Tach signal questions

TheRedBlur · Jan 16, 2009

These questions are for a distributor system:
-Is the tach signal a pulse/frequency signal or a changing voltage signal?
-Does it change at a linear rate or logarithmic?
Also, what ranges should I be looking at.

Any help would be very appreciated.

jrichker · Jan 17, 2009

The tach signal is a pulse. The factory pickup spot is pin 4on the computer, IDM. The wire color is a dark green/yellow for 88-91 cars. Aftermarket tachs use the same wire, but pick up the connection point on the ignition coil.

Diagram courtesy of Tmoss & Stang&2birds

TheRedBlur · Jan 17, 2009

Jay, thanks for the input on the signal being a pulse! What I'm trying to do is wire up a progressive LED rev counter. So it'd be helpful for me to know the ranges of Hz from idle to redline and if it is a logarithmic progression or not, that way I can ballpark the resistors per desired rpm and adjust accordingly. If all works out I'll have a working diagram and a total price.

jrichker · Jan 17, 2009

For an engineering guy, you seem to be short some basic understanding of how things work.

The Mustang has a 4 cycle engine, which means each cylinder fires once every 2 revolutions of the crankshaft.

Since there are 8 cylinders, there are firing 4 pulses per revolution of the crankshaft, which at 600 RPM is 2400 pulses per minute.

At 6000 RPM there are 24,000 pulses per minute.

The increase in pulses per minute is linear.

In order to get any useful information, your sampling time can be no more that 1/10 of a second, or 100 milliseconds. The engine crankshaft speed can change radically in 100 milliseconds.

TheRedBlur · Jan 17, 2009

actually, an engineering student. Thanks for the help!

bubba-dough · Jan 18, 2009

what microcontroller do you plan on using for this?

TheRedBlur · Jan 18, 2009

I don't have my stuff handy, but I think it's an LM3914 to do the LED bargraph, and an LM2917 to covert the pulse to a voltage

bubba-dough · Jan 18, 2009

Thats not exactly what I was asking, so I guess you don't plan on using a microcontroller. I would consider using one to control you driver ic. This would make an easy reprogrammable interface that would enable you to have all kinds of programming/reprogramming options for your led's.

TheRedBlur · Jan 18, 2009

I'm new to all this, sort of a project just to get myself familiar.

stang&2Birds · Jan 19, 2009

Is this for a class??

Are you an Electrical Engineer student or a Computer Hardware Engineer student? Seems like you're an EE to me. Also, what year Engineering student are you? Have you done programming, microprocessors, micro-controllers, PICs, ADCs, DACs, etc?

IMHO, your design is "okay" for an first-year EE. But, not at all acceptable for a Hardware Engineer.

For a Computer Hardware Engineer student, doing a PIC-based solution is what would be best. It would actually touch on many different technical areas (DSP, real-time, interrupt, polling, latency, accuracy, code size, debugging real-time code, fractional compute time & accuracy, etc). But, it's also a pretty simple thing to do and there are many PIC code examples that you can use.

Eventually, once you graduate and start working, it'd be nice if you can do the above in Xilinx Spartan 3 with a PicoBlaze and some level translators (needed for the 5V input signal).

Knowing DSP, FPGAs, DSP, micro-controllers, floating point issues, analog interfacing, and so on are the areas that will help get you a job.

IMHO, using the LM3914 and LM2917 is more high-school level stuff than college level stuff. And, considering that mammoths still roamed the Earth when I taught for 8 semesters, started a brand new course (new course number, the whole bit), and overhauled 2 existing courses when I taught, I don't think that something that was overly basic back then is something you should be spending much time on as a College-level Engineering student.

Also, if doing the PIC stuff isn't what you like, then don't even think about going into Hardware Engineering. EE would be fine. But, even now, many EE's have to program and know a lot about DSP. Using Matlab is pretty standard for a lot of EE stuff today.

Last: You don't have to use a PIC. There are other choices. But, the PIC is the most common and where you'll also find the most examples and help. I like the Motorola micro-controllers, the Cypress PSoC, and the FPGA-based solutions.

A Cypress PSoC would be good if you're an EE. But, that's really more of a 3rd or 4th year level student project because of the complexity involved in dealing with the PSoC.

stang&2Birds · Jan 19, 2009

$59 + S&H
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en534506

BTW: I like and use digikey (#1 electronic component supplier a zillion years in a row) when I order stuff.
http://parts.digikey.com/1/parts/1518206-kit-starter-mplab-dspic-dsc-dm330011.html

MPLAB Starter Kit for dsPIC DSCs View attachment 276679
Part Number: DM330011

The MPLAB Starter Kit for dsPIC Digital Signal Controllers is a complete hardware and software tool suite for exploring applications based upon Microchip’s dsPIC DSCs. With a built-in debugger on the board, simply install the software and connect the USB cable to the PC. Start up MPLAB IDE and you are in full control, able to run the sample programs, and to download and test your own applications. The board is designed with dsPIC DSC with 256 KB of flash memory, a high-fidelity audio codec, microphone input and headphone/speaker outputs, and is powered from the USB connection to the PC. Also on the board are reconfigurable switches, potentiometers, a temperature sensor and a 4 Mb serial EEPROM to store data such as audio samples.
Features

