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RF sensor

  1. Feb 20, 2006 #1
    I'm looking to make/buy an RF(radio frequency) to detect and display the level of an incoming RF signal. This is going to be used on a radio controlled vehicle. Any suggestions and info would be greatly appreciated.
     
  2. jcsd
  3. Feb 21, 2006 #2

    berkeman

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    What frequency and what modulation are you trying to detect?
     
  4. Feb 21, 2006 #3
    I'm not too sure about that. I'm just looking for some general sensors for it. It is supposed to detect an rf signal from the remote control for the radio controlled vehicle.
     
  5. Feb 21, 2006 #4

    berkeman

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    Well, the simplest (and least reliable) is just to use an antenna and a tuned coil to pick up the carrier frequency, and amplify and rectify it to give you a level indication. Depending on what kind of performance you want, you will be amplifying a lot of noise as well, and it will be a little hard to tell the difference between the noise and the signal. This may be sufficient for your initial needs, however.

    The next better step is to do some of the demodulation that the RC vehicle receiver does, and look for the actual radio control signal's characteristics. I don't know much about RC encoding schemes, however, so there might not be much extra to look for if the carrier is just a steady CW signal. If they use a digital modulation scheme, then you will be able to lock onto the code to know that there is a signal there.

    I'd say it would be best for you to understand the RF characteristics of the RC signal, and then maybe take apart an RC receiver to see what-all they are doing. BTW, most RC vehicles use an undersize antenna (because they can't fit a 1/4 wave monopole onto the vehicle at the 30-40MHz carrier frequency). But check to see if in your project you can use a full-size antenna. That will give you a huge amount of gain in signal/noise.
     
  6. Feb 22, 2006 #5
    www.mackrackit.com/djm/LF411 Reciever.html

    Here is a simpler AM Reciever that might help. It basicly amplifies the incoming RF through that OP amp. If your transmitter isn't AM then you shouldn't use the low pass filter at the output.

    I am not sure how you want it to detect, you could place an LED on the output.
     
  7. Feb 22, 2006 #6

    berkeman

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    The problem with that receiver is that it is for the "AM Band" in the US, which is around 1MHz. And even that low, the LF411 opamp they show is running topped out. The RC bands are more like 30-40MHz, I think (I forget exactly), so a vanilla opamp isn't going to help in terms of gain. The amplification will need to be done with higher-speed amps or with discretes or a combination. And again, you will need to be very careful about shielding the input stages from the amplification output stages, and you will need to pay attention to power supply splitting and filtering. Otherwise your amp will sing like a songbird (and ignore the input signal).
     
  8. Feb 22, 2006 #7
    Oh I didn't even think about that higher frequency range.

    Even though, does maverick99 need a reciever that demodulates an FM or AM signal, or does he simply need something to detect a frequency?
     
  9. Feb 22, 2006 #8

    chroot

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    maverick probably does not need to actually demodulate the signal; he probably just needs to measure the power over a range of frequencies near the carrier.

    A notch filter (or a tuned antenna, as berkeman suggested) would select a range of frequencies from the spectrum. Next, an amplifier and rectifier convert the RF signal to a DC voltage proportional to carrier amplitude.

    Truth be told, you more than likely don't need to build much of anything -- the entire receiver is already present in your RC car. All you'd need to do is reverse-engineer the receiver to find which wire carries the baseband signal, then use a voltage-follower and rectifier to measure its average power.

    - Warren
     
  10. Feb 23, 2006 #9
    Could anyone supply me with a schematic that would work the best for the 30-40Mhz incoming signal? I'm basically wanting to detect the rf signal and light an LED when it detects it.
     
  11. Feb 23, 2006 #10

    berkeman

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    I googled radio control car receiver schematic, and got a surprising number of hits. Check some of them out or re-google... This first one is a gem:

    http://www.commlinx.com.au/Models.htm

    http://jap.hu/electronic/rf.html & click on 27MHz receiver schematic

    BTW, having the schematic is really just a small part of the work. If you can get a kit from Radio Shack or somewhere, that will help you a lot because the layout work will have been done for you, including any power supply splitting and filtering. Have fun!
     
  12. Feb 23, 2006 #11

    berkeman

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    Actually, chroot's suggestion may be the best for you. Do you just want to light an LED when the car is detecting a control signal? As chroot says, just figure out which signal coming out of the car's receiver is a carrier detect indicator, and buffer that to turn on your LED.
     
  13. Feb 23, 2006 #12
    I will probably end up doing both and see what the output is. I specifically want a circuit that will detect the signal and light an led when the car is within range of the rf signal controller.
     
  14. Feb 23, 2006 #13

    chroot

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    maverick,

    I doubt that the RC controller actually broadcasts a continuous carrier, even when none of its buttons are being pressed (though I am not sure of this). In that case, the only time your LED would light up is when you're pressing a button on the controller. Of course, when you're pressing a button on the controller, your car will be moving.... so it all seems a bit redundant.

    - Warren
     
  15. Feb 23, 2006 #14
    Actually I build a circuit that would detect an RC controller, and it did brodcast continuously even when the buttons weren't being pushed.

    Although I didn't have it tuned to the frequency of the RC contoller
    (47MHz)... when I turned it on, my receiver Led would light up.

    This is the RF detector that I built:
    www.mackrackit.com/djm/LF411 Reciever.html
    you just need to change L1 and C1 to tune the circuit, (like using a 1uh coil and a 20 pF capacitor will give you 35 MHz) and omit the low pass filter on the output.

    Does this help?
     
  16. Feb 23, 2006 #15
    I think this will work for what I need. I'm sure I will be back for more questions but thanks for the schematic!
     
  17. Feb 23, 2006 #16

    chroot

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    What the hell, McCormick? We already told you why this receiver won't work -- the LF411 has a gain-bandwidth product of 3 MHz! There's no way this op-amp can amplify a 30-40 MHz signal! Why did you post this again?!

    - Warren
     
  18. Feb 23, 2006 #17
    Ok no need to get mad.

    I though he just said that all he needed it to do was turn on an Led, so all the op amp needs to do is amplifly when it recieves a signal.

    The LC series tuner will block unwanted frequencys and when the right frequency passes the op amp will amplify and light an led.

    Sorry but I don't see any problem
     
  19. Feb 23, 2006 #18

    chroot

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    The frequency in question -- in the 40 MHz ballpark -- won't go through the LF411 op-amp at all.

    Do you even know what "gain-bandwidth product" means?

    - Warren
     
  20. Feb 23, 2006 #19
    It won't go through the op amp at all? I built that circuit and tested it using a 47MHz RC and a 400MHz walkie talkie.

    No actually I don't really know what gain-bandwith product means:blushing: . Maybe my ignorance on this is why I don't understand why the LED will flash very fast when the signal is 47MHz and why it comes clear onwhen the signal is 400Mhz.

    Although it does work cause I have built and seen it work.
     
  21. Feb 23, 2006 #20
    McCormick, on your schematic, is R2 just a resistor in the feedback line? I was just confused on what the wire above the 100k label was.
     
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