Looking for the right Proximity Sensor for the right Price(RIGHT!!)

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I am toying with a project for sensing when the mail man/lady pulls his car up to my mailbox. This is basically on street side with no sidewalk. The aproximate distance from the mailbox post will be in the neighborhood of 4ft - 5ft(120cm - 150cm). A simple motion sensor will not work as I would get false signals when cars pass by at a slightly greater distance(6ft - 7ft or more). The sensor I found, whose data sheet is linked below has a range of up to 300cm is really far too expensive($52) but probable get the job done.

THE JOB
The detector senses that the mail car has arrived and triggers a geared motor
to extend the mailbox out. another sensor sensing that the mail box door has been opened, then closed(LDR or something like that) determines that the mail has been delivered and retracts the mailbox.

Living in the North-East there shoveling out snow in front of a mailbox can be a pain in the ars, I thought this could help me out since it is too painful given my medical issues.

one issue I see up front with this is that if the mail person in the mail car sees the mailbox so far away he/she might try to get closer, thus when the box extends a certain distance the box may hit the car. I would need more sensor capabilities to move the box to within a certain distance of the vehicle. It should also work at night.



Does anyone have any suggestion on the right detector for the job and at a lower price??


http://document.sharpsma.com/files/gp2y3a003k_e.pdf
 

SgtWookie

Joined Jul 17, 2007
22,221
Interesting project idea.

First thing that came to mind is that vehicles passing at "normal" speeds will be present for only short periods of time. Rather than worry about how far the sensor can pick up signals, seems to me that you'd want to simply start a timer, to see if the sensed presense of the vehicle is stable for a second or thereabouts.

But then, what if an 18-wheeler truck happens by, going 10MPH? If the truck is 60 feet long, it'll take roughly 4.1 seconds to pass by your sensor. If you waited just 1 second, your mailbox would extend, and possibly get torn off the post.

Seems like you also need to detect if a vehicle approaching is slowing to a stop; that'll require a little bit of processing, and another sensor.

Once the vehicle has stopped by the box, the sensor will need to determine just how far to extend the box, leaving perhaps 8"-12" of space between the vehicle and the mailbox door. You wouldn't want the box getting any closer than that, or risk damage to the box or the mailman's vehicle; causing damage to Government vehicles is a rather serious matter.

Your sensor would have to be low enough to sense the side of the vehicle, yet high enough to clear a possible snowbank.

There are kits about for ultrasonic transducers; for robotics-related projects. I'm afraid I don't have the time to research that at the moment, but there are quite an array of them out there.
 

SgtWookie

Joined Jul 17, 2007
22,221
What about something like this: http://blast02.lns.mit.edu/targets/ion/manuals/Allegro_3503.pdf

The postal van is made of steel, yes?
One can't depend upon the postperson's vehicle being made of steel. The "Jeep" type vans did have all-steel construction, but I think they're making much of the bodies from aluminum nowadays.

Where I live, postpersons are independent contractors and drive a myriad of vehicles. One of them drives a Saturn, which has polycarbonate body panels. Another drives a Buick, which has polycarbonate fenders and aluminum door panels.

A Hall-effect sensor is an extremely useful device, but I'm afraid that it wouldn't work well in this application
 

beenthere

Joined Apr 20, 2004
15,819
Another thing to consider: The Post Office simply won't deliver to a non-approved box. Check to see if a mailbox like the one you are thinking about is going to be acceptable. Be a big waste to get it working right and then find it couldn't be used for mail.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
The postal van is made of steel, yes?
The "Postal Truck" in my town is the drivers own vehicle, which for the most part is a Ford Escape.

Interesting project idea.

First thing that came to mind is that vehicles passing at "normal" speeds will be present for only short periods of time. Rather than worry about how far the sensor can pick up signals, seems to me that you'd want to simply start a timer, to see if the sensed presense of the vehicle is stable for a second or thereabouts.
I think maybe both.


