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Need help with RF

Discussion in 'Radio and Wireless' started by Ino, Dec 5, 2016.

  1. Ino

    Ino

    3
    0
    Dec 5, 2016
    Hello,

    I'm a beginner with electronic engineering, and this is a project I'm doing at my house. I could use a little help with the details.

    The system is relatively simple, I'm just not educated enough yet. I have two switches. Both switches start in the closed position. When one switch opens, it will start a countdown from 5 minutes. If either switch toggles, the countdown stops. If at the end of 5 minutes, one switch remains open, and the other closed, I want an RF transmitter to activate, sending a signal to the horn of my car. The horn should rapidly beep, with two second interrupts thereafter, until the system is deactivated by placing both switches in the same position.

    I am trying to do this project at very low cost.

    I already have the switches. Will a 555 timer work for my countdown, and for the two-second interrupt? (I don't need digital display, or recall, just a simple counter) Also, where can I find an RF transmitter that will fit the bill? It needs to be low power, and programmable.
     
  2. the great potato

    the great potato

    47
    0
    Dec 2, 2016
    i have no clue what this would be used for besides a joke of some sort... but this sounds like something those andrino chips would do. Just throwing it out for an idea.
     
  3. the great potato

    the great potato

    47
    0
    Dec 2, 2016
    if you need a car alarm thee like 50 bucks on ebay, i have one never used still in package ill sell for a good price =P
     
  4. hevans1944

    hevans1944 Hop - AC8NS

    4,247
    1,997
    Jun 21, 2012
    That's not much of an explanation. Where are the two switches located? How far apart are they? Who or what actuates and deactuates the two switches? How far is the transmitter from (presumably) a receiver in the car?

    This could be a job for a PIC connected to the car receiver to control the horn beeping (beep beep beep ... two second delay... beep beep beep... wash, rinse, and repeat) after receiving an RF trigger signal from the transmitter. Another PIC to monitor the two switches and implement the five minute delay would also be a simple solution to your "switch logic" and would control the transmitter.

    There are hundreds (if not thousands) of "key fob" transmitters and receivers available. Range varies considerably. Unlicensed versions (which is no doubt what you need) have strict limitations on frequency band, duty cycle (FCC regulation), transmitter power, and antenna gain (if any). Transmitters must be "type approved" to be legal in the United States, but most of the Asian vendors somehow get around that or ignore it completely.

    So, your first task is to find a key-fob transmitter and receiver that provides at least two control channels (one to start the horn beeping and the second to turn it off) and has sufficient range for your application. A good place to start looking is here. Next, you have to mod the key fob to get at the switch contacts so you can substitute your own switches for the conductive elastomer switch contacts in the key-fob. You can obtain a receiver with logic level or relay contact outputs, so hooking that up to the PIC in the car not a big deal.

    To avoid draining the car battery overnight, the PIC should be programmed to "sleep" most of the time, waking up periodically to check the receiver output for a horn beep command. So, a few milliseconds while awake to check the receiver, then sleep for a few hundred or more milliseconds, drawing only a few microamperes of current while asleep. Depending on receiver current draw, you may want the PIC to enable power to the receiver only while the PIC is awake. Alternatively, you could latch the receiver output (so the PIC "sees" it when it wakes up) and power the receiver continuously. All depends on how much current you are willing to draw from the car battery when nothing is happening.

    Or if microprocessors are not your cup of tea, leave the RF receiver powered all the time, add a couple of 555 timers and perhaps some CMOS logic to beep-pause-beep your car horn. But purchase the transmitter and receiver first, to find out if the range of the pair is sufficient for you. The rest of the project is just logic. We can help you with that whichever way you choose to go.
     
  5. the great potato

    the great potato

    47
    0
    Dec 2, 2016
    so let me get this right .. you want a switch to ... you arm it and walk a way it gos off in 5 min ? that just what im getting. There is easy ways to do this if thats the case.
     
  6. Ino

    Ino

    3
    0
    Dec 5, 2016
    Hevans 1944,

    This helps. All of the components of the project will be inside of the car, to include the transmitter. The transmission shouldn't have to travel more than a few feet to the car's CPU. The whole project should be very low power, hence the simplicity of the system. The only radio-controlled part of the project will be the vehicle horn.

    I have some experience with simple circuits, but I usually have a diagram to work off of. I do not have a diagram for this project. I imagine it will take some trial and error on my part. As for a micro-processor, I have almost no experience dealing with them.

    I'll be going with the 555 timer for a couple of reasons. First, it should be simpler than using a PIC, and second, it will be cheaper if I have to reproduce the project multiple times.

    I will work on getting the pieces parts, and go from there. Thank you very much for your reply.
     
  7. Ino

    Ino

    3
    0
    Dec 5, 2016
    Not quite. I want an alarm to go off after 5 minutes if I leave the second switch on, and turn the first switch off.
     
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