I've posted about this before and need some help. Basically what I'm doing is trying to create an FM transmitter for emergency vehicles that broadcasts a message or alarm to civilian vehicles that are in the same area as the emergency vehicle. I've decided to go about this using an fm transmitter instead of arduino, but i really need help on every step. Any responses are greatliy apprediated.
I think you will find it illegal to build and / or use an FM transmitter that transmits on the emergency wavelengths. In fact any sort of radio transmitter requires a license or "type approval" to build or operate.
Ham radio licenses allow you to construct your own transmitters and operate them at certain frequencies but most countries require some sort of exam qualification that proves you know what you are doing.
Sorry to be so negative but I don't think it is a practical project.
Here you go http://www.ramseyelectronics.com/hk/
I know you CAN transmit on the AM/FM frequencies without breaking the law, but mike is right, its illegal (and the FCC will hunt you down) to broadcast in the standard AM/FM spectrum were all used to with any kind of reasonable power.
Also of note, the max power output you can use without being illegal is pretty damn pathetic whatever it is.
Also, for a practicality sense of your idea, it would sort of need to broadcast over ALL frequencies to actually alert anyone.
That wouldn't be so hard, but then you would want to override their favorite radio station at the same time. For that you will need a decent amount of power id say, but at the same time you don't want to annoy everyone. Maybe you could have special antennas or something; but all in all, i see many inherent flaws in the idea. Well, without changing everyone else's infrastructure too, you know?
There is like what 200 channels in the FM broadcast band, how is your transmitter going to be able to send a message to all of them at the same time. If done sequentially it will take a long time to get the same message out to all channels. Then you might also need to be able to cover all the AM stations as well. I don't think you have a good understanding how radio frequency communications works.
Now there is away to transmit a noise signal that can override all other stations for a certain radius, however it won't be able to carry any meaningful information.
This is fundamentally a project with no practical solution.
Lefty
I can think of several dead-easy ways to make this a successful project. Your demo will require special hardware in the test cars. But ultimately, the hardware required would mostly already exist in most cars; auto manufacturers (assuming we still have them) would need to update the firmware in their radios and maybe a little hardware.
A car radio could always be listening to a specific frequency. It would be listening for a specific RDS or sequence of tones. As soon as this is detected, the car radio would shut down any other audio (CD, phone, other radio station, etc) and alert the driver that an emergency vehicle is approaching.
This could be done with other radio bands, but using the FM/AM radio band would mean that this system would easily integrate into current car radio designs.
It's a senior project, it's there to demonstrate a team's design and documentation skills, not necessarily the practicality of rolling the system out over the entire country.
"I can think of several dead-easy ways to make this a successful project. Your demo will require special hardware in the test cars."
Yes, of course if the project had the scope and means to add or modify the equipment in all the car in the immediate area then there are probably several solutions. However the OP's question was pointed at designing a FM transmitter to do the task.
Lefty
It's up to the person doing the project to decide where their project scope begins and ends.
OK I looked at the link for the transmitter and it said:-
Note: The FM100B and FM100BEX are do-it-yourself learning kits that you assemble. The end user is responsible for complying with all FCC rules & regulations within the US or any regulations of their respective governing body. The FM100BWT is solely for export use and can only be shipped to locations outside the continental US or valid APO/FPO addresses or valid customs brokers for documented end delivery outside the continental US. The end user is responsible for complying with all rules and regulations of their respective governing body. Orders for the FM100BWT are processed manually and verified to be export compliant prior to order acceptance. There are no exceptions.
As a paper exercise this would be fine but actually transmitting would not be especially as the original question said:-
I've decided to go about this using an fm transmitter instead of arduino, but i really need help on every step.
There are other ways of doing this job without the need for a separate transmitter by using the existing messaging system built in to the FM standards. These normally display just the name of the station but can be altered by the broadcaster to transmit scrolling text messages. Therefore there would be no need to modify everyone radio. Unless of course you live in a third world country that still uses radios that conform to standards that went out 20 years ago.
My understanding of this student's project is that he wants an emergency vehicle to have the ability to interrupt the regular broadcasts on the FM band (87.8 - 108 assuming the original poster is in the United States) for other vehicles in the immediate vicinity. This is certainly possible and - off the top of my head - here's how I would approach it.
