CTCSS "Sniffer' - Don't Leave Home Without This in Your Older Rig!
There are many excellent 2 meter FM radios that were designed before
CTCSS became popular within the ham radio repeater community, and they
are either relegated to simplex work, for sale for cheap on ebay or
just sitting in a dust bin somewhere. I recently purchased
nice Kenwood TR-7800, a TR-7850 and a TR-7950 and decided to add CTCSS
capabilities to all of them. I did it and they all work, and work well!. More conversions may follow.
Using a PIC16F628A
microprocessor, an OP-AMP and a handful of discrete components, I
designed a circuit that will automatically track of the current
received CTCSS frequency and then use it when transmitting.
For the lack of a better term, I call this circuit the SNIFFER. Furthermore,
when the radio is turned off, or the power is disconnected, the last
used CTCSS frequency will remain stored in the PIC's memory. Neat, huh?
2. How Does It Work - Check the Schematic
The PIC uses an interrupt driven, period measurement proprietary algorithm to determine if
frequency sample is a valid CTCSS tone. If two consecutive
identical samples are received, the tone is considered valid.
Whenever a valid CTCSS tone is received, the LED will
On the transmit side, the PIC produces an easily filtered,
near perfect sine wave output. It took a very long time to develop
this software mainly because I had to learn both PIC programming (I'm a
Freescale guy), develop the concepts, program, debug, simulate and then
finally do a series of on-the-air tests.
The CTCSS tones (hz.) hard coded in the PIC include:
3. Hardware Considerations
The initial SNIFFER design is 'housed' in a P/C
measuring 1.5 inches wide by 2.5 inches long. As you'll note, this is a bit large for installation
directly into the radio (my primary goal), but I was able to squeeze
one into a TR-7850. Of course, this board could easily be
in one of the smaller Radio Shack plastic cases and then affixed to the
side / bottom of your favorite radio.
I've developed a smaller
P/C board measuring less that 1 inch wide by a little over 2.5 inches long. This
should be able to be fitted very nicely into some of the Kenwood radios that I've
seen like the TR-7400, the TR-7930, the TR-7950 (etc) and undoubtedly many, many others.
sided tape both insulates the bottom of the P/C board from the chassis
and holds them nice and firmly.
4. Illustrative Conversion
a) TR-7800 and TR-7850 Radios
This section will provide a very brief overview of how I upgraded my
TR-7800 and TR-7850 2 meter FM radios (essentially the same design
except for the PA stage which is larger in the TR-7850) for those of you who are willing to dig into
your older radio.
to most other radios of this era would be similar since they all share
basically the same FM Discriminator design
First of all, find a suitable location to mount the SNIFFER board.
In my conversion, I 'double sided taped' it to the area
containing the helical resonators as shown in the larger board / smaller board
pictures, and this works fine. If you are using the larger board and can't
fit it into your radio, then you can mount it in Radio Shack's smallest
With the SNIFFER mounted, connections need to be made to 13.8 VDC and to ground, to
the PTT circuitry, to the discriminator, to the radio's CTCSS output
point, to the LED indicator and to the LOCK switch. I drilled out one of
the speaker mounting screws on the top and placed ('crazy glued') a
small red LED (another Radio Shack 'special') in it. It's flush
with the top, can be readily seen and can easily be removed should you
desire to restore the radio to its original condition.
For the discriminator connection, I just made a connection to the
top of R47, a 3.3K resistor. I simply scratched off some of the
insulation and soldered a 30 gauge wire to it. Again, the picture
will show the location.
The CTCSS output connection is made with miniature coax to the point shown in the larger board / smaller board pictures.
The PTT connection is made topside, as well, to the orange wire shown in the larger board / smaller board pictures.
For the LOCK function, I used the TR-7800 / 7850's existing TONE locking
buttons. I removed (and taped) the existing leads and soldered 2 -
30 gauge wires and ran them back to the CTCSS 'Sniffer'. When the
LOCK button is pushed in, the SNIFFER will no longer scan for CTCSS
tones and the transmit CTCSS frequency will not change, even if the
radio is powered off and removed from all DC power.
Note: As a visual reminder, the LOCK LED will remain lighted.
b) TR-7930 and TR-7950 Radios
The conversion of the TR-7930 and TR-7950 are more asethetically pleasing as
the smaller P/C board can be mounted behind the front
panel in the space that the elusive Kenwood TU-79 was designed to
occupy. While the Kenwood tone board was screw mounted, the SNIFFER
installation does equally well with double sided tape.
As the SNIFFER board needs to be mounted in the area under the multi-wire connectors
(so the speaker won't glomp down on it when the top of the radio is
installed), it would be prudent to make the wire connections - especially the coax to the CTCSS input - to the
SNIFFER board BEFORE installing it. These connections include the
power and ground leads, the PTT and LOCK leads, the LED leads...... I used 30 gauge wire wrap connections.
(switched 13 VDC) - Connect the power lead to the
middle pin (brown) wire on J3 on the underside of the radio.
While this connection could also be made 'topside', I found this
to be easier. Before connecting, verify with
your meter that you have identified the correct point.
- Ground - Use small gauge 'hook-up' wire and connect to the corner chassis screw and tighten it securely.
Pickoff - Connect a 30 gauge wire to the bare wire on the P/C board just below R-99 and
run it to the Discriminator In pin on the SNIFFER.
