Tone Keyer For
Morse Code Programs
By N1HFX
Recently, I received email from an
amateur wishing to key his transmitter from the tone output of a
Morse Code generator program. Although it is possible to connect
to a COM port on the back of a PC using a diode-transistor
configuration, this results in another precious COM port being
used up. The tone keyer is an ideal alternative and works very
well.
The circuit in Figure 1 is actually very similar to a circuit I
used for an earlier VOX system. The circuit uses a 741 Op Amp as
a comparator. Approximately 1/2 of the supply voltage is set for
pins 2 and 3 of the Op Amp. The voltage across pin 2 is actually
slightly higher than pin 3. This results in the output voltage at
pin 6 to remain low with no signal.
When an audio tone is applied to
C1, the voltage across pin 3 of the IC is slightly higher than
pin 2 causing the output at pin 6 to go high. Because the output
of the IC is pulsating in accordance with the tone frequency,
capacitor C2 filters out the 700Hz output. Diode D1 prevents C2
from discharging through pin 6 when it goes low. Resistors R6 and
R7 form a voltage divider network since the voltage from the IC
could be as high as 1.5 volts in a low state. This keeps the
voltage at the base of Q1 below .7 volts in a no signal
condition. Transistor Q1 keys the transmitter by causing the PTT
line to go low when a signal is present.
This circuit will work well using the output from amplified
speakers or even connected to your PC speaker. The components are
tailored for a 700Hz tone. If a lower frequency tone is used, C2
may need to be increased in value. R4 should be adjusted to just
before the PTT line goes low. Some additional adjustment may be
needed for best results.
The above circuit works well and does not tie up another one of
those precious COM ports since many computers only have two
available. If a high power transmitter is used, place this
circuit in a shielded enclosure to prevent RF from affecting its
operation. I have not tested this circuit at speeds higher than
20 words per minute. Of course, if you are able to copy at that
speed then a circuit like this is probably not needed.
DE N1HFX