But then, what if an 18-wheeler truck happens by, going 10MPH? If the truck is 60 feet long, it'll take roughly 4.1 seconds to pass by your sensor. If you waited just 1 second, your mailbox would extend, and possibly get torn off the post.
Not too many 18-wheeler's go down my street and I don't plane to stick the mailbox in the middle of the road, maybe 12 - 15in further out than it is now but also further back that it is in the retracted position.


Seems like you also need to detect if a vehicle approaching is slowing to a stop; that'll require a little bit of processing, and another sensor.
If we can get a fairly accurate distance I wouldn't have to worry too much about vehicles that are further away and thus assume that if in range for 1 sec. go ahead and extend, and if mailbox door has not opened and or closed within 1 min, retract


Once the vehicle has stopped by the box, the sensor will need to determine just how far to extend the box, leaving perhaps 8"-12" of space between the vehicle and the mailbox door. You wouldn't want the box getting any closer than that, or risk damage to the box or the mailman's vehicle; causing damage to Government vehicles is a rather serious matter.

Your sensor would have to be low enough to sense the side of the vehicle, yet high enough to clear a possible snowbank.
That is of concern. I will have to measure how high the snow comes up the post and how high a car door is(aprox 2-2.5ft) [/QUOTE]


Another thing to consider: The Post Office simply won't deliver to a non-approved box. Check to see if a mailbox like the one you are thinking about is going to be acceptable. Be a big waste to get it working right and then find it couldn't be used for mail.
I've seen some pretty wacky set-ups around here, some people have swinging arms that extend longer in winter, the box will be unchanged, just how the box gets to be within reach. But I will talk to them about it.

And, while I will need to get electricity to the post I will add a "Mails Here" indicator to the works. I have a wireless device setup now that I converted from a wireless driveway motion sensor to a chime once and Latched LED flasher, but the only problem I am having is that the 9V battery in the motion sensor goes dead after about a week....REALLY DEAD! Funny thing is that's the half of the remote circuit I didn't make changes to.
 

Distort10n

Joined Dec 25, 2006
429
I think this would require some sort of RFID with cooperation from the postal service. There must be some intelligence built into the sensor. Detecting if something is there or not is not good enough. What would you do if someone just happens to park in front of your mail box? :eek:
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I think this would require some sort of RFID with cooperation from the postal service. There must be some intelligence built into the sensor. Detecting if something is there or not is not good enough. What would you do if someone just happens to park in front of your mail box? :eek:
First off, that would be illegal, but not impossible. Secondly, if they park that close to begin with then they deserve a good bump, that is if the protracting box does not function properly and stay 12in - 15in from the trigering source.

__
Forgive my ignorance here, but would RFID pinpoint a specific distance and would it transmit through a few inches of snow?
The IR sensors I have been looking at will more than likely get plugged with snow from the plow, not burried, but covered with flying snow even at 3ft. http://www.acroname.com/robotics/parts/sharp_guide.pdf

__
Also if I were to walk up to the mailbox I would set it off if there was not a minimum distance for me and a max distance for cars, such as a trigger range of 1.5ft - 2.5ft.
 

SgtWookie

Joined Jul 17, 2007
22,221
I suggest that infrared sensors would be too difficult to implement a reliable range detection scheme at such short distances. Light travels at roughly 186,000 miles per second in a vacuum, so your resolution requirements would be ridiculously small.

Ultrasonic detection would be a more likely approach, as sound travels at around 700 feet/sec at sea level, depending upon a variety of things such as temp and humidity. (Surprisingly, humid air is less dense than dry air.) However, for such short range, audio distance sensing is quite do-able. Short pulses will help a great deal to prolong battery life. The detection range can also be controlled by pulse duration; the shorter the pulse, the shorter the range. Focusing the audio energy using parabolic reflectors will at once extend the range, and reduce the power necessary to produce a detectable return.