Board includes integrated debugger / programmer
USB powered
dsPIC33FJ256GP506 DSC with 256 KB Flash and 16 KB RAM
16/24/32 bit codec with a maximum sampling frequency of 48KHz
Low cost audio capture and play back circuitry using the 12 bit ADC and PWM Audio
Microphone and line level inputs with adjustable input gain
100mW headphone amplifier with digital volume control
2 switches and 3 LEDs for user application purposes
4 Megabit serial flash memory for application use
Temperature sensor
CD contains MPLAB IDE with full editor, programmer and debugger; MPLAB C Compiler; code examples and user’s guide
All tools provided for developing and prototyping speech and audio application & algorithms

Package Contents

dsPIC audio board with integrated debugger
MPLAB Starter Kit with CD containing MPLAB IDE, MPLAB C Compiler for dsPIC DSCs, application notes, and sample programs
USB cable

bubba-dough · Jan 19, 2009

TheRedBlur said:
I'm new to all this, sort of a project just to get myself familiar.

oic, so i take it you are probably a first year student. If you are doing electronic engineering, you should get into the different languages and micro-controllers in your second year. You should love those classes after you get the hang of them.

Good luck....

TheRedBlur · Jan 19, 2009

Yeah I'm a first year EE, so we haven't gotten into circuits yet. I just thought I'd tinker around a bit to get familiar. Anyways, I'll throw something together soon, it'd be nice if I could bounce the idea off you guys.

bubba-dough · Jan 19, 2009

yeah get something together and let us know how it goes. if you get stumped let us know. also, are you using any circiut design software? if you don't have any there are a lot out there that are free to try. I prefer NI Multisim, but thats because thats what we use at work plus its what I used when I was in school. This will allow you to "virtually" build the circuit and test it on your pc without physically building it. I am sure you will get a student version at school but it will be very limited on component choices. here is a link to what I use... NI Multisim for Professionals

stang&2Birds · Jan 19, 2009

Yea, IMHO, Multisim kicks *ss for analog simulation.

For me, it's great.

For a first year EE, with no high school electronics experience, then I agree, what you first chose is a good way to get into "real life" circuits. However, I'd STRONGLY suggest that you get some oscillator circuit or programmable oscillator to test out your circuit. There should be a number that you can buy for under ~$25 that have a variable output, but not super accurate. That's fine for what you need.

For a simple and cheap oscillator, one of the best ways is to use a 555 and some resistors and a pot. Simple and cheap to bread board.

555 Timer/Oscillator Tutorial
YouTube - Changing the freq. of a 555 with pots.
LM555 Timer Circuits

BTW: Buying a cheap USB 'scope or a cheap digiscopes (~$400) is a very good idea!

For $50,
DI-148 Data Acquisition Starter Kit - USB Data Acquisition system
But, limited to 240 samples/sec without software upgrade.

Or, for only $25 if you have a free RS-232 port:
Data Logger Starter Kit
Again, limited to 240 samples/sec without software upgrade.

Instead of the dataq junk, I'd suggest something like:
$169 Sample Rate 10 kS/s
NI USB-6008 - Products and Services - National Instruments

or
$279 Sample Rate 48 kS/s
NI USB-6009 - Products and Services - National Instruments

Good Luck, and HAVE FUN!

stang&2Birds · Jan 19, 2009

BTW: Maybe in a year or two, I suggest that you start checking out something like the Cypress PSoC. It's the "first programmable analog array". (Take that statement with a grain of salt.

). But, it is very neat. And, you can do a lot of very neat real-life circuits with it. Plus, it's widely used. Hard to say if they will still own the market share in that feild in 4 years. That's a long time. But, my guess is that the PSoC will be around for a while.

Programmable System - on - Chip - PSoC - Microcontroller - Mixed Signal - MCU - 8 - bit - Cypress Semiconductor

FYI:
http://www.cypress.com/?docID=2556
AN2087 - Motor Tachometer Speed Calculation Using Hardware Timer Capture Feature
Utilize the unique PSoC® architecture to capture, filter, and store the period of a fan tachometer signal.
The PSoC MCU captures and measures tachometer (tach) signals from fan and other types of motors. This is critical when performing automatic speed control and speed error detection and correction.
...
The PSoC digital blocks provide a hardware timer capture, which is a special feature used for Motors come in a wide variety and have different features and capabilities. For this design, a voltage controlled variable speed motor with a tach output feature is selected.
...
First, a Programmable Gain Amplifier (PGA) is used in the PSoC to provide a connection block. From the PGA the signal goes to a Low Pass Filter (LPF) to remove the high frequency components that are common on motor tach clock for the capture timer.

TheRedBlur · Jan 19, 2009

Still greek to me.
I found this site and looked up the datasheet for the LM2917, sort of made sense. The site says that the tach-in is protected. How does it work?

View attachment 276541

:edit: I don't at all plan on revving to 8K RPM, so I will have to adjust that one. I guess it goes by (Vin)(R1)(C1)(input)

:scratch:

4pulse/rev (thanks jrichter), I want the range from 5300RPM-6300RPM.. So that'd be 21200pulse/min-25200pulse/min. Which leads me to 353.3Hz-420Hz..

LM3914 uses a 0-5v input signal source

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