I am trying to prototype a sensor + location reporting chip that will do the following:

  1. Sense if a object is present over the top of it (a vehicle to be specific)
  2. Reporting the location of the chip (Not of vehicle)

The second option may be hardcoded into the chip and should be reprogrammable. These chips would needed to be installed in large numbers and there will be several such chips installed in close proximity. Each will report its own location to a centralized server (via wifi?)

That centralized server will then send the locations and busy/empty slots to a remote server which will then update some database.

I know the above description is very broad in terms of technical aspects but I need a starting point. Another thing to consider here is that I need to keep the cost of such chips lower considering that there will be 100's of such chips needed.

  • 2
    \$\begingroup\$ What sort of sensor technologies have you looked at? How close would you be placing the sensors? Is there likely to be sensor-field overlap? \$\endgroup\$ – Andy aka Jun 2 '14 at 10:55
  • \$\begingroup\$ imagine a sensor placed in center of a parking slot and then the chip needs to detect the presence of a stationary vehicle! \$\endgroup\$ – beNerd Jun 2 '14 at 10:57
  • \$\begingroup\$ ultrasonic sensors may help? see the video here: bannerengineering.com/en-US/products/8/Sensors/892/… \$\endgroup\$ – beNerd Jun 2 '14 at 11:19

Detecting vehicles above a roadway is usually done by what is essentially a large metal detector. A coil of wire is embedded in the pavement. Together with other circuitry, a oscillator is created such that the oscillation frequency is sensitive to the coil inductance. Since frequency can be measured very accurately (50 ppm is cheap), even small changes of inductance can be detected. Detecting a steel car chassis 1 foot above the loop is routine. This is how most sensors for automatic traffic lights work, for example.

As for how to get the data back to a central location, you haven't given us enough details. You mentioned parking spots, which implies a fixed installation that needs to run for a long time, so running wires for power seems appropriate. While you're providing the wired power, you can run some communication lines at the same time. There are various options, but CAN sounds quite appropriate as a first reaction. Your bandwidth requirements are quite low. CAN running at 125 kbits/s can easily cover something the size of a parking lot. The limit with CAN will be the number of nodes the bus can handle electrically, which is usually 80-95 nodes per bus, depending on exactly what transceivers are used. Even with a separate CAN bus for each 80 parking spots, the extra master nodes will be a tiny fraction of the overall cost.

  • \$\begingroup\$ is there a way we can use ultrasonic detectors for this purpose? This may keep the cost low \$\endgroup\$ – beNerd Jun 2 '14 at 14:23
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    \$\begingroup\$ @bene: Perhaps, but I'd worry about how to keep them from getting covered up by dirt, oil, snow, etc. Magnetic sensors are largely impervious to that. Also, loops of wire are pretty cheap. The eletronics should be a small portion of the overall installation cost, regardless of what technology is used. It would also seem the cost to have to maintain or fix the system would again dwarf the parts cost. Then there is the cost of the system being unreliable. This is not a place to be penny-wise and pound-foolish. \$\endgroup\$ – Olin Lathrop Jun 2 '14 at 16:06
  • \$\begingroup\$ I completely understand your point now. Makes sense. I was looking at this: instructables.com/id/… \$\endgroup\$ – beNerd Jun 3 '14 at 7:37

It appears as though you would like a device that is cheaper and has a smaller footprint than what Olin suggested. I am answering based on your provided specs. I would not expect any RF sensors to be as weather-resistant or accurate as what is currently done all over the world to detect stationary vehicles.

Since you are discussing building a 'prototype', I would encourage you to experiment with several sensors from different manufacturers -- the solution you desire may require more than a single sensor per device depending on your desired precision and accuracy. Possibilities may include ultrasonic, infrared proximity, or pressure/vibration sensors that can all be purchased on Sparkfun.

With regard to to the wireless transmission of sensor information, there are several things to keep in mind. First, range is an issue if you're talking about large parking lots which may prohibit 802.11 (wifi). You may wish to consider Zigbee or 802.15.4 radios (as are used in electrical power smart meters). Additionally, you may have problems trying to transmit with a large vehicle on top of the radio -- I would suggest either isolating the transmitter and/or antenna away from the vehicle (say, on a light pole).

I agree with Olin that running power may be necessary. Depending on your access, you may be able to pull this from electrical units local to the parking lot (which may also be associated with light pole). However, if you are looking into low power units (as it seems you are), batteries with mounted solar chargers may suffice.

As you said, your question is very broad and opens up a lot of possibilities and potential limitations. Please post or message if you have more specific questions.


If you want to detect only the presence of any vehicle there, i would suggest Giant Magnetoresistive Sensor or short range Radar sensor (used for crane collision avoidance) instead of IR or ultrasonic transceivers.

As far as reporting of the location is concern, use the digital addressing to interface all the sensors with a main station. As Lathrop sir suggested 802.15.4 would also be very efficient to make it ease mobile setup.


  • \$\begingroup\$ do you need something giant here?? Further how mangnetoresistive sensor would work in this case. How you plan to apply a external magnetinc field? \$\endgroup\$ – beNerd Jun 5 '14 at 5:13
  • \$\begingroup\$ Basically this is a sensor. There are considerably two types of magnetoresistive sensors available, AMR and GMR. Definitely magnetoresistive sensor would work for your case. The application of any external magnetic field will only be needed if we required linearity in GMR. If linearity will require then go for AMR. The AMR sensors consist of magneto-resistive elements oriented on a wheatstone bridge, the resistance of magneto-resistive elements change when exposed to a magnetic field. These elements are made of permalloy thin films that achieve a resistance of approximately 1000 ohms. \$\endgroup\$ – KushaL Jun 5 '14 at 19:28

For the problem of monitoring a lot of car parking slots, I'd guess you don't want to have to dig up every one to install an induction loop or other sensor.

I'd look seriously at using a few cameras and running OpenCV or similar to recognise when there's something in a space or not.

IP CCTV cameras are cheap now, and the infrastructure is easy & cheap. One or two HD cameras mounted on a single pole could monitor hundreds of spaces, and that makes the wiring even easier.

For object recognition they don't need to be great cameras and you can in fact get better results by using them in night-vision mode (IR sensitive, mono picture) as the contrast is much better. ANPR cameras typically run in this mode with fairly bright IR illuminators as number plates are highly reflective and you don't care about colour data - it's possible the floor of your car park is quite bright (concrete) or dark (tarmac) in the IR spectrum, which makes the recognition easier.

As you are detecting stationary objects your image capture & processing requirements are pretty low, you can get away with very slow updates from the cameras and a central PC or whatever doing the processing & spitting out updates. If it updated every few seconds it'd be plenty.

To make the software task even easier you could paint marker dots on the floor, if they are done in reflective road-marking paint and you use an IR-sensitive camera & illuminator it pretty much becomes a job of counting the bright dots against a dark background.

  • \$\begingroup\$ For some reason, I am skeptical about this approach! In the first place, it involves image processing which may add certain delay if we talk about some real time. Second is that proper angles may be required during camera installations. Correct me if i am wrong. Regarding painting a spot, will it not be covered by dust, grease etc? Will the above solution be accurate? \$\endgroup\$ – beNerd Jun 5 '14 at 5:34
  • \$\begingroup\$ ANPR cameras manage very good accuracy on fast-moving vehicles, this is many times easier as the spaces don't move. How often do road markings become completely obscured? Unlikely to be a major problem. How quickly do you need to update the count of parking spaces? More than once a second? Most cameras run at 25 or 50fps and object recognition software can keep up with this to track people or vehicles moving around. All you need here is to track covered/uncovered dots or object present/not present. \$\endgroup\$ – John U Jun 5 '14 at 10:58
  • \$\begingroup\$ ...correct angles will of course be needed (the camera(s) need to be able to see all the spots) but this is not difficult - a camera mounted high on a pole or building can see a long way. Compare the effort of installing a small number of IP cameras plus a PC to run image processing with the effort of installing a custom-sensor in every single space plus power/communications with all of them, it's quite a practical proposition. \$\endgroup\$ – John U Jun 5 '14 at 11:01
  • \$\begingroup\$ ...standard CCTV cameras are also already rated for working outdoors (a harder job than you may expect). PTZ ones could move to zoom in on each space or group of spaces in turn before the PC grabs a snapshot for processing. \$\endgroup\$ – John U Jun 5 '14 at 11:03

Can I suggest that this design would be a carpark space coordinator? My local shopping centre is Westfield Hornsby, NSW, Sydney, Australia. It has beam sensors on overhead horizontal poles that beam at an angle towards the rear of the space like a shop door beeper. Any car parked would block the signal giving an 'occupied' report. Additionally, the pole has a dual colour LED at the end for shoppers who want to find spaces (red for occupied, green for available). There is an LCD display outside each building entrance that displays the current occupancy [I think it says something like "145 / 300"] I would have to guess that there would be a MUX/DEMUX to reduce cabling as there doesn't seem to be much - definitely not 300 wires. I am pretty sure this particular design would be hard-wired, but there is no reason why it cannot be re-designed for outside / WI-FI. HTH

  • \$\begingroup\$ can you enunciate a bit more on the beam sensors? What type of beam sensors are they? Where these sensors report their location? Some server? \$\endgroup\$ – beNerd Jun 4 '14 at 4:55
  • \$\begingroup\$ @anon May I ask, why have you created 3 separate accounts? \$\endgroup\$ – Nick Alexeev Jun 4 '14 at 5:17

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