Speedometer / Tachometer (part 2: electric boogaloo)

[Howard_Casto]

Ok I've been milling over this stuff for over a year now.  My ultimate conclusion is that unless you are into pc sims or really like bling on a cab it probably isn't worth it but here is a summary of my findings for those playing the home game:

Probably not practical options:
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Real instrument cluster pulled from car -  This obviously is the most realistic choice, but the interfacing to a computer can be cumbersome and this won't fit on an arcade conversion.  Also you are stuck with the min/max values as printed on the gauges.  Keep in mind that depending upon the cluster you choose, things can get pricey.

Scratch built analog gauges - This is extremely expensive.... a stepper/servo along with an appropriate driver board can be 30 bucks a pop, so you are probably better off hacking up a used car cluster.  Most of the same pitfalls of a ready made cluster apply here.  The only benefit is you can take the individual gauges and make them to fit your dash in an arcade conversion situation. 

Practical options:
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7 segment display - You can get these pre-made online for around 5-10 bucks a pop complete with 8 digits, a row of leds, and some buttons on an Arduino compatible pcb.  You can daisy-chain several together and they only take up 2 data lines on your avr.  Some programming is required, but libraries are readily available.  It's very low overhead on your pc and your avr to drive these.  The downside is that the ready made boards are quite tiny (which could also be a plus) and although you can roll your own via buying the individual parts and larger digits, it will cost you more time and more money. 

Dot matrix display -  These are pretty much exactly like the 7 segment flavors except it's basically a low res monochrome display.  1.25" 8x8 modules can be had for as little as $1.50.  Several can be connected together for a seamless, larger, display.  They are driven via an avr with the same data lines and low overhead.  This flexibility makes it a great for conversions and scratch built cabs alike.  In addition you can display whatever you want.... a needle, numbers.... animations ect.  The only real downside is the low resolution.

LCD/LED monitor - Yeah so just use a second monitor and display the graphics to fake whatever gauges you want.  This method certainly has the most flexibility, but rgb/vga monitors in the needed size can be quite costly, you will probably need two (one for tacho and one for speedo) so that displays can fit around the wheel, some games/emulators kill the secondary monitors when going to fullscreen, and even with dedicated video cards driving multiple displays can add significant overhead to your pc.  It's certainly an option if money is no object, but otherwise it's probably too expensive.

[Howard_Casto]

Anyway, my first attempt at making something that will work is a dot-matrix display:

http://www.ebay.com/itm/171578345542?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I got two of these for 12 bucks.  I will arrange these in a square which will make a 2.5"x2.5" display for each gauge.  I got them in red because they are considerably cheaper, but if I like how it turns out I might upgrade to blue or white. This will be driven via the same avr mentioned in my rig build thread so I'll release the source once I'm done.  The library for these allows you to send frames via comma delimited arrays of 0 or 1, so I'm making an app that converts monochrome bitmaps into code.  That'll make it easier. 

I really was leaning towards lcd monitors, but believe it or not even finding a "micro" monitor as large as 2.5" was difficult.  They seem to come in three sizes.... postage stamp, car monitor and full on monitor.  Also most of them are composite only, which means I would have to put janky old video cards in the pc to drive them.  It just wasn't worth it. 

[SailorSat]

I really was leaning towards lcd monitors, but believe it or not even finding a "micro" monitor as large as 2.5" was difficult.  They seem to come in three sizes.... postage stamp, car monitor and full on monitor.  Also most of them are composite only, which means I would have to put janky old video cards in the pc to drive them.  It just wasn't worth it.

Depending on the price range one could go with these: http://www.ebay.com/itm/4-0-inch-TFT-LCD-module-w-Font-IC-480x320-ILI-arduino-DUE-MEGA-3-2-2-5-STM32-/111146197614 (just an example)

Although I cannot quote about write performance at all.

[Howard_Casto]

35 is a bit pricey imho, especially considering a person would need two of them. 

Anyway though, I looked into those, I guess I should add them to the list.  The serial ran monitors can barely be driven by the average avr.... the resolution one can drive reliably is comparable to the dot matrix displays and the processor requirements even for that are rather steep.....  you would probably need a dedicated avr for each monitor.  There are ways around this... graphics can be stored on a sd module and pumped to the display, for example, but things are getting really complicated if you go that route. 

It's an option though.  Personally I'm just too lazy and cheap for this particular one though. 

*edit*  It looks like the one you linked to has embedded flash memory, so that's one less complication.  Sorry, I haven't had much sleep.  It looks like the icons have to be flashed at design time though, so that could be a pain in the butt.

[Howard_Casto]

Ok so my dot matrix displays came in today.  Unfortunately they are not exactly as advertised.  By the auction pics I thought I was getting 4 individual modules linked by jumpers.  What I got was 4 pcbs that hadn't been cut apart, essentially making a continuous board.  It should be possible to cut them loose though and the build quality actually looks better than the previous model, so I can't complain too much. 


I'm not feeling super great today, so I'll work on getting them plugged in tomorrow. 

[Howard_Casto]

*update*

So I actually bothered to take them out of the package tonight and while the pcbs are fused together as a single board, the led matrices can be unplugged from the board.  So with jumper wire and a clever mounting solution a person could re-arrange these without much effort.  I think I'll go ahead and separate them anyway as the pcbs have really nice mounting holes.   

[Howard_Casto]

More FYI stuff.....

It turns out these things are extremely power hungry.  If you don't have an external power supply they won't work, but instead of lighting dimly or not working at all, they just do random glitch stuff on the display before eventually powering down.  I thought I had defective units so this is just a quick heads up. 

The libraries for the dot matrix displays aren't real great, so I might have to write something myself.  I'm still experimenting atm. 

[Howard_Casto]

So I've tried various matrix libraries.  They are all pretty competent for displaying things on a single display, but they all have various issues on multiple displays.  The main thing is you can't re-order the modules, so things look odd when a function tries to scroll text or what have you.  So I think I'm going to mix-and-match the best features of each library and write a bit of my own code to allow you to define a virtual screen space similar to how windows does it.  Anyway, this is holding up my project so I'll continue work on a separate Arduino and add it to my upcoming driving rig sketch when it's done.