Hi

I would like some comments on my attached schematic, it is for a digital dashboard for a car. I understand digital electronics, but analogue stuff kills me.

The circuit has to run in a car, which im told is very electrically noisey, what changes if any would i need to make for this.

The dip switches in the circuit make up a 6 bit ladder type thing (R10-R15), am i right in thinking that if i read the output from this, as an analogue voltage, i SHOULD be able to decode it into what switches were set, and which ones unset.

The inputs to the circuit are 12v (well about 14.2 with the engine running), are the resistors - for example R1, R2, and R7 ok for reducing the voltage/current reaching the PIC?

Are there any other hardware related problems that you can see in the schematic, or that you would foresee with the project?

I like to try and do things myself, and usually succeed, with my breadboard ect, but this will be my first project that i will be producing on a properly manufactured PCB. (anything i should be careful with here for example component location or track placement?)

Sorry about so many questions, but if you have any for me, please ask.

i can provide any additional info you need to help me.

best regards

graham

 

Your DIP switch ladder won't work. You need a proper ladder with divide resistor, like R-2R. Probably better if you just use a shift register to read the switch instead of ADC.

 

hardware related problems

As n9352527 pointed out your resistors at the dip switches needs a resitor to VSS to form the devider so you can read the analog value in.

I also foresee PSU problems. Auto's can generate extreme noise, and 7805's cannot handle the extremes and may tilt. To eliminate this, from 12V in, place a diode (1N4007) in series with a 15R and a cap 100uF to GND, then another 15R and 100uF cap to gnd, then a 15R to the regulator input with yet another 100uF to GND so you will have 3 filter stages. I would also replace C1/2 with 100nF and add a 1uF parallel to C2.

Not sure what C3 / C4 does, if they are random 100nF caps sitting close to components supply pins it's a good idea.

A note on 7 segments - they alone don't work too good for tacho's, while an engine rev's up fairly fast the 7 segment becomes a blur, and if you slow it down you're out of rev's before you can see it. To solve this you may want to add a LED bargraph for the tacho, the digital would be fine when you cruise.

 

Hi

Thanks for the very informative posts.

Am i right in thinking that the resistor for creating the potential divider at the dip switches goes between the +5v and the left hand side(on the schematic) of the dip switches?

I have have went down this route for reading the dip switches purely because i only have 1 free pin on my pic, and its analogue.

(well not really true) - you see im an ebay hog, and i buy all my bits in bulk, and as part of that i got some dip switches(lots and lots) but they are 8 way ones, and i may as well use them. Most of my configuration is done in software, but when i started laying out the board i discovered that i only had 8 way dips and 1 free pin, so i looked for ways to read them - for future use, if you know what i mean.

I have read the comments about the input, and will impliemnt them, i have also been advised to replace the 7805 with a LM2937, which can handle a 60v load dump, and generally better tollerate the automotive enviroment. From what i have read about it, it is a direct replacement for the 7805 in terms of pinout ect, just needed some changes to the cap values ect. Can you foresee anyproblem with this change too?

Sorry just noticed i dont have any values in for C3/C4, but yes they are a ceramic and eletrolytic cap in parallell and close to the supply pins for the IC's.

As for the overall display and use of 7 Segs. It goes kind of like this:

The connector marked 'Input' J1 is connected to 12v ign wire, vehicle gnd, a tacho wire (on some cars a replacemnt or additional sensor may be required), a speed sensor wire, as before additional sensor may be required.

The speed and revs signal can be either 12v or 5v depending on the settings of dip switch 1 and 2 (which bypass the potential divider if 5v signals are used)

J2 'Control' is a ALPS incremental encoder with switch

J3 'LCD' is the lcd (the control is used to navigate menu's on here)

J4 'Shift' this is a row of leds, although only the first 5 are used (future proofing - connect all 8)

j5 'Speed' this is a set of 3 '7 seg' displays reasonably big ones, the ones i use just now are about 2 inch tall

j6 'Revs' this is a row of leds for display of revs (Very configurable - either one LOOOOONG line , or as i use them just now , 16 Bi-Colour leds, so that as you approch the rev limit they change color)

I have built one of these using an OOPIC, and breadboard ect, but i now want to make a 'proper' version, with my ultimate goal being a 'Make-Your-Own-Dash' kit, where all my stuff is on this one little board (about 2"x2" till i figure out smt), and a few connectors with tails so you can choose your own led colours ect.

