The Automatic CTCSS "Sniffer' - Don't Leave Home Without This in Your Older Rig!

tr-7850 picture

1. Introduction

    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 very 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 the 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 flicker.  

   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:

    67.0            69.4            71.9            74.4            79.7            82.5            85.4            88.5            91.5            94.8            97.4
   100.0          103.5          107.2           110.9         114.8          118.8          123.0         127.3          131.8          136.5            141.4
   146.2          151.4          156.7           159.8          162.2         165.5          167.9         171.3          173.8          177.3           179.9
   183.5          186.2          189.9           192.8          196.6         199.5           203.5        206.5          210.7          218.1           225.7
   229.1          233.6          241.8           250.3          254.1         

3. Hardware Considerations

    The initial SNIFFER design is 'housed' in a P/C board 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 mounted 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.  

Note: Double sided tape both insulates the bottom of the P/C board from the chassis and holds them nice and firmly.

4. Illustrative Conversion Details

    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.  

    Note: Connections 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 plastic enclosure.

   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.
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 SNIFFER 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.

Important Note:  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.

   Note: 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.

7. Components

   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 helpful.
Note: 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

10. Service Manuals

    73's - Joe - K3JLS