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Twin touch paddle with
capacitive sensing, version 3 with integrated keyer |
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Good keyer paddles are often master
pieces of mechanical engineering. One needs also good |
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machinery and tools + good skills if
you want to make your own. |
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As mechanical engineering is not my
strength, I have tried to go around those mechanical |
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challenges and replace them with
electronics. Over the years I have tried many such solutions, including
infrared sensing. |
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The results have been at best only
moderate. So I have continued using commercial mechanical paddle even my |
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preference is home brew. |
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I have seen some articles in the
net about capacitive keyer paddles. Those have been done with charge transfer
devices QT113 |
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manufactured by Quantum Research .
One such article was written by KG4JJF and published in QST 3-2007. |
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I wanted to test the capacitive
principle as well. But I did not have QT113's in my junk box. Therefore I did
something similar |
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with CMOS logic circuits. The
results were so good that I want to share my solution here. |
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My core component is CMOS dual
NAND-gate with Schmitt-trigger inputs. It is
used as an oscillator, monostable |
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multivibrator and NAND gate, even
two functions at the same time. |
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The schematics explain it all. X1
and related components make a square wave oscillator. Its frequency is about
13kHz and . |
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pulse ratio 4 - 6 at point
"In". During the shorter,
negative going pulse paddle capacitance is charged via R0. |
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Depending on paddle capacitance
value, voltage "Out1" at gate X2 may reach logic 0 or 1. |
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"Out1" is read at the end
of charging period. Timing pulse "Out2" for that is generated from
up going edge of oscillator signal "In". |
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If paddle capacitance is low, less
than 20pF, Voltage "Out1" is logic 0 and gate X2 gives logic 1
during reading. That do not |
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activate the monostable X3 and no
signal to keyer is generated. |
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If paddle capacitance is high, more
than 30pF, Voltage "Out1" is logic 1 and gate X2 gives logic 0
during reading. |
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Logic 0 at X2 output triggers
monostable multivibrator made around X3, which gives a positive pulse, 110uS
long. |
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This pulse is 1.5 times the cycle
length of the oscillator, which means that continuous logic 1 is maintained
as long |
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as paddle capacitance has its high
enough value. The multivibrator drives MOSFET X6, which conducts and drives
