An Improved PL Tone Encoder



A significant amount of interest
has been generated since my last article on a PL tone board.
Because of this interest I decided to improve on the old PL tone
design. Figure 1 is an inexpensive PL tone generator with
significantly improved performance. It has a much more stable
tone frequency as well as the elimination of the loading problem
on the output. The transistor used in the old circuit would cause
the tone frequency to change at high levels. This month’s circuit
eliminates all those problems.

Among the changes in this circuit
are the use of a 78L05 IC voltage regulator instead of a resistor
and a zener diode. This device looks like a transistor but has an
extremely stable output voltage. The most important change is the
use of an Op Amp as the output buffer. I originally tried using a
Field Effect Transistor for buffering. Although it worked well,
the FET did not have enough output. The 741 Op Amp reigns supreme
once again.

The only rigid requirement is that
capacitor C1 be stable over a wide temperature rate. Use only
Mylar, Polyester or Metal film for capacitor C1. The tone
frequency is determined by the following formula:


The specified values should allow
any tone between 100 to 300 Hz. Increase R1 or add another
resistor in series with R1 for lower tone frequencies.

The output of the PL tone
generator should be connected to the deviation potentiometer in
the transmitter circuit and not the microphone input. (Check
schematic diagram of transceiver.) If the circuit reduces the
transmitter audio, install a resistor between the output of the
circuit and the transmitter. Use a value in the range of 1K to
1Meg (start with 100K) depending on what yields the best results.

To align the frequency of the tone
generator, use another transceiver with CTCSS decode capability
for the desired PL tone frequency. Attach a dummy load to the
transmitter and key down the transmitter while turning R1. As
soon as the correct frequency is obtained, the squelch will open
on the transceiver with CTCSS decode. Now adjust R5 for the
correct audio level of the tone. The tone level only needs to be
load enough for the other receiver to decode the tone and not
loud enough for others to hear. If the tone can be heard easily
then the level is probably too high.

The LM567 IC and the 78L05
regulator are available from JDR MicroDevices (1-800-538-5000).
All other parts are available at Radio Shack. The author will
make a limited number of parts kits available for the usual low
price of $5.


An Improved PL Tone Encoder


LM567 Tone
Decoder/Encoder IC

741 Op Amp

78L05 5 volt low power
regulator (DM105 Marking)

10K 15 turn trimmer pot

1K Resistor (Brown Black

10K Resistor (Brown Black

100K Resistor (Brown
Black Yellow)

10K trimmer pot

1 microfarad mylar,
polyester, or metal film capacitor (105)

10 microfarad
electrolytic capacitor (observe polarity)

.01 microfarad capacitor

.1 microfarad capacitor

An Improved PL Tone Encoder #2
An Improved PL Tone Encoder #3



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