Be careful when making this connection. Use a low wattage, fine tipped
soldering iron. If necesssary, place some aluminum foil around the
neighboring components so that you don't damage them. This connection point is in the recovered audio path just before the radio's high pass filter and provides a higher signal level to the SNIFFER than would a direct connection to the discriminator itself.
- CTCSS - RG-174 miniature coax - 7 inches. Connect both the shield and the inner conductor to the SNIFFER board and the center conductor on other end to pin TO (Tone Out) on the other side of the circuit board. Do not connect the shield to this connector. Pin TO is the end most pin on the connector to which Kenwood's fabled TU-79 would connect.
- the PTT connection is also made beneath the chassis to
the ST (start) lead of J10. The PTT connection is required as the
PIC16F628A processor keeds to 'know' when a CTCSS tone output is
- LED - run a twisted pair of wires (30 gauge is a good choice) from the LED pins on the SNIFFER
board to a top panel hole created by removing one of the speaker
mounting screws and enlarging the opening. Crazy Glue the LED in the hole.
- LOCK - run another twisted pair of wires (30 gauge is a good choice) from the SNIFFER
LOCK pin and ground to a switch mounted on the top, close to the
speaker. While the TONE button in the older Kenwood FM rigs (like the
TR-7800 and TR-7850) could be easily 'repurposed' for this task, the
TONE button in the TR-7930 / 7935 is just a non lockable push button
switch that signals the radio's microprocessor!
While some of the less frequently used buttons such as
the two PRIORITY
lockable switches could be used (after modifying the
internal wiring), I opted to leave their functionality alone and just
install a new LOCK
switch (Radio Shack SPST) on the top. Twist the LED and LOCK pairs together, securing the top near
the speaker bracket.
Slip a cable tie around the leads coming from the
board, and leave enough 'slack' in this wire pair (and the LED leads described below) so that the top cover
can be removed for radio servicing / alignment, etc.
When chosing a mounting location for the switch, ensure that the
switch - when mounted - will not touch any of the P/C board components.
For that reason, a subminiature switch is probably the best choice.
Verify all your connections and then test it out.
c) Yaesu FT-221 Memorizer Radio
Works, but has yet to be documented............
5. Testing It
After installing the SNIFFER board into your radio and making all the
connections, you should check out the receive functionality first.
Tune your radio to a nearby CTCSS equipped repeater (or use your
own 2 meter HT) and verify that the LED will flicker during reception
when there is a carrier present but no speech. The low pass
filter in this design is not that great, but all you need is one or two
LED 'flickers' to ensure that the CTCSS tone has been properly
detected, decoded and stored in the PIC's EEPROM.
To adjust the CTCSS output level, you can either use an FM
deviation meter or another 2 CTCSS equipped 2 meter radio set for CTCSS
receive. Key your transmitter and adjust R12 until either the deviation is proper or when your 2 meter radio opens up its squelch.
Just to be sure, make a contact or two to verify that your
deviation level is proper and that the fellow at the other end of the
connection cannot hear your transmitted CTCSS tone.
6. Using It
Using the SNIFFER is simplicity itself. With the LOCK
switch off, tune your radio to the channel of interest and wait for a
conversation. As soon as the LED flickers, the tone-in-use has
been detected and stored in the PIC's EEPROM and you are ready to
transmit. If you desire to hunt around for other conversations on
the band with the intention of returning to this particular frequency,
just depress the LOCK button and subsequent CTCSS tone scanning will
cease. The LED will stay lit to remind you that a CTCSS tone
frequency has been stored.
If you go back to the
original frequency, your radio will transmit the proper CTCSS tone.
If you want to transmit on other frequencies, just release the
LOCK button and you're good to go.
The CTCSS frequency will remain stored in EEPROM just as long as the
LOCK button is depressed even when the radio is powered down and then
powered up again.
Here's a spread sheet
showing the costs of the components from Mouser and those that I'll make
available - the P/C board and pre programmed
microprocessor. E-mail me with any questions.
8. Board Building
Here are just some suggestions on populating and testing either of the
two circuit boards. Please keep in mind that a fine tipped
soldering iron will be required and that a magnifying glass may also be
- Start by installing the 2 integrated circuit
sockets first. If you use the snap-in sockets, be very careful as the
pins are easily bent during the insertion process. If one of
the pins happens to get bent and this is missed during the soldering
process, it will be a very time consuming job (PITA) to remove it. I've
it helpful to straighten the pins a bit before insertion.
all the capacitors (including the 2 electrolytics), followed by the
resistors, the 2 diodes and then the one trimmer resistor (R12).
- Note that the R13 input level trimmer shown in some of the pictures is NOT USED in this application and is bypassed.
- When installing the 4 Mhz crystal, leave a bit of space under it so that it does not hit the surface mounted trace.
- Install the mounting pins (for the power, ground, PTT, LOCK, LED and CTCSS output) last.
- Double check all soldered connections to ensure that nothing was missed.
Before installing either integrated circuit, temporarily connect a 13
volt DC power source (watch the polarity) and verify that +5 VDC is
present at pin 14 of the PIC16F628A and at pin 8 of the LM-358 op amp.
9. Other Likely Candidate Radios
- Install the 2 integrated circuits and you're good to go!
10. Service Manuals
73's - Joe - K3JLS