There are only 101 channels in the "FM band" and due to FCC channel spacing, a maximum of only about 50 channels could be in use at any given location (assuming the local FM market were saturated and ignoring the possibility of pirate broadcasters or private vehicles using mp3 player FM modules). In reality, many areas of the United States have fewer than 50 FM stations available in the area.
I would set-up a unit in the emergency vehicle with a digital FM receiver (like the AR1000 module from Sparkfun) that would scan for all broadcast stations available in the area in question. An Arduino would record every broadcast station picked-up by the digital receiver. The Arduino would then control and array of, oh, let's say 10 digital FM transmitters such as the NS73M (also available from Sparkfun). The Arduino would set each of the 10 Transmitters to one of the frequencies picked-up by the FM receiver and broadcast a 3 second message, "Warning, Emergency Vehicle approaching!". Then the Arduino would set the transmitters to the next 10 frequencies recorded by the FM receiver and repeat the message. If we assume a saturated market of 50 FM stations and 10 transmitters broadcasting a 3 second message, the entire transmit operation would take about 15 seconds plus let's say about 5 seconds to scan the FM band for broadcast frequencies. That's not too bad and it could go faster if you make the emergency message shorter or add more transmitters.
It's perfectly legal to broadcast on the FM band in the US under the FCCs part 15 rules; that means that your power level must be below 250uV/Meter at 3 Meters. Most people don't bother to measure that and just try to keep the transmitter output to 7mW or so (and really probably less than that - the NS73M modules output 2mW max). That's enough to drown out full-power broadcast stations if you're really close to the receiver as you might be in an approaching vehicle on the highway - especially if your transmitting antenna is outside of your vehicle. This is certainly an aspect to thoroughly test though.
This approach is not very elegant (imagine 10 separate 30 inch antennas sitting on top of an ambulance - and who knows what it would do to the other emergency services equipment on-board the vehicle), but that's how I might start investigating a project like this. It's a neat project idea.
To add to cairn's observations (which I agree with), I would imagine that once the prototype works for the 10-meter case, then the interested municipal players could go through the regulatory process with the local and federal authorities. They'd set up a "pilot program," where they have a permit to broadcast to say 1000 meters distance within one particular part of a larger town. They'd do some tests for about six months, open up for comments (that almost nobody ever participates in), and then roll it out to larger and larger areas.
The reason I suggested something that required modification to the radio is because I don't know anyone who actually listens to the radio anymore in their car. They listen to CDs, MP3s, XM or Sirius, talk on the phone, or leave it off. The radio would need to know there is an emergency and automatically turn on and make the notification.
You are not required to transmit on all channels in the FM band. Most commercial radios utilise a superheterodyne receiver system. Basically to reduce image frequency interference. The radios utilised an Intermediate Frequency, which is provided by adding or subtracting the incoming RF signal to a local oscillator frequency, and mixing it. This IF is usually 10.7 MHz for FM or 455 Khz for AM. All one has to do is, transmit on 10.7 Mhz and regardless of what frequency the radio is tuned to, it will be overridden. All you need really is a HF Transmitter FM modulated set to 10.7 Mhz and what you are suggesting is possible.
Most superhet receivers have very good IF rejection specs, say -60 db or better? It would take a tremendous amount of transmitter power to 'burn' through the IF of car receivers which also have the advantage of a lot of RF shielding that would make IF pass through that much harder. I don't think this is a viable solution.
Lefty
I agree with you that it does take an amount of power, but his project is not about ghosting a radio station for an entire suburb, my understanding is he wants the emergency vehicle to announce its pending arrival in the area. A small transmitter, ought be able to do this for a good 200 metres or so. It is after all how the Emergency Warning System in Sydney tunnels work. Obviously the more power the more effective the system. I agree with you there. But he is only trying to do this in a transient manner to warn immediate traffic. Having been an Ambulance Officer, and having to beep my horn at idiots who arent taking notice of lights and sirens, I think he has a great idea. The power problem is an issue. But, it is probably cheaper than trying to do this with multiple radio transmitters. Albeit that was a cool idea too. I agree with you in re shielding, but, shielding and emi protection only works so far. If you can scan a car radios LO then you can get past it.