Solution for RF unit/circuit that transmits signal in 200m range

[zerodegreec]

Hello all. An interesting "problem" came up and I thought it would be interesting to explore.

What I am looking for is a unit/circuit that will transmit a signal (at least 200m and not line of sight). Also a receiver that detects the signal and will sound an alarm.

In effect I am looking to make some sort of proximity sensor.

Any ideas?

 

[biff44]

Re: Beacon / RF question

If you are willing to get a license from your local government, and you can transmit a couple of watts on a protected frequency, it would be trivial.

If, on the other hand, you want to do this unlicensed and low power....200 meters is pretty far.

The first problem is that there will be many other frequencies present in an unlicensed band, so you have to differentiate your signal from the other false signals. One way would be to employ a long coded modulation sequence at the transmitter. In the receiver you would use a correlation receiver (like a matched filter to that specific sequence) to determine if the signal was present or not. The correlation receiver will add "coding gain" to the received signal, and bring it up out of the background noise/jammer signals without needing high transmit power.

You are trading off lower transmit power for a much longer packet size.

Scope this out:

Ideas for HF Modulation and Coding

Another much cruder way would be to use 13.56 MHz (most countries allow high RF powers at this one frequency to use in induction heaters, etc), so you can get high power. Unfortunately, since the frequency is low, the antenna size will be long (at least 1 meter).

 

[RF_Jim]

Re: Beacon / RF question

Hello all. An interesting "problem" came up and I thought it would be interesting to explore.

What I am looking for is a unit/circuit that will transmit a signal (at least 200m and not line of sight). Also a receiver that detects the signal and will sound an alarm.

In effect I am looking to make some sort of proximity sensor.

Any ideas?

Can we get some 'refinement' on what it is you wish to do - a) detect motion or something within 200 meters, or b) send that kind of info (alarm activation) over a 200 meter path?

There are possible (as well as impossible) solutions to both, and it would help to refine the requirement a little bit more.

Jim

 

[zerodegreec]

Re: Beacon / RF question

Can we get some 'refinement' on what it is you wish to do - a) detect motion or something within 200 meters, or b) send that kind of info (alarm activation) over a 200 meter path?

There are possible (as well as impossible) solutions to both, and it would help to refine the requirement a little bit more.

Jim

No problem, we would like to find a cost effective solution to a safety concern for Rally racing. We want to have a beacon transmit a signal if the car is stopped on the stage, crashed or some other danger. The occupants can turn the beacon on, and this will send out the signal. The next car on the stage (typically 1 to 2min behind) will get an audible and or visual alert that they are coming up to a possible blocked road. At the speed we race at, 100m is the borderline for distance to react to something around the bend or over the next crest.

There are a few systems on the market that do this, but they are all either VERY expensive and or have issues with reliability/poor support from the manufacture.

Think of it as a panic button that any receiver in the area would receive.

 

[RF_Jim]

Re: Beacon / RF question

No problem, we would like to find a cost effective solution to a safety concern for Rally racing. We want to have a beacon transmit a signal if the car is stopped on the stage, crashed or some other danger. The occupants can turn the beacon on, and this will send out the signal. The next car on the stage (typically 1 to 2min behind) will get an audible and or visual alert that they are coming up to a possible blocked road. At the speed we race at, 100m is the borderline for distance to react to something around the bend or over the next crest.

There are a few systems on the market that do this, but they are all either VERY expensive and or have issues with reliability/poor support from the manufacture.

Think of it as a panic button that any receiver in the area would receive.

Okay, understand, kinda like Sentinel Safety device, less like IriTrack device ... descriptions for others can be found here:

Dakar Tech; Iritrack, GPS, Sentinel, Emergency Beacon... - ADVrider


You have an interesting project/goal in mind.

In the USA one might recommend using MURS (2 Watt power level at five frequencies near 152 MHz) frequencies or FRS (.5 Watt power level, but restrictions on how the radios are built, like requiring a built-in antenna, but this is not such a big handicap to overcome) or CB (5 Watt AM at 27 MHz, but, too much interference with sun spot cycle on the upswing).

There are also bands like 900 MHz available if Spread Spectrum (1 to 2 Watts power) techniques are used. And even 2.4 GHz, but line of sight starts to become an issue on 2.4 G partly because of frequency (absorption in vegetation, etc)

Seems like something could be worked up, especially today with chips that do a good job of signal strength measurement (if your system uses signal strength to roughly 'compute' a distance for protection distance) or maybe 'beacon' your GPS coordinates and drivers coming up would receive this and then their unit would calculate how close your were. This last approach would require a GPS receiver, but, could be engineered to give repeatable, and predictable results.


Jim

 

[zerodegreec]

Re: Beacon / RF question

Okay, understand, kinda like Sentinel Safety device, less like IriTrack device ... descriptions for others can be found here:

Dakar Tech; Iritrack, GPS, Sentinel, Emergency Beacon... - ADVrider


You have an interesting project/goal in mind.

