Basic Pulse generation help

[john2k]

I have a speedometer/instrument cluster from a car that I am doing some bench tests on. I've got the cluster powered up by referring to the pinout diagram. I've got turn signals emulated too. However, I want to emulate speed and according to the pinout diagram, there are two speed related pins and the pins are described as:

Speed signal (Output) - Pulse generation (Waveform 1 Diagram below)
Speed signal (Input) - Pulse generation (Waveform 1 Diagram below)

I've attached the waveform diagram below. Can anyone tell me if there is a easy and cheap way to emulate this so that I can simulate speed and send this data to the cluster? I'm not sure what the difference between input and output on the cluster pins are either.

 

[bertus]

Hello,

What is the variation of pulse sequence width A?
It looks like you basicaly need an oscillator that creates the shape.
An 555 could do that.

Bertus

 

[john2k]

Hello,

What is the variation of pulse sequence width A?
It looks like you basicaly need an oscillator that creates the shape.
An 555 could do that.

Bertus

Sorry I have no idea I've never worked with pulse generation before. This is the only documentation I have. I think I just need to provide the pulse generation power to the Speed Signal Input pin because the output pin takes that reading and sends it to other parts of the car from what I have seen. But seeing as it's only 1 pin for the pulse generation power, does that mean it shares the same ground as the rest? Do I get something like the pic below?

 

[bertus]

Hello,

Looking at the diagram, I see a pulse of about 80 mS in the Toolsetting.
This means that the pulse frequency is 1 / 80 mS = 12.5 Hz.
Looking at the PCB you show, it can likely not go low enough in frequency.

Bertus

 

[john2k]

Hello,

Looking at the diagram, I see a pulse of about 80 mS in the Toolsetting.
This means that the pulse frequency is 1 / 80 mS = 12.5 Hz.
Looking at the PCB you show, it can likely not go low enough in frequency.

Bertus

What kind of PCB ready made kit can I look for that will work? The diagram says One wheel revolution generates 4 pulses. As the vehicle speed increases, the width indicated by (A) in the illustration narrows. So, if I wanted to simulate speeds like 100mph then what kind of pulses would I need?

 

[bertus]

Hello,

The speed versus pulses is dependend on the wheel diameter.
A smaller wheel will produce more pulses as a larger wheel.

Bertus

 

[Hunter64]

According to the given information the signal has a fixed duty-cycle and the frequency depends on the speed.

Assuming the circumference of a wheel is 2m you get a pulse every 50cm. All you have to do now is calculate the number of pulses/s at a given speed in km/h.

100Hz = 100 pulses/s = 360000 pulses/h = 360000x0.5m/h = 180000m/h = 180km/h

 

[john2k]

Thanks, I have ordered the following NE555 Adjustable Module Duty Cycle Pulse Frequency Square Wave Signal Generator. I will see if I can get anywhere with that once it arrives. Any ideas if this will work? As there is only a single pin on the connector dedicated to the speed signal input, how do I wire the pule generator?

 

[bertus]

Hello,

That looks usable.
The lower ranges can be used.

Bertus

 

[majoco]

I wouldn't get too worried about the "output pulse generation" at the moment. See if you can get the speedo part working first. A "Function Generator" can do all that you need as far as generating signals is concerned. I suspect that the signal coming from the wheel speed sensor is not a nice clean square wave so the speedo circuitry may be doing a bit of signal conditioning for all the other electronics that need to know the road speed.

 

[john2k]

Thank you. I'm waiting for this NE555 Adjustable Module to arrive, once it arrives i'll do some experimenting with it and report back.

 

[twister]

You might need a 5V zener diode because it looks like that circuit will put out 12V.

 

[bertus]

Hello,

@twister , Did you look at the diagram in the openings post?
I see that the signal amplitude is about 12 Volts.

Bertus

 

[majoco]

From the pulse generator specs....

Input voltage to the module (VCC) is variable from 5V to 15V DC and draws approximately 15mA at 5v (35mA at 12v) with an output amplitude of 4.2V peak-to-peak to 11.4V peak-to-peak based on the supply voltage.

So 12volts Vcc in will give 11.4volts p-to-p signal - I see no problem.

 

[twister]

Hello,

@twister , Did you look at the diagram in the openings post?
I see that the signal amplitude is about 12 Volts.
I see the waveform of 5V in the first post.
Bertus

 

[twister]

I see a 5 volt waveform in the first post.

 

[bertus]

Hello,

@twister , Next to the diagram it says 5 V/div.
I see about 2.5 divisions, so 12.5 Vtt.

Bertus

 

[twister]

Hello,

@twister , Next to the diagram it says 5 V/div.
I see about 2.5 divisions, so 12.5 Vtt.

Bertus

Ok, I missed that completely!

 

[Thymopathy]

May I know the variation of pulse sequence width A?

 

[majoco]

Interesting exercise - I did it a couple of times as it didn't look right. I just measured my "17 inch" wheel and its just over 26 inches diameter which makes it 163 inches circumference, 13.6 feet. and so 5280/13.6 = 388 revolutions per mile, 38,800 revs per 100 miles, 38,800/3600 = 10.7 revs per second and as there's 4 cycles per rev. then 42Hz at 100 mph! Probably the circuit will trigger on the leading edge as shown on the 'scope drawing, so I would doubt whether you need to preserve the 1:1 mark/space ratio.

Which leads me to another interesting question - when you're going down the road at 100mph, which part of your car is going fastest?