Field Strength Meter for Key Fob

Discussion in 'The Projects Forum' started by JDR04, Apr 16, 2012.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Hi Folks, is it easy enough to build a circuit to indicate if a car alarm key fob is transmitting. I dont need to know its frequency etc just that it is transmitting and some sort of way of determining the transmitting strength?

    Would like to build it for a spare time project if not too complicated. All circuit suggestions are welcome. Thanks a Lot - JDR04
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    If you have a DMM you might be able to get away with just a detector diode and small filter cap arrangement but these things probably transmit less than a mW, so you may need a tank also. If you want a self contained LED arrangement a tuned MPF102 JFET would make an acceptable front end. This could drive a BJT or two to light the LED. BTW, both of these concepts would require very close proximity to the clicker.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    It's called a car :p

    But seriously, I think you'll need to specify a lot more about the transmitter (especially the frequency) since I doubt they're all the same.

    If you want the device to work for ANY transmitter, there is such a thing as an EM radiation detector. Kinda like a bug detector. Might work?
     
    JDR04 likes this.
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I forgot to mention that my search returned 315MHz as the most common frequency. The MPF102 falls within this range.
     
  5. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks guys, I should have considered the frequencies first before posting so we can be clear on that one. As you can see I'm based in the UK and I'm sure the freq. will be different so I'll look into it a bit more. I'll find the frequencies and the mW if I can and post soon. Thanks again for your input-JDR04
     
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