Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Car digital displays Tacho and speed

Status
Not open for further replies.
Hi all. im new here and have a basic knowledge and wanting some help for a project im doing.

I want to convert my dash to a digital / led one.
I want the tacho as a circle of leds and if possible a number on a lcd/ led digit display in the middle and the same for the speedo, speed by leds and i numerical readout in the middle.
Would be good to do the same with the oil pressure/ water / voltage and oil temp. but ill work on one thing at the time as i think the basic circuit for each item is the same with slight modifications.

I have been looking at the 2 Chips , LM3915 led driver and lm2907 but not sure how to hook them together and how to output to more than more than 10 leds, would prefer about 20 leds and a 4 seg display for tach and 3 seg for speed.

Any help is greatly appreciated from you good experts.


Thanks in advance
Stuee
 
You have to specify what model and make of car you have, attach pictures of the actual dashboard and how's connected.

If your car is old, I hope your speedometer is not linked mechanicaly: that would render the project very difficult. If is electrical, the chances are better.

The tachometer is much more promissory, because is always electricaly driven, so its easier to project a circuit for that.
 
Hi,
The speedo is wire driven BUT.... because its a 4wd and the conversion i did i have to buy a VSS that's on the transfer box to the ECU that if i remember correctly has a diagnostic led on the box that flashes per revolution..

The Tacho is just wired. But i tried hooking the wire from the new engine coil pack to the original wire on the original tach that went to the neg coil but it never did anything :(

thanks
 
Hi on this website Stuee,
The LM3915 has logarithmic steps and is used for sound and light levels. The LM3914 has linear steps and is used as a voltmeter.

You might need a magmetic variable reluctance pickup to breaker points converter circuit so that the tachometer will work. Maybe the other way around.
 
If you do a search there are a few circuits for speedo/tacho on this site - including one I helped my brother build - but this would REALLY be easier with a microprocessor - I have been doing these conversions for a while now - and doing it in logic is possible but you need to divide your pulses quite a bit and you cant really calibrate it very well.

I work as a taxi-meter installer, so I deal with the speed pulses on all types of vehicle almost everyday - and I wouldnt even consider hard wiring a speed display (do you fancy re-building your circuit just because you want to change your wheels)
 
As I said, the speed think is hard, but here is an idea I had during breakfast

Why you don't strip the old speedometer, put somekind of electrodes on each number on the scale, and replace the ¿pointer? (don't know the english name for that, the orange stick that moves on the dial) with a small electrified metal rod... So acording to the speed, determinate electrodes are touched

its an electro-mechanical approach, and doesnt need strange calibrations
 
This is not the way to go for several reasons.

Given that you want to do digital work use a microcontroller. It will take real time to learn how to program and use it but the return will be tremendous.

3v0

As I said, the speed think is hard, but here is an idea I had during breakfast

Why you don't strip the old speedometer, put somekind of electrodes on each number on the scale, and replace the ¿pointer? (don't know the english name for that, the orange stick that moves on the dial) with a small electrified metal rod... So acording to the speed, determinate electrodes are touched

its an electro-mechanical approach, and doesnt need strange calibrations
 
This is not the way to go for several reasons.

Given that you want to do digital work use a microcontroller. It will take real time to learn how to program and use it but the return will be tremendous.

3v0

Microcontrollers are great, I wish someday learn how to use them, but my learning curve is so flat :(

If you read the previous post 3v0, you will see that the problem is not how to process and display the data, but adquiring it in first place

Why do you think the electro-mechanical method using the existing speedometer is not feasible?

here is another idea: if you dismantle the existing speedometer, it must have some kind of circuit to convert the pulses into voltage that drive the dial. What about using this pulses, instead of the signals entering to the instrument from the engine bay?​

(it would be nice to see any Discostuee input)
 
Microcontrollers are great, I wish someday learn
here is another idea: if you dismantle the existing speedometer, it must have some kind of circuit to convert the pulses into voltage that drive the dial. What about using this pulses, instead of the signals entering to the instrument from the engine bay?​

I guess that will be a good idea. U dont have to worry about how thy take the output from the Engine. U got a dial rotating according to the speed. So wat else u want? :D Ok if thrz nothin to do at all, just use the arm to change a resistance level and that u can use as a input to ur system.
 
