Ken put an end to expensive car damage

Not satisfied with the low-tech method of suspending a tennis ball from a string, Ken Swanstrom devised a better garage locator device from a laser pointer. This simple circuit is powered by a 6V DC wall wort, adjusting the output to yield 3V DC as a substitute for battery power. A CdS cell “looks” at the garage door opener light and actuates a relay, turning on the laser pointer when the garage door opens. And voila… no risk of front-end damage anymore.

This Gadget Master was originally sourced from our sister publication, Design News.

Garage Locator

There are many methods used to locate automobiles in a home garage space, the most common of which is suspending a tennis or golf ball on a string from the ceiling. The ball is usually located so that it just touches the windshield when the car is in place. This ensures the car is inside the garage door, but not too far so as to reduce clearance in front.

The problem with this method is that it is car specific, since the ball is suspended in a fixed position, and since the ball is usually hung in the center of the parking space, it is very difficult to adjust when the car is in place, or when another car is parked in the same space.

To improve on a locating method, I thought of using commonly available laser pointers which could be mounted to the garage ceiling in front of a car. These lasers are normally powered by two internal “AA” or “AAA” cells, which together yield 3vdc. I built a simple circuit powered by a 6vdc wall wort, the output adjusted to yield 3vdc, as a substitute for the battery power. I then needed to turn on the laser pointer when the garage door was opened. So as to not to tie into the garage door opener circuits, I used a CdS cell to “look” at the garage door opener light and actuate a relay, turning on the laser pointer when the garage door was opened.

Brackets were fabricated using brass tubing and flat stock, available at most hobby shops. One bracket attaches the laser pointer to the ceiling at a location 3 feet in front of the car, allowing easy adjustment when the car is in place. Tubing, of a diameter that is a slip fit over the pointer, was soldered to flat stock, which in turn, was attached to an “L” bracket, providing attachment to the ceiling. This tubing also served to depress to the “on” button of the pointer.

The other bracket, also comprised of brass tubing and flat stock, locates the CdS cell approximately ¼” from the door opener light housing. The CdS cell was placed inside the tubing to eliminate interference from ambient light.

The only modification to the pointer was to drill a small hole in the pointer end cap so an insulated wire can pass through. A wooden dowel of a diameter and length that duplicates the batteries’ diameter and total length was drilled through and fitted with a brass screw which also was drilled through, to which the wire was soldered, this assembly then became the center contact.

A small brass tab was fabricated that had a small, drilled hole to which a wire could be soldered, and a larger hole that just cleared the threads of the pointer cap. When the cap was tightened down, this tab formed the other contact of the pointer circuit.

The relay control board was housed in a standard 4″ x 2″ electrical box with appropriate grommeted holes for the various wires.

Note that with most laser pointers, the batteries are inserted with the positive end towards the end cap, and thus, the brass tab attached to the end cap is positive. Also, most laser pointers incorporate a circuit to prevent damage to the laser diode in the case of battery reversal, so when the 3vdc is applied, if the laser does not light, simply reverse the wires.

One adjustment is necessary – check that the voltage across the laser supply terminals is a maximum of 3vdc and adjust R2 accordingly.

Part list

Quantity Item Part description
1 R1 180 ? 1/4w
1 R2 1k ? 15-turn pot
1 Ry1 5vdc reed
1 Q1 NPN transistor 2N4401
1 D1 1N4001
1 CdS cell
1 6vdc wall wort
1 Terminal strip

Other components:

Laser pointer(s)
4″ x 2″ electrical box with cover (modified with various holes)

Fabricated parts:

Laser pointer ceiling brackets
CdS brackets
Dowel with brass screw
Brass contact tab for end of laser pointer




  1. “Wall wort”?
    One for the botanists?

  2. If you must use a laser pointer, then shining up from the floor would be better. Then it suits any car – you just drive in until the car blocks the beam, as seen on the ceiling.

  3. The headline seemed promising but the details of the implementation beggar belief. This circuit is not worth publishing. The CDS cell could potentially pass a large current in bright sunlight – it’ll almost ceratinly fail after a short time in service, possibly taking the tranistor with it. There is no schmitt trigger so the current in the relay coil will continuously vary as the ambient light level changes. Better circuits than this were published in Practial Electronics over 30 years ago!

  4. Tolak, you can stop being hysterical about a laser pointer blowing a hole in your head. <5mW is incapable of causing physical harm.
    And as for the rest of you…
    Is it not utterly obvious what you’re doing with the laser pointer ?
    For the hard of thinking I’ll state it right out where you can ignore it because I’m not pandering to your stupidity :
    Place the laser pointer so it makes a dot on the hood of the car where you can see it.
    How else can you make use of this cool toy ?
    And how many other cars are you wanting to park in your single garage anyway ?
    If you have a collection of them you can afford a valet to park them for you and the entire issue is academic.
    Come on guys do you really need a manual for something equally as trivial as putting on your trousers ?
    Actually forget that, if there were a manual you’d be suing the manufactureer for not telling you to take them off before ironing them or something.

  5. It is not clear what the laser points at! And how the system can be used for different cars without adjusting?

  6. Sledgehammer to crack a nut comes to mind.

  7. I would rather not have a laser pointer just sitting there, potentially in my line of sight. I value my eyesight ore than that.
    I have a tent peg on a string in my garage, positioned so that it touches the front or rear bumper (fender) of whatever car is put in the garage. Its very easy to see the string move when you touch the peg. Providing the car will actually go in the garage, it leaves a set space (set by the distance of the string from the wall) and the rest of the space is available around the other end of the car.
    I can’t imagine wasting time on such a potentially dangerous gadget, life is too short.

  8. Details how the lamp is powered and mounted, but not how the pointer beam helps stop the car before it touched the wall or intrudes too much into the garage.

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