user8054: I would like to make a sensor that detects when my car is clear of a certain point. So, when backing in my car to my garage, I would have a beam and sensor. When the beam is broken, an LED light would turn on and then when the beam gets through, the light turns off and I know that the car will clear the garage door.
I am pretty sure I have to use either an Arduino or Raspberry Pi, but I'm completely new to home automation. There are a few other things I'd like to do but this is a good starter. 
I don't know where to start.

user7741: I am pretty sure that Arduino would be enough for this project since the proper computation should not be that complex. Arduino Uno seems to be the simplest to start, at the same time not being too big for your application domain. However, you could also consider some smaller Arduino model, there would not be any "computational power" problem probably. 
You could use an Infrared Proximity Sensor to check whether a car is present or not, here is a link that could be useful for this: http://arduinomega.blogspot.it/2011/05/infrared-long-range-sensor-gift-of.html
In this way, the system would light ON one or more LED(s) whenever the measured distance turns out to be less than some threshold value that you decide (maybe using a test-driven approach), and then light it/them OFF once the distance returns over the fixed threshold.
Hope this will be helpful.

user8076: I cant make out from the picture if you have consider this thought.
Install sensor at height so car tail breaks the beam.
Also It might not be ideal solution if you have multiple car lets say one Range rover SUV and another hatchback. 
You might want to consider having multiple sensor from ground to around 4-5 feet.

user8058: I would recommend using laser and LDR. This will be very efficient and will not cause any problem due to light or ultrasonic deflecting. It will be precise by mm if you do thing right way. I made a simple porject back in 2005 using laser, LDR and relay. 
Here what you need to do. 
Get a laser, any laser will work. www.ebay.com/itm/6mm-650nm-DC-5V-5mW-Mini-Laser-Dot-shaped-Diode-Module-WL-Red-Copper-Head-Tube-/291089398593?pt=LH_DefaultDomain_2&hash=item43c647bb41
Get a PhotoResgister www.ebay.com/itm/Reliable-Hot-New-20-PCS-Photoresistor-GL5528-LDR-Photo-Resistors-Light-Dependent-/161434780484?pt=LH_DefaultDomain_2&hash=item2596434344
Put ldr in the dark box and drill a hole in it so that no other light can enter 2mm diameter hole will work. 
Connect the LRD to your board. Fire laser and change status of indicator led by the value of ldr. 
Check this link for refrence www.instructables.com/id/Arduino-Laser-Tripwire/?ALLSTEPS

8054: <p><img src="https://i.stack.imgur.com/2kYvA.jpg" alt="enter image description here">I would like to make a sensor that detects when my car is clear of a certain point. So, when backing in my car to my garage, I would have a beam and sensor. When the beam is broken, an LED light would turn on and then when the beam gets through, the light turns off and I know that the car will clear the garage door.</p>

<p>I am pretty sure I have to use either an Arduino or Raspberry Pi, but I'm completely new to home automation. There are a few other things I'd like to do but this is a good starter. </p>

<p>I don't know where to start.</p>
 7741: <p>I am pretty sure that Arduino would be enough for this project since the proper computation should not be that complex. Arduino Uno seems to be the simplest to start, at the same time not being too big for your application domain. However, you could also consider some smaller Arduino model, there would not be any "computational power" problem probably. </p>

<p>You could use an <a href="https://www.sparkfun.com/products/8958" rel="nofollow">Infrared Proximity Sensor</a> to check whether a car is present or not, here is a link that could be useful for this: <a href="http://arduinomega.blogspot.it/2011/05/infrared-long-range-sensor-gift-of.html" rel="nofollow">http://arduinomega.blogspot.it/2011/05/infrared-long-range-sensor-gift-of.html</a></p>

<p>In this way, the system would light ON one or more LED(s) whenever the measured distance turns out to be less than some threshold value that you decide (maybe using a test-driven approach), and then light it/them OFF once the distance returns over the fixed threshold.</p>

<p>Hope this will be helpful.</p>
 8076: <p>I cant make out from the picture if you have consider this thought.
Install sensor at height so car tail breaks the beam.
Also It might not be ideal solution if you have multiple car lets say one Range rover SUV and another hatchback. 
You might want to consider having multiple sensor from ground to around 4-5 feet.</p>
 8058: <p>I would recommend using laser and <a href="http://en.wikipedia.org/wiki/Photoresistor">LDR</a>. This will be very efficient and will not cause any problem due to light or ultrasonic deflecting. It will be precise by mm if you do thing right way. I made a simple porject back in 2005 using laser, LDR and relay. </p>

<p>Here what you need to do. </p>

<p>Get a laser, any laser will work. www.ebay.com/itm/6mm-650nm-DC-5V-5mW-Mini-Laser-Dot-shaped-Diode-Module-WL-Red-Copper-Head-Tube-/291089398593?pt=LH_DefaultDomain_2&amp;hash=item43c647bb41</p>

<p>Get a PhotoResgister www.ebay.com/itm/Reliable-Hot-New-20-PCS-Photoresistor-GL5528-LDR-Photo-Resistors-Light-Dependent-/161434780484?pt=LH_DefaultDomain_2&amp;hash=item2596434344</p>

<p>Put ldr in the dark box and drill a hole in it so that no other light can enter 2mm diameter hole will work. </p>

<p>Connect the LRD to your board. Fire laser and change status of indicator led by the value of ldr. </p>

<p>Check this link for refrence www.instructables.com/id/Arduino-Laser-Tripwire/?ALLSTEPS</p>
