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Need help with 4-digit LED digital tachometer circuit

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martianent

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I have been looking for a while into building my own custom digital instrumentation for my car. Mostly because I don't like the costs associated with the pre-built stuff ($400 for some gauges! And there is a full set out there for $600!) and the fact that there aren't any drop-in clusters available for my car.

I bought a book, called Building Your Digital Dashboard, Volume 1 by a guy by the name of Christian Forget, who runs a site and company that sells conversion parts for Knight Rider KITT clone cars, called Jupiter Electronics. I have the schematics and parts lists for 5 out of the 6 instruments, but I feel that the tach could be a little better, seeing as how it only shows 3 digits. I also noticed, while coming up with my schematics, that some parts of the Jupiter design seem to be incorrect, namely the multiplexing on the LM3914N IC's, which he has in parallel for the signals instead of connecting pin 9 of the first 3914 to pin 1 of the next. My plan, instead of using the CA3161E and 3162E 3-digit ADC and driver in the schematic for the LED numerical display, is to replace them with a Maxim ICM7135 and ICL7212. I have drawn up a schematic in ExpressSCH and need someone to look it over before I buy and fry these expensive chips and the board that goes with them ($66 by my count). Can someone point out to me any mistakes I may have made before I get too far into Express PCB? I think I followed the datasheets for the LM3914Ns, ICM7135, ICL7212, and the others properly, but I'm hoping someone with a practiced eye can tell me if I did or not. I'm not a novice when it comes to soldering, so putting these together should be easy when I get the boards made and the components together.

Oh, and the reason the SCH is so squished is because I only had an 8.5x11" space to work with. I know I needed more room.
 

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What a horribly complicated way of building a tach. Besides, it will have an accuracy of +-5% at best. I would use a PIC micro, and measure the period between pulses of either the distributor or other pulse train. To display RPM, it is very easy to convert pulse period into frequency by doing a division, which is trivial if you use a C compiler. You can easily get 10rpm resolution with an update rate of 10Hz or faster.
 
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There was a nice digital readout tach in an issue of Nuts & Volts a few years ago. It uses commonly available parts with no microcontrollers (which would be my preference along with MikeMI), if you want it I'll scan it for you. I don't think N&V would appreciate me posting it publicly.
 
The other whole point to building the tach this way was to avoid the microcontroller... Programming the ECM (engine controller) is going to be hard enough when I get that squared away, and I'm not versed in BASIC, C, or any other programming language.

LTX, if you need an email address for that, let me know.
 
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The other whole point to building the tach this way was to avoid the microcontroller... Programming the ECM (engine controller) is going to be hard enough when I get that squared away, and I'm not versed in BASIC, C, or any other programming language.

LTX, if you need an email address for that, let me know.

hi,
This ICM7217 makes a simple 4 digit counter
 

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LTX, that digital tach doesn't have quite the resolution I'm looking for. Even the one I'm building from uses the 3rd digit of the 4-digit display (say, 650x10 for redline).

StudentSA, I look forward to seeing what you have now. Although it may need to be modified for my GM HEI system (very tiny input). Have you thought about using an LM7805 as a power filter? Those can be set up to 15V.

ericgibbs, Is it my understanding, from the datasheet, that the input of the ICM7217, the COUNT pin (I'd guess?), would come straight off of the ignition TACH output, which would also be split for the outer ring LED "dot" display?
 
[QUOTE
ericgibbs, Is it my understanding, from the datasheet, that the input of the ICM7217, the COUNT pin (I'd guess?), would come straight off of the ignition TACH output, which would also be split for the outer ring LED "dot" display?[/QUOTE]

hi
Look thru MikeL posts for good tacho take off circuits..
 
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I would like to know what the purpose of the 2 LM3914s are for.

The best way to make a tach without a uC that I can think of would be to use a counter that counts the number of revolutions, and a clock pulse would enable a display momentarily to show the number of pulses logged, then that same clock pulse would clear the counter and start over. All this would have to happen at a rate of about 10Hz.

But looking at the circuit that you've posted, I can't see anything wrong with what you've made.
 
I had a second look at your circuit, and it is very, VERY important that you reverse the polarity of C4 other wise you risk having it blow up on you. The output voltage of the MAX1044/ICL7660 is a negative voltage when reffernced to ground.
 
Okay, I'm going to revisit this for a moment.
I've changed my plans... Now my plan is to go with the original circuit, using an LM2917 feeding the dual LM3914s for the outer ring display (important as I need to show redline somehow), and also feeding a CA3162E that drives a CA3161E that drives a 3-digit 7-seg LED display. Still want the last digit, however, and I'm thinking that I want the 3-digit LED OFF when engine speed is below where the engine is running and that I would like a last digit fixed at Zero when the ignition power is turned on. I'll see if I can get a diagram made of the circuit as is drawn in the book so everyone can see, but, how hard would it be to have a 4th 7-segment LED set to always display "0"?
 
Yes, this is my second reply in like 10 minutes... Didn't realize I already had a circuit drawn up (had to restore my laptop since then and thought I might have either not drawn the file or lost it). This is the same circuit as the 3-digit tach, with the exception of a couple of resistors (R3 in the original circuit and R6-8 in this second diagram, which I still need to do some calculating on). Just looking to tack on a 4th LED that constantly displays "0" here with the CA3161 turned OFF with no indication when the engine is not running.
 

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While I can appreciate the direct approach as easier; consider that once the uC learning curve is surmounted, you can have All your sensor data processed/displayed by the same uC.

I've got 2 motorcycles with instruments I built on them: velo / tach / oil press / volt / oil temp / coolant temp displayed on a character type LCD, the other is standard info stuffed into existing form factor (pix encl).

I'm getting vision issues to soon require migration to graphic LCD. Once I learn gphx LCD control, it'll be a Lot more fun. Season to taste... <<<)))
 

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Well, you also have a rather small space there... My instrument cluster covers half of my dash board, which leaves me a lot of room. If I knew programming, I'd use a microcontroller and add a few more gauges (exhaust temp, fuel pressure, air temp, air/fuel mixture, vacuum/pressure). I have books on C, but haven't really done anything with them yet due to lack of C writing software.

Do you have any more info to share?

:edit: Almost forgot to mention that the ECM I will be using to run the car (MegaSquirt II) supports a CAN bus, if that may or may not make things easier. I don't currently have the ECM configured that way, but it wouldn't be to hard to modify it as I'm the one who put it together in the first place.
 
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