History
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Since the time that I originally made this circuit, commercially made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.
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