Analog and digital speedometer

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Analog and digital speedometer

Postby x_byte » Tue Sep 13, 2011 2:06 pm


i'm looking to develop an app that shows the rpm and speed of a car. I am retrieving all the data through a can network over bluetooth, but i want show them in a fancy speedometer, that shows the rpm of the car with the needle and the velocity in km/h on a "digital screen" at the bottom of the speedometer.

I searched on the web and i found that i could eventually do that with adobe air.

Has anyone another shot to do that without flash ?

Best regards
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Re: Analog and digital speedometer

Postby MichaelEGR » Tue Sep 13, 2011 4:52 pm

It sounds like you may not do much coding, so yeah there is a bit involved of course, but here is some essential details on a way to implement what you are trying to do. Unfortunately I don't have much time to help more.

The easiest way to accomplish this is to create a native app with a custom graphic component for the fancy speedometer with the Android 2D graphics API. At its simplest you could have an image provide the background for the speedometer then simply draw a line for the needle. You could also conceivably use an image for the needle and rotate the image accordingly.

If the speedometer is linear from 0 to max speed things are fairly easy to determine the rotation of the needle. Let's say it will rotate on the top half of a circle for speeds 0 to 200 km/h. 0 is PI (radians angle) and 200 km / h is 0 (radians). You can determine the point along the unit circle in screen coordinates (0, 0; upper left corner - 2D API is oriented this way) by applying x = cos(<angle in radians>); y = -sin(<angle in radians>). You can treat this (x, y) as a unit vector / normalized direction. You can multiply it by a scalar value to increase the length.

For a linear mapping of speed to angle in radians for rotation around the top half of a circle you'd calculate PI - PI * (speed / max speed).

For a review of general trig and a picture of the unit circle and how radians relate check this out:

Good luck.. :)
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Re: Analog and digital speedometer

Postby roopa » Fri Sep 23, 2011 8:06 am

he analog speedo in my 1971 Datsun 1200 was notoriously unreliable, and the hotter the ambient temperature, the higher it would read!

After scouring the web for digital speedo schematics, I found one, and set about constructing it. After sorting out some teething issues with the electronics, and redesigning parts of the circuit, I built a prototype, and installed it in my Datsun. It functioned perfectly in my Datsun for about 5 years, and was removed when I sold the Datsun.
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Re: Analog and digital speedometer

Postby millerni456 » Thu Oct 13, 2011 3:14 am

I have a working analog like joystick in my application and here's how I did it.

I have two Opengl Squares that render two textures. The base of the analog and the thumb or actual stick of the analog. I enable GL_BLEND and pass the GL_SRC_ALPHA_MINUS_ONE for the blend function like this:


this enables transparency in textures.

After that I used a little trigonometry to position the thumb:

First I need the mouse clicks position and the middle of the analog base.

P1 = mouseLoc; P2 = analogBaseCenterPoint.

Then I need to find the distance from these using the distance formula:

P1.y - P2.y divided by P1.x - P2.x = distance.
This distance will determine the value(speed) of the analog.

I then need to find the change in X and Y separately to create a vector.
The same points will be used and I'll create a vector which the two points

Vector( changeInX, changeInY);
Then I normalize it, so its coordinates will divide to be within 0 to 1.
Normalizing can be done by dividing each component by the vectors magnitude ( or length).

So now i have normalized coordinates.

Using trig I can now plug these into arcsin or arccos functions to get the angle (or direction or velocity of the analog)

I also use this and the distance to determine where to draw/translate the analog thumb.

So... I use this method for finding the angle:
where X = the normalized X coord
and Y = the normalized Y coord.
The angle will be the angle of direction. (between 0 and 360 degrees).

float angle =0;
angle = (float) Math.acos(x);
else if(x<0&&y>0)
angle = (float) Math.acos(x);
else if(x<0&&y<0)
angle = (float) (Math.abs(Math.asin(y)) + Math.toRadians(180));
else if(x>=0&&y<0)
angle = (float) (Math.asin(y) + Math.toRadians(360));

So... now that I'ved got the angle I need to create a 3-component velocity vector.
I do this again using trig and using the sin and cos functions

float velX = (float) Math.cos(angle) * distance; //multiply by distance to tell how much. cos functions return a value up to 1.
float velZ = (float) Math.sin(angle) * distance; //same as cos function but for Z value. (character moves along xz plane).

Create my final movement vector:

Vector( velX , 0, velZ) //y = 0 because character doesn't move up(jump) from the analog.

Hope this helps and is relevant to what your are trying to achieve. Good Luck and Happy Programming!
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Re: Analog and digital speedometer

Postby loosi007 » Sun Jan 01, 2012 3:49 pm

Nice post thanks for sharing........
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Re: Analog and digital speedometer

Postby samstephan9 » Fri Jan 06, 2012 12:20 pm

I am also interested to know that please help us I will make an speedometer too.
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