already checked that black is ground. But if this is the case then you should not be getting 1.6V on the white cable when the turbo button is pressed, you should be getting 0V shouldn't you? This is what I am confused about.

By the way the turbo switch should probably be a 2-way switch, meaning that the middle pin connects to either side depending on whether the switch is pressed or not, so you could use it with the current design as a backup plan.

[RetroLoom]

So the 1.6v is present on white when the turbo button is not pressed. So I guess this voltage is sort of floating when it is not connected to ground? I am not exactly sure why this is but I can try probing around a little more on the display board to see what is happening.

Ok if I am understanding correctly, you are saying that the middle pin / black wire would connect to TurboSwCM but left off of ground, the white wire would connect to TurboSwOn, and the unpopulated pin would connect to TurboSwOff?

The motherboard also has a LED pinset for the turbo LED, but in this configuration I guess it makes sense to also plug it directly into the MHzDisplayBlaster instead?

[RetroLoom]

As I am studying the schematic to adjust to this configuration I am thinking it will be best for me to ditch the original 3 pin connector and crimp my own dupont ends that match your original pin set instead. Dedicate a 2 pin connector for power, then a 3 pin to handle the turbo switch. This will make things less confusing and lesson the possibility of accidently sending 5v where it shouldn't go.

[scrapcomputing]

We can keep the 3-pin connector, but we need to to change the schematic a bit. I think I understand your switch works, so let me work on your branch for a bit and I will get back to you.

[RetroLoom]

Ok sounds good! I made a few changes in an attempt to have a better arrangement that will fit the clearance on the back side of the PCB. Feel free to make any changes you think are appropriate. I will let a photo of the backside so you have an idea of what space is available for connectors in the back.

[RetroLoom]

Also, I adjusted the display and the cut holes position to what I think should be accurate. I am not certain it will be perfect until I order a sample PCB and give it a try, but after double checking with the calipers I think it is correct. Right now the components all lean heavy to one side, but I am also trying to consider how the routing is going to work with this. I feel like it is already getting a little complicated.

[scrapcomputing]

I am using a different version of kicad so I don't want to mess up your files. Here are the changes I made:

1. The 39 Ohm resistor is replaced by a 100 Ohm resistor and connects the cathodes to ground.
2. Pins 2 and 3 of the displays are now powered by the 7404 NOT gates as shown
3. The white cable (turbo) is pulling down a 1K resistor and is connected to the input of three of the NOT gates

Please note that I have not checked the orientation of the 3-pin connector, so it may need to by flipped, depending on the footprint you are going to use.

I am not super confident that it works correctly so if I were you I would try to make some prototype to check that everything works as expected.