Microcontrollers are great, I wish someday learn how to use them, but my learning curve is so flat :(

For the most part computers and uC's are not complicated. Just a lot of very simple things you have to know all at once :) I have tought kids as young as 11 to program PICs. (with a bit of help)​

If you read the previous post 3v0, you will see that the problem is not how to process and display the data, but adquiring it in first place

All you need to do is have a uC count reveloutions that are directly proportional to the vehicle speed. Drive shaft, spedo cable etc. Use either a mangetic or optical pickup. Either is fine. The uC end of it is a common tach like application. Same for tach execpt your count is based on engine rotations.

A uC would very handy to light both the LED circle and the 7 segment displays.
Why do you think the electro-mechanical method using the existing speedometer is not feasible?
It could work but it is the wrong solution. How many points are you going to use. How are you going to decode the speed from the last point(s) to make contact. It is large. You ruin the original spedo.
There is another idea: if you dismantle the existing speedometer, it must have some kind of circuit to convert the pulses into voltage that drive the dial. What about using this pulses, instead of the signals entering to the instrument from the engine bay?

Yes but the original spedo in a box or sell it on Ebay. The solution I am talking about counts pulses, as I suggested there are several ways to get them.

xxxxxxxxxxxxxxxxxxxx 3v0
 
Last edited:
Have to agree with 3v0 on this one, a mechanical solution WILL NOT WORK!!

it is as easy as counting the pulses,

on a cabledriven system you either add a magnetic picup, or you cut the sleeve and insert a redily available pulse adapeter that connects to the wire directly

on a more modern system when the car has a digital pulse, you tap that - either behind the speedohead, or pin1 of the ISO connector on the radio, it will either be pulsed ground, or pulse between 5v/12v and ground (model specific)

on a very modern car you *hope* they left in a pulse wire, if not you go down the magnetic pickup route, or buy a CAN adapter such as the CANM8 which plugs into the wiring, and gives an output of approx 1Hz/mph, or you build a CAN adapter.

I dont want to piss on anyones fire, but this is where you get the speed pulses from - end of.

So the real question goes back to how to process it, which needs to be calibrateable.

to give you an idea - *MOST* vehicles generate approx 3600 pulses over an approved measured mile give or take a few hundred, a new vauxhall vectra however generates near 50000 pulses over the same distance.

so built in logic, the speedo would *only* work in this vehicle, and the pulse dividers/freq to voltage chips etc would need to be re-wired every time you got new tyres, whereas with a micro running the show it could work in ANY car, and be calibrated at the push of a button.

(my latest digital speedo changes from blue to red when you exceed the speedlimit, but i need a better mapping system as the speedlimit data is out of date now)
 
thanks everyone for all the input. like i said i do have the cable driven speedo that is original but as i changed the engine the ecu was also changed which required me adding a vss to the transfer case then the orig speedo cable goes through that, It has the 3 wires which are B+, neg and signal. Could i use that.
i think i might aim for the fuel gauge and tacho first as they are both none existence due to different pump and engine. can anyone shed some light on the tacho situation even to get the basic one working for now. from the new engine magnetic pickup with the old points signal on the tacho. thanks again
 
yup youu can use the signal wire from your new vss without any problem.

i dont know about points, but do you have a rev counter?

if so there will be a rev signal behind the clocks
 
i tried hooking the original rev counter (points type) to the new engine signal(magnetic pickup type) and it didn't even move a single bit. As i have read they are 2 types of signal, frequency and voltage.
 
Calibrating the spedo is easy enough. Setup the uC a a counter, zero it and drive through a known mile. Then reprogram the unit to show MPH instead of counts. Scaling of all inputs is simple with a uC.

As I said earlier there are several ways to get a sensor/signal for tach. On older cars one could pick up the distributor pulses on the low side of the ignition coil. Worst case you stick a bit of reflective foil on the harmonic balancer (or pulley bolted to it) and watch it go buy with a optical sensor.

If I recall right the fuel indicator is a variable resistor. Use it as half of a voltage divider and read the center voltage. Scale it so the output is about
zero to 5 V and read it the the uC ADC. If you have a working gauge you can
read the voltage at the input terminal with a full and empty tank. Then measure in the same wire but with the gauge disconnected and find the resistance to gnd for a full and empty tank.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top