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Morse sounder Introduction When communication conditions are difficult
because of weak (speech) signals, high QRM or QRN levels, Morse code usually
is at its best allowing communication to go on long after other operation
modes have been exhausted. The soft 'bleeps' appear to be nicely tuned to our
biological hearing system, allowing us to still communicate at signal levels
far below the 'noise'. No wonder that the first wireless communications could
succeed with the primitive systems of those days. Morse code therefore still
is very much alive today at radio-communications between amateur stations. Morse sounder When practicing Morse many years ago, it was
not just sufficient to recognize these signals, but I also needed to practice
generating Morse signals as well. To get the hang of the Morse rhythm and the
right length of the pause in- between characters and words it appeared to be
very useful to compare my own Morse-writing to that of others, preferably
masters at the trade and / or Morse code lessons from course material on tape
and from club stations. The latter I also recorded on tape to practice
where-ever possible. Using all available support tools, I was able to pass
the official exams for recognizing and making 12 words per minute in less
than five month time. The simple one-IC Morse code sounder / play back device
I was using at the time is the one in the next figure. The circuit for the Morse code sounder is
very simple and is build around a very cheap hex-inverter with hysteresis,
HEF 40106, that is very permissive to the power supply (5 - 15 V.) and is
available in abundance.
In the lower left part of the diagram is the
audio-generator, consisting of a single inverter with hysteresis. When the
output of this gate is at logical '1', the capacitor 3n3 is being charged by
the set of resistors. As soon as the input is reaching the switching level,
the output will go to logical '0', with the capacitor being discharged by the
same resistor network. Discharging will continue until the second (lower)
threshold has been reached and the cycle is restarted. Oscillation frequency may be set by the Switch gate The audio-oscillator signal is fed to two
logical gates, each consisting of two diodes. When the Morse key is down, the
gates will open en the audio signal will be passed on to the left and right
selection switches. The diodes may be just any type without special
requirements. In the indicated position the audio signal
will be passed on to two buffers, each consisting of two gates in parallel
for enhanced drive capabilities to the headphones to allow a very wide range
of impedances. A potentiometer in each of the output channels may
independently set the audio-level, which is very convenient when comparing
your Morse code to an external channel, especially when operating in unison
mode to the external channel. When applying a headphone with local controls,
the 2 kOhm potentiometers may be of course be omitted. External channel In the upper left-hand side is the external
input. An analogue audio-signal at 'line' level ( 0 dBm audio level is 1 V.
in 600 Ohm) is quite sufficient to operate the input gate. At the output the
external signal will automatically have the same level as the internal
oscillator signal. Selection switch Selection switches L and R may independently
select external or the internal signal to each of the headphone elements.
When selecting a double throw, three position tumbler, the power supply
(battery) may be switched off as well in a logical sequence (internal
-off-external). Even when not switching-off the power supply, the circuit
will draw very little current with no sound from the head-phone. Bob J. van Donselaar, on9cvd@veron.nl
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