RF Project

Discussion in 'Wireless & RF Design' started by davidGG, Nov 7, 2013.

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  1. davidGG

    Thread Starter Member

    Dec 22, 2012
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    Hello.
    I understand there is a projects forum, but most of the radio threads die fast so I'd like to post it here :)

    I have an AM transmitter and receiver. It operates at about 1.9MHz.
    What I want to do is send a modulated signal that contains something like 1010101011. Once this signal is received by the receiver, I want to receiver to check if the signal received matches 1010101011, if it does, it lights up an LED.
    How would I go about this?

    I've already constructed an AM receiver/transmitter. I have been able to successfully transmit audio. I'm fairly sure I can generate a binary hi/low signal using a 555 timer or something like that.

    My issues arise in having the AM receiver check for the signal and making an LED light up once that signal is received.
    I have no idea on how to do this.

    Any suggestions would be appreciated.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Before I offer any help, I have a question? Are you in compliance with the FCC rules regarding the operation of an unlicensed transmitter which operates in a band assigned to radio amateur use? (Disclosure: I am a ham, I use the 160m band, and I take a dim view of unlicensed operations).

    You can buy inexpensive pre-made modules specifically designed to operate in frequency bands which meet the requirements for unlicensed operation...
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    Your notion of a "modulated signal" that contains a string of bits is a bit fuzzy. I'm also a ham and we have several ways of doing this. You don't say what the data rate is and this will have a significant impact on potential solutions.

    I'm also concerned about your lack of understanding about the unlicensed operation of a transmitter. So what's the story?
     
  4. davidGG

    Thread Starter Member

    Dec 22, 2012
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    I have a technicians license. Yes I know most of the technician frequencies are above 30MHz but my transmitter is very low power, it barely goes from one room in my house to the other, maybe 15 feet max.
    Plus my oscilloscope is only 20MHz :(.

    I am doing this for educational purposes, I like to see the signal on my scope every step of the way. I know there are pre-made transceivers which I can set up to a microcontroller, I have a few OOK transceivers and they work.

    I just want to test a method that doesn't involve a pre-made transceiver using AM modulation. Later on I plan on doing FM, then move onto serial data, images, and anything else I can think of

    I don't care for an extremely fast data rate, I don't plan on sending serial data of any sorts (at the moment). I just want the circuit to make an LED go high/low.

    Any suggestions?

    By the way, electrons flow from negative to positive :cool:

    Edit: It seems as if it is a big deal for your two that all radio operators have a license no matter what. If you don't believe I have one I can PM you my call sign if you PM me yours since a call sign gives away a lot of personal information.
     
    Last edited: Nov 7, 2013
  5. tshuck

    Well-Known Member

    Oct 18, 2012
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    What's the frown for? A 20GHz bandwidth is a pretty penny!;)

    How are you demodulating the signal?

    By the way, electrons move toward a more positive charge because they are negative. The way this is conveyed is either conventional current flow, or electron current flow...know the difference...
     
  6. davidGG

    Thread Starter Member

    Dec 22, 2012
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    Sorry I meant to put 20Mhz! If only I had a 20Ghz scope.

    What do you mean by how am I demodulating the signal? I am using an AM receiver if that helps.
    I didn't design my own receiver. I just started learning RF electronics on my own from books/youtube.
    I got my receiver from here:
    http://www.zen22142.zen.co.uk/Circuits/rf/amrec.html
    (I didn't use the antenna show in that guide, I am using a salvaged car monopole antenna)
     
    Last edited: Nov 7, 2013
  7. davidGG

    Thread Starter Member

    Dec 22, 2012
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    Anybody?

    I just need help with a circuit that can check if an incoming signal is the same as another predefined signal.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That definitely sounds like a job for a microcontroller or microcontroller module like a basic stamp, arduino etc.
     
  9. davidGG

    Thread Starter Member

    Dec 22, 2012
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    This is primarily for aa school project and one of the requirements is a low cost. However I am really interested in this stuff. I was able to do this very easily using a AVR microcontroller and an RF (OOK) module, but things get a little expensive.

    By the way, can I do this using just a microcontroller without an RF module?
    Example: I program my μC to wait for a binary 101010101. Hook up the RX pin to an AM receiver/demodulator and send 101010101 from my transmitter.
    Will this work or do I NEED a RX/TX module?
     
  10. gootee

    Senior Member

    Apr 24, 2007
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    There are lots of ways. You get to decide.

    The first thing to decide is how you will represent a 1 and how you will represent a 0. It could be "audio or no audio". Or it could be 3 kHz tone = 1 and 1 kHz tone = 0. Or you could send DTMF tones. And on and on. Some will make the rest of the scheme easier to implement and some will make it very difficult.

    Then you need some way for the receiving side to determine what is received, i.e. It needs to know whenever a 1 or a 0 is received, and then do something. Maybe you could use what is called an Envelope Detector circuit, which in its simplest form is just a series diode followed by a resistor and capacitor in parallel, to ground. You can tune the R and C to make them work well with particular audio tone frequencies. You can square-up the output with a Comparator or a Schmitt Trigger circuit, or some kind of latch. If you don't have and use LT-Spice, I would go to linear.com and get it.

    Then, since your sequence is longer than 1 bit, you have to STORE that many bits, somehow. For so few, maybe just a string of flip flops would work. But you probably could more-easily use what is called a Shift Register, or some other type of multi-bit memory. You could just shift all of the previously-received bits down to the next position, each time a new bit is received.

    You also need to store the "key", i.e. the sequence you're looking for.

    And you need to compare the key word with the currently-stored received word, after each new bit is received, to see if all of the bits match. That could be done with simple static logic gates. The output of that could turn on the LED.

    Alternatively, and maybe simpler, if you were allowed to give a "start" or "reset" signal, to tell when the code sequence was about to start, then you could use a "binary combination lock" circuit, like we had to design in our first digital logic lab course, back in the 1970s. You could still use that type of circuit, if you had a third type of transmission that would signal a reset to the receiver and the post-processing circuitry (OR, a special bit sequence). The key is then inherent in the circuit, and each correct bit makes it farther through the circuit until the last one being correct makes the output go high (or low).

    I don't remember the details of the combination-lock circuit. But it used 7400-series logic chips; probably just flip flops and basic gates, and a clock strobe signal, for which we had a (debounced) pushbutton circuit but which you would need to create whenever a bit was received.
     
    Last edited: Nov 18, 2013
  11. Papabravo

    Expert

    Feb 24, 2006
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    These are hardly convincing arguments for violating the rules that you studied to get the license. I would advise you to find a different way. Either upgrade your license to General or experiment on a band where you have privileges, like the 10M band. An oscilloscope is not really a great tool for RF work and I never found looking at one illuminating except for answering the binary question "Is it beating up and down or not". There are cheaper better ways to do this.
     
  12. brucef

    New Member

    Jan 22, 2016
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  13. brucef

    New Member

    Jan 22, 2016
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    This might be far and away more involved that you were looking for but I believe you would need to marry the transmitter to a modem circuit. In the early 1990's, I used to repair 900 MHz transceivers that modulated RS 232 serial data using FM and used FSK (Frequency Shift Keying) thus modulating the carrier. You would then need a duplicate setup to receive and demodulate to carrier.
     
  14. bertus

    Administrator

    Apr 5, 2008
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    @brucef ,

    Did you notice the warning when replying?
    This is a very old thread.
    When you have questions, start a new thread refering to this one.

    Bertus
     
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