In the USA one might recommend using MURS (2 Watt power level at five frequencies near 152 MHz) frequencies or FRS (.5 Watt power level, but restrictions on how the radios are built, like requiring a built-in antenna, but this is not such a big handicap to overcome) or CB (5 Watt AM at 27 MHz, but, too much interference with sun spot cycle on the upswing).

There are also bands like 900 MHz available if Spread Spectrum (1 to 2 Watts power) techniques are used. And even 2.4 GHz, but line of sight starts to become an issue on 2.4 G partly because of frequency (absorption in vegetation, etc)

Seems like something could be worked up, especially today with chips that do a good job of signal strength measurement (if your system uses signal strength to roughly 'compute' a distance for protection distance) or maybe 'beacon' your GPS coordinates and drivers coming up would receive this and then their unit would calculate how close your were. This last approach would require a GPS receiver, but, could be engineered to give repeatable, and predictable results.


Jim

Yes we are looking for something like the Sentinel used in Dakar (this is how it came up actually). We cant get in touch with them so far.
All the bells and whistles are available from companies like **broken link removed** but the problem is the price. What we need is a system that costs somewhere in the $150 to $200 range to be put into everyone's cars. It would be nice to have the distances and direction with fancy lights and screens. But the reality is we would ultimately like a early warning system. The most dangerous time for us crews is the time in between the car stopping and deploying the caution triangle 100m back or where ever it needs to be in order to warn the next car. And once we are ready to continue on the stage, running back to the car with triangle in hand. With the electronic beacon, at least the drivers coming up at race speed have a chance to slow down.

This does appear to be over my technical ability... I am only a part time solder smoke eater on weekends and evening.

 

[E Kafeman]

Re: Beacon / RF question

Cheap and reliable is often hard to combine. Coverage is a problem if it is mounted in a rally car as the cowered distance can be almost infinite on an open field and relative short if there is an hill in between RX and TX unit. If mounted in a car, where to place the antenna? A car upside down, in a ditch with muddy water, will not cover much if it is a roof mount antenna. I have been a rally driver and have had all types of experiences related to this problem, including one or two ditches. Who should operate the radio? My guess is that if activation not is automatic, will it often be forgotten in a stressed situation. In worst case is driver and co-driver unconscious. Battery cables from the car can be broken for several reasons so it is no good power source for a reliable transmitter. That was the problems I can see right now.
A cheap RF system that can give the wanted terrain coverage is PMR446/GMRS. Can be bought including battery and charger for less then **broken link removed**. It is a good start to buy basic parts instead of develop the whole wheel. For another 40$, add GPS module and a circuit that can include a unique number of the car in a transmitted message and maybe an possible to send SOS=> stop other cars from starting on this stage, need for ambulance.
Better batteries, a box, big enough to make antennas embedded and a quick mount for the box so it will be sitting well protected behind the cage. Here will it be both protected but still in most cases will it not be too RF-shielded. Add push buttons, charging indicator, charging from car battery. vibrations sensors (no vibrations=possible auto-alarm, no sound=auto-alarm, no movement according to GPS=auto-alarm), All these extras are rather simple and low cost but important both in design and function if it should be a reliable system that also is easy to handle.
A important design part is how to use same unit in receiver mode and calculate incoming information, based on own GPS position, and that way calculate if any other car is within the warning-range and how this alarm should be presented. A flashing lamp need to be strong in daylight but can be an danger in the night if the light is to strong. Sound is a problem but maybe some type of general biasing sound sent to existing intercom can be something. Text display is a bit more exclusive, but a nice complement. Automatic resending of other cars important messages is another possibility and other functions that also RallySafe use.
As I see it can material cost be total held rather low, well within your budget, but someone must be thinking twice how and what functions that this radio protocol should handle, how it should handle if two transmitters send information at the same time. I have not either any idea if it is legal to use these frequencies for transmitting digital information, or if it is better to select another system that better can be accepted world-wide, gives similar coverage and not is too occupied.

A warning system based on that a received carrier is strong enough will never be reliable, expect uncertainty in the range of several kilometers if a coverage of 200 meters is minimum. In worst case can it give false security if its system not takes whole functionality in account.
For a simple walkie talke can it always be accepted that it not always gives expected coverage and noise kills a part of the message, but if your system inform the driver that there is no obstacles behind next hill, you have a certain responsibility as designer that the system both in hard and software have a high rate of correctness.
From safety view is it else better to not give any information at all. I know as driver, that if the technical information says that nothing is blocking behind next bend or crest, do I dare to drive faster, and is less prepared for the unexpected, such as a car with a not enough functional warning system is blocking the road.

 

[zerodegreec]

Re: Beacon / RF question

Thanks for the reply.

