An Inexpensive PL Tone Encoder


An Inexpensive Tone Encoder

If your local repeater has got you singing the
PL tone blues, then read on. For less than $10, an inexpensive PL
tone generator can be constructed that will bring new life into
that old non-PL rig.

An Inexpensive PL Tone Encoder

Figure 1 illustrates an easy circuit that can
be constructed with mostly Radio Shack parts on any general
purpose circuit board. The only rigid requirement is that
capacitor C1 be stable over a wide temperature range. Use only
Mylar, Polyester or Metal film for capacitor C1. The frequency is
determined by the following formula:

FREQUENCY = 1/R1C1

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 this PL 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 resister between the output of the circuit and the
transmitter. Use a value in the range of 1K to 10K depending on
what yields the best results.

The heart of the PL tone generator is the ultra
stable 567 IC which is readily available from JDR Microdevices or
Ocean State Electronics. All other parts are available at Radio
Shack.

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 which turning R1. As soon as the
correct frequency is obtained, the squelch will open on the
transceiver with CTCSS decode. Now adjust R4 for the correct
audio level of the tone. The tone level only needs to be loud
enough for the other receiver to decode the tone and not loud
enough for others to hear it. If the tone can be easily heard
then the level is probably too high.

IC’s are Available from JDR MicroDevices (1-800-538-5000) or
Ocean State Electronics. The author will make this chip available
to RASON members for about $1 each if enough inquiries are
received.

Parts List

R1
10K 15 turn trimmer pot (271-343)

R2
470 ohm � watt resistor

R3
10K ohm � watt resistor

R4
1K ohm trimmer pot (271-280)

C1
1 uF Mylar, Polyester or Metal Film Capacitor
(272-1055)

C2, C3
10 uF Electrolytic Capacitor (272-1025)

C4
.01 uF ceramic Capacitor (272-131)

C5
4.7 uF Electrolytic Capacitor (272-1024)

D1
1N4735 6.2 volt Zener diode (276-561)

Q1
2N3904 or similar transistor (276-2016)

73 –



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