Imagine for example that you have a cutie little pink vw bug, you may want your rev counter to be pink leds in the shape of a heart that runs around your speedo.

I have been working on this for a while now when i have spare time but cash has always been my problem, so thats why im trying to get this right as i cant afford a mistake on PCB and have to get it made again.

I have attached a few pictures of bits that i have done, to see if it helps visualise it.

 

Sry almost forgot, the lcd displays useful info like mileage, trip distace, and im hoping to add other bits and pieces to it later.

And the aim is to no longer make one big board for my car as pictured above, but to make a small universal module for any car so that only the plastic mouldings (if reqested) would have to be made. The picture attached to this post shows most of the bits on my breadboard.

 

I have designed and built a digital dashboard.The main unit is based around the PIC16F877 and the LCD modules are based around the PIC16F628.The displays are as follows:- Volts (0 to 17), Amps (+/- 80), water temp (0 to 130), oil temp (0 to 200), vacum / boost (0 to 30in/Hg and 0 to 30 PSI),revs (0 to 10,000), MPH (0 to 150 + miles), fuel tank 1 (0 to 8 gals), tank 2 (0 to 8 gals) And oil presure (0 to 100 PSI).All the data from the main computer is sent down 1 wire in serial form at 19200, So all the display boards are just pluged into a 20 way ribbon cable.It can be programmed with limits that are sent to another computer in the car, this one deals with the coded ignition, car heating lights and alarm.The alarm sends out a signal over radio link every second and is recieved on a small pocket reciever, it has about a 250 mtr range.I have had no problem with noise, but I was very carefull in the design of the power input section.Daniel.

 

Hi

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I have been working on this for a while now when i have spare time but cash has always been my problem, so thats why im trying to get this right as i cant afford a mistake on PCB and have to get it made again.

I have attached a few pictures of bits that i have done, to see if it helps visualise it.

You can make home-made prototype versions of your PCBs, very cheaply and very quickly and easily, using the "toner transfer" method on this page:

http://www.fullnet.com/~tomg/gooteepc.htm

For lowest cost, also use the home-made etchant solution recipe, there.

You can find cheap "blank" PCBs to use, on ebay, if you search for things like FR4 and "copper clad". Or, you can buy them at www.mouser.com , fairly reasonably.

For your finished commercial PCBs, you will still want to use a professional board fabricator, such as http://www.4pcb.com (whom I have used, recently, and like, a lot), so you can have things like soldermask and plated-through holes.

Note that NOT having plated-through holes on your home-made prototype PCBs can be a bit of a pain, for 2-sided boards, since you can't solder top-side trace connections to pads that are underneath a socket or component, for example, in which case you have to create a neaby "via" pad, through which you solder a wire, on both sides, or else use a component lead for that, if there happens to be one there.

You should probably submit the Gerber files that your PCB layout program produces to 4pcb.com's free "freeDFM" service. It will then automatically email you with a report with (and images of) examples of possible manufacturability problems on your board, a PDF of the Gerber artwork of each layer, and a price quote. Interestingly, in the price quote, you'll note that raising the quantity doesn't change the total price by very much. So, even though the default 5 and 10 board quotes might seem high, the price per board goes WAY down if you buy 20, 50, 100, etc., at a time. Molex connectors might end up costing more than the boards, especially if you use gold-plated ones. (And there are, no doubt, cheaper places to get boards made. Theirs is just the most-painless one I've found.)

By the way, for the resistive "D-to-A converter" in your schematic, you might want to add a capacitor or two to ground, after the resistors, so it'll also act as a low-pass filter, for better noise suppression for your analog input, "just in case".

Good luck!

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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