I think that the system would be used as an addition to the safety procedures already in place. The what ifs are endless. False positive is the worst thing that can happen. If the alarm goes off without there actually being a car in distress. the crew begins to doubt the unit and then its a total waist of everyone's time and money.

This project is beyond my technical ability (to do it properly). If someone is looking for a "project" I can provide a test group and potential market that is for certain. If the price per unit could be kept under control, and reliability proven. It would be a very valuable item. In my area alone we would be looking at 30+ cars. Plus if its adopted as a required piece of safety gear our race series has over 100 active cars.

 

[E Kafeman]

Re: Beacon / RF question

From safety point is any false result bad but in this case is it very simple, signaling "free road" and it not is the case is dangerous wrong. Compare with similar systems for other types of communication. Take for example a railroad, signaling green for train driver when it not is so, is much worse then the opposite, by a factor 1:10000000 or so,
The responsibility for this error is not at the train driver, it is the designer of the system logic that not had put enough effort in evaluating all possible "what ifs". All these "what ifs" are endless also for trains or air traffic, and a safety system must be very very very very evaluated as it else easily becomes the opposite. By clever design can uncertainty in all these whatifs easily be reduced in an early design level and without any additional hardware cost and over all better functionality.
Is it a less serious system as it is intended for a rally car? Is it not so important to give all these "what ifs" a serious classification as it only is a complement to other systems? No, it is an engineer that have been careless and not feel responsible for the work he have been doing. It is not about waist in time and money, it is how to use existing money and time as effective as possible.
Is it something that you later want to sell as safety related, design it with such functions. If it is worthless does it not help even if it costs a fraction of a system with some thinking behind.
Have been involved for many years in car electronics both in serial production and competition cars. Mostly EMC and motor control electronics. A lot of safety related "what if" also in these fields.
I am already involved in to many similar projects, some also related to rally cars and safety. Currently working on a new system for wireless intercom with a protocol special designed for that type of needs that occurs in rally cars involving several radio systems (short and long distance, video and audio) and solve problems related to compatibility between these and other existing systems. Almost all work around this is theoretical planning how to take care about all these "what if", communication security, and also EMC such as, what happen if the driver have pacemaker and place his helmet with antenna at his chest (common situation).

 

[RF_Jim]

Re: Beacon / RF question

Thanks for the reply.

I think that the system would be used as an addition to the safety procedures already in place. The what ifs are endless. False positive is the worst thing that can happen. If the alarm goes off without there actually being a car in distress. the crew begins to doubt the unit and then its a total waist of everyone's time and money.

This project is beyond my technical ability (to do it properly). If someone is looking for a "project" I can provide a test group and potential market that is for certain. If the price per unit could be kept under control, and reliability proven. It would be a very valuable item. In my area alone we would be looking at 30+ cars. Plus if its adopted as a required piece of safety gear our race series has over 100 active cars.

zerodegreec,

What is described as being desired is along the lines of the "Automatic dependent surveillance-broadcast" system proposed and being implemented for aircraft by the FAA (currently in Alaska) and is the step after present TCAS (Traffic Alert and Collision Avoidance System) system:

Automatic dependent surveillance-broadcast - Wikipedia, the free encyclopedia

In effect, it is a system (as I alluded to briefly in last post) which receives the broadcasts of the GPS position of all A/C in the vicinity ... each receiving A/C can than use that data to plot possible intercept course (if that is likely).

The poster E Kafeman has touched on more than just a few of the important items in this area as well, esp in the area of reliability and user reaction to a device which might prove to be less than totally reliable, and he presents some 'test cases' where proper device operation might be problematic for a given installation configuration (e.g. roof antenna mount impacted adversely during a vehicle roll-over scenario).

In any case, I think it is doable, and doable reliably for all but the worst-case test case scenarios; for non-accident scenarios I can see attaining 100% reliability given various 'tricks' of the radio trade: proper coding/format of the over-the-air protocol (e.g. GMSK/FM or SS/FH); ARQ Automatic Repeat Request bay another unit in the case of garble (e.g. as when an Ethernet packet is incomplete, etc) for the desired reliable path distance of 200 to 500 meters.

The next step would be a 'brass board' test system using some modules/radios hacked together to demonstrate the concept before getting funds committed for size/cost reductions and any specific 'coding' done .. should this perhaps be done along the lines of an open source project?

Minimally, for test purposes, an Arduino Mega talking to the GPS module (via serial) and interfaced with a cheap FRS (462.550 MHz FM transceiver); direct access to the FM transmitter modulator (VCO in the PLL) and receive discriminator (FM demodulator) would allow GMSK (Gaussian Minimum Shift Keying or FM freq shift keying) to send the 'data' to the other units ... the actual protocol and rate of GPS 'beaconing' (transmitting coordinates for a possible course-intercept scenario from a 'stricken' or broken down driver) all of course, to be determined ...


Jim