user3533: my garage door is pretty narrow, about 10 cm (4") on each side of my car's rear mirrors. So every time I drive to garage, I need to go very slowly because I am afraid I might not be lined up correctly. 
I am thinking about using Arduino and some sensors to help me. Output is pretty simple, just two leds, one for each side, showing me if I should turn little right or left to have perfect lineup to center. This I can handle with no problem
What I am asking for is idea how to measure the alignment of my car. I know how to use ultrasonic sensors for measuring distance. But I am not sure they are suitable for this project, because I want preferably to put arduino and sensors in my garage, not in my car. 
Any idea how to work this out ?

user313: It seems like an ultrasonic sensor on each door-post should do it. You need, I'm guessing, 30cm to the sides of your car as its nose enters the doorway. Smooth the sensor reading, compute a difference and display the sign of the differences as right/left lights. Keep a small dead-band around zero-difference or you'll drive the driver crazy!
Do Ultrasonic sensors have the range and accuracy you need for the job? Time for a feasibility study.
Update:
Is 10cm plus the length of a mirror = a couple of feet? But even if so, you have to keep to +/- 10cm of the centerline to prevent striking the mirrors, so you could project your sensors out into the doorway by an amount the still keeps them > 10cm away from the car. You could mount them hinged (like the mirrors are) and padded to limit any damage from the occasional accidental touch.

user10757: It is very hard to determine position accurately there is a new project on kickstarter called Pozyx, you can look at that but that's expensive, to be honest this sounds like a job for hanging tennis balls or strings or something like that. But if you have a lot of money to blow and you just want a project go for it, you may be able to do this with ultrasonic sensors but I reckon that would be painful. Anyway good luck

3533: <p>my garage door is pretty narrow, about 10 cm (4") on each side of my car's rear mirrors. So every time I drive to garage, I need to go very slowly because I am afraid I might not be lined up correctly. </p>

<p>I am thinking about using Arduino and some sensors to help me. Output is pretty simple, just two leds, one for each side, showing me if I should turn little right or left to have perfect lineup to center. This I can handle with no problem</p>

<p>What I am asking for is idea how to measure the alignment of my car. I know how to use ultrasonic sensors for measuring distance. But I am not sure they are suitable for this project, because I want preferably to put arduino and sensors in my garage, not in my car. </p>

<p>Any idea how to work this out ?</p>
 313: <p>It seems like an ultrasonic sensor on each door-post should do it. You need, I'm guessing, 30cm to the sides of your car as its nose enters the doorway. Smooth the sensor reading, compute a difference and display the sign of the differences as right/left lights. Keep a small dead-band around zero-difference or you'll drive the driver crazy!</p>

<p>Do Ultrasonic sensors have the range and accuracy you need for the job? Time for a feasibility study.</p>

<p>Update:
Is 10cm plus the length of a mirror = a couple of feet? But even if so, you have to keep to +/- 10cm of the centerline to prevent striking the mirrors, so you could project your sensors out into the doorway by an amount the still keeps them > 10cm away from the car. You could mount them hinged (like the mirrors are) and padded to limit any damage from the occasional accidental touch.</p>
 10757: <p>It is very hard to determine position accurately there is a new project on kickstarter called Pozyx, you can look at that but that's expensive, to be honest this sounds like a job for hanging tennis balls or strings or something like that. But if you have a lot of money to blow and you just want a project go for it, you may be able to do this with ultrasonic sensors but I reckon that would be painful. Anyway good luck</p>