I'm working on a project; just coming back when I take a quick break or have an idea.
 

hgmjr

Joined Jan 28, 2005
9,029
Perhaps a good thing to implement as part of the design is a contact switch that will act as a failsafe detection scheme in case the mailbox extends too far out and contacts anything in its path. Also a rubber bumper that extends out would help avoid any damage to a person or vehicle in the event of a runaway malbox.

hgmjr
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I purchased this ultrasonic movement detector kit a while back:
http://www.electronickits.com/kit/complete/surv/ck203.htm

While it simply detects the presence or absense of movement, it could be made to work in this application with modifications. I'll have to dig out the schematic later; this isn't something I can produce right away in the middle of this other project.

Understood... thus far your helpful replies have been great.
 

Distort10n

Joined Dec 25, 2006
429
I suggested an RFID option, perhaps something along the lines of TollTags. Those have a distance of 20' or more, and must be fairly accurate as automobiles are speeding up to 60 mph past the sensors.

Simply detecting distance or movement is not good enough in my opinion. You have to worry about other vehicles, not to mention people simply walking by.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I suggested an RFID option, perhaps something along the lines of TollTags. Those have a distance of 20' or more, and must be fairly accurate as automobiles are speeding up to 60 mph past the sensors.

Simply detecting distance or movement is not good enough in my opinion. You have to worry about other vehicles, not to mention people simply walking by.
Somehow I don't think that the postal drivers will want to carry or stick some strange device in their car just for me.
 

thingmaker3

Joined May 16, 2005
5,084
One of them drives a Saturn, which has polycarbonate body panels. Another drives a Buick, which has polycarbonate fenders and aluminum door panels.
Excellent observation. Any magnetic sensor would have to be low to the ground, picking up the chassis.

I've seen some pretty wacky set-ups around here, some people have swinging arms that extend longer in winter, the box will be unchanged, just how the box gets to be within reach. But I will talk to them about it.
Hmmm... perhaps what you actually need is a snow sensor...

Simply detecting distance or movement is not good enough in my opinion. You have to worry about other vehicles, not to mention people simply walking by.
That's why I was thinking about magnetics. People and stray dogs don't have all that much iron in their blood, but vehicles do have that much iron in their make-up.

Still, the "snow sensor" might be the best option. IR LED and photoresistive sensor, perhaps?
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
Hmmm... perhaps what you actually need is a snow sensor...

That's why I was thinking about magnetics. People and stray dogs don't have all that much iron in their blood, but vehicles do have that much iron in their make-up.

Still, the "snow sensor" might be the best option. IR LED and photoresistive sensor, perhaps?
I'm confused with respect to the "Snow Sensor" You might have to explain that one some more.

The magnet idea would probably pick up every car.

Maybe we ought to have a contest to see whose sensor design will be the most accurate! hook them up the the post with some counter circuitry and see which one would count correctly. A bit too expensive for me though.
 

SgtWookie

Joined Jul 17, 2007
22,221
Gee, I forgot about this topic - sorry 'bout that.

But I stumbled across something that seems to fit your requirements :)

Note: external link to Parallax, a manufacturer of "Basic Stamp" uP's and other things
http://www.parallax.com/Default.aspx?tabid=86
Once that page comes up, click on the "Object Detection" link, and then the PING))) Ultrasonic Sensor link. Basically, $30 for an ultrasonic detector that has a range of about 3/4" to 10 feet.

You could use a Basic Stamp to control/sense it, but they're kind of pricey. You could use another brand of microcontroller, like a PIC, to trigger it and read the responses.

You'll still need to cover it with some kind of thin membrane to keep critters and moisture out. Thin Mylar sheet stock might be a good bet.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
Gee, I forgot about this topic - sorry 'bout that.

But I stumbled across something that seems to fit your requirements :)

Ultrasonic Sensor link. Basically, $30 for an ultrasonic detector that has a range of about 3/4" to 10 feet.

You could use a Basic Stamp to control/sense it, but they're kind of pricey. You could use another brand of microcontroller, like a PIC, to trigger it and read the responses.
Thanks Sarg! I will be looking into these soon.
 

Bob O

Joined Jul 3, 2007
9
I have not read all the post so I may be out in left field here.
Why not sense when the mailbox door has been opened and then closed?
This would remove all types of vehicles and weather from the problems.

HTH
 
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