Darth Bagel

So to set up some background, I just moved into a new house a week ago (yay!), and it has a 1-car garage. Naturally, my wife instantly laid claim to it, and while her car *does* fit in the garage there's only a very few (I'd guess 3-4 total) inches to spare. Naturally, rather than a low-tech tennis-ball-and-string approach I thought I'd take the opportunity to create some needless technological terror to guide the parking process.

My initial thoughts centered on range-finders, primarily ultrasonic ones. However, I'm not sure they've got the accuracy I'd need for this project, especially at the desired ranges. An IR reflection object-detector sort of thing also occurred to me, but I don't want to take the chance of ambient light causing trouble with the calibration, and I also want to be able to accomodate a vehicle in various states of cleanliness.

But I don't really need range. The information I need boils down to a boolean "is the car in the right place yet?" Through some form of lunacy I thought of using a laser mounted either on the ceiling or on a (fairly sturdy) mount well above the car, directed at a phototransistor placed on or near the ground. This keeps the costs minimal ($5 at most?) and the design nice and simple. For now at least.

If you don't care about background, start reading here.
So now the plan, electrically, is to shine a laser onto a phototransistor. When the transistor can see the beam, either no lights or a green light will be on. When the transistor can't see the beam, a red light turns on. By using LEDs for the lights hopefully I can keep power usage low enough that I can just leave it on all the time, otherwise I'll have to put some thought into an easy way to turn it on and off.

So first the laser. I was thinking of even trying to save the cost of a red laser pointer by salvaging the laser diode out of an old CD drive. I like this for two reasons. First, it means I don't have to buy anything. Second, based on what I've seen the standard CD-ROM laser emits a wavelength around 780nm, while all the red laser pointers I see are 700nm or less. 780 gets me much closer to the peak responsivity of an IR phototransistor, and may even allow me to use a transistor with the black-ish ambient light reject lens material. However, I'm worried about its ability to produce reasonably focused light at such a distance since a CD drive obviously doesn't need very long range optics. More importantly, I read on one website that 780nm may be quite bad for the eyes, even just the reflected light. Can anybody possibly comment on either of these concerns?

The transistor is a fairly straightforward issue, and I really only want to ask if anybody has a particular device they'd like to suggest. I was looking at some Sharps on Digikey but they all said obsolete or limited quantities, so I'm not sure if they've been supplanted by a new device I've yet to find.

If by some twist of fate you're still reading after all that, bravo and thanks for your help/comments/mockery.

edit: forgot to mention, I'm located in Rochester, NY, USA.

audioguru

When the laser shines on the snow on your car then you or your wife will be blinded.

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ericgibbs

Hi,
Have you considered an airpipe, same method as used on some filling station forecourts,, the 'ding, ding' type.

As its a 4 inch gap clearance, you could use an old 6 inch thick bed mattress, fastened the garage end wall !

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ClydeCrashKop

You don’t want to leave a laser on constantly due to limited life span. An IR LED might work nicely in that dark environment.
I would suggest using in to trigger one of those sound recording chips with screeching brakes and crash sound.

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Torben

Hi Darth!

I would really recommend reading through Sam's Laser FAQ. The page on diode lasers is at http://repairfaq.ece.drexel.edu/sam/laserdio.htm

I don't know that I would feel comfortable with the CD-ROM laser; from the FAQ it would seem that most of those are near IR and pretty much invisible. You run the risk that if the beam is bounced into an eye, you will not get a blink/avert response and won't even know that the lens of your eye is focussing the beam into a burning spot on your retina. Unless your car is matte black then reflections will probably be an issue.

Personally I'd go with the ultrasonic range finder for this application. My very simple one measures from 2.54 m down to about 9 cm and has a 1 cm resolution. I chose the lower limit just for simplicity with my code and circuit.

I'm sure this has occurred to you, but just in case: make sure there's some indication to the driver that the circuit is operating properly before they get critically close to the wall. If the thing is blindly trusted and power happens to be out/the battery is dead/something else goes wrong, then whammo: time to break out the spackle. Like, display the distance to the wall, instead of flashing a light when the car is in position. If it works fine for a long time and you get in the habit of waiting for that light before hitting the brakes, the first time it doesn't work it would be easy to hit the wall. That's the nice thing about the string and tennis ball approach over those laser dealies you find at Sears: the battery won't die.


Just some random ideas,

Torben

Darth Bagel

Can you point me towards a decent rangefinder, or would I have to build one for myself?

Torben

Building one is certainly doable if you're into it, but if you're a hobbyist like me it can take a while to work out the kinks. You could also go for something like the Maxbotix LV-EZ1: http://www.sparkfun.com/commerce/pro...roducts_id=639

I haven't used them myself but they look good on paper and Servo magazine sure seems to like them. If you don't want to use a microcontroller to get the output they also output an analog voltage which is proportional to the measured distance, so you could sense that instead.

Or you could check out some of the other listed proximity sensors at Sparkfun; they may have something more applicable to what you want. They have an IR proximity detector which goes from 10 cm to 80 cm, again with analog voltage output, for about half the price of the LV-EZ1.


Torben

blueroomelectronics

Why fix something that's not broken. The tennis ball works like a charm.

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