800MHz EDACS Trunk System Scanner Advance

I came up with this circuit during my freshman year in college.  The county that I live in converted all of its radio services to a GE - Errickson EDACS trunked radio system.  I had recently purchased an 800MHz capable scanner, but soon became dismayed by trying to chase a conversation as it advanced across the trunked frequencies.  At the time, trunking scanners for the EDACS system were not yet available.  So, what do I do?

Well, the first thing I noticed was that the communications queue was setup that the next transmission on the system was on the subsequent channel.  Therefore, to get the next transmission, all one had to do was advance the scanner one step in its scan list.  Under normal circumstances, the scanner would do this on its own once the carrier dropped.  The problem was that the system would have about a three second tail at the end of each transmission with several 'beeps' (which I later determined to be high level (space) tones from the 9600 baud modems which every radio contains to pass data related to the system's operation.  When one of these tones was transmitted, it was a sure sign that the transmission was completed.  Fortunately, the first one would come immediately after each transmission ceased.  I later learned that these same tones were used by the system to signal the user radios to close their squelch for the same reason.  I reasoned that if I could decode this tone and signal the scanner to resume scanning, I would be in business.

Since the time that I originally made this circuit, commercially made scanners capable of monitoring an EDACS trunk system have become available, essentially obsoleting the original purpose of this circuit.  Even more, the main IC used, the LM567 has also been obsoleted by National Semiconductor and is no longer in production  (if you need a couple, contact me, I have about two dozen left and would be willing to sell them to you at a very reasonable price).  The basic circuit does have other applications as well.  If you would like to activate an alert when certain tones are transmitted, say for instance a fire page or weather alert, this circuit will do the job nicely.

  Circuit Description

Click here to view the schematic

At the heart of this circuit is a LM567 tone decoder IC (now obsolete as previously mentioned), which is a phase locked loop frequency detector.  The RC combination of  C4 and R1, R2 set the center frequency to accept as the frequency to detect.  These are the values that will require adjustment to adapt this circuit for another application, if desired. C2 sets the bandwidth, or the range of frequencies to accept as the desired frequency to detect. Try not to make this too wide or otherwise the circuit will falsely detect a tone when none is present.  C3 is to provide DC uncoupling from the audio output.

Also shown in this circuit is an LM340T which is nothing more than a 5V voltage regulator. The other semiconductor package is an optoisolator.  By using the optoisolator, I didn't need to get intimately involved with the scanner's circuitry, just mimic the switch action, making the circuit compatible with a wide variety of scanners available at the time.  For the purpose of setup and monitoring, I added an LED to indicate when a tone was detected.

©2004  Ray Meyer, N9PBY