How can I make a garage door laser sensor? 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.
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&amp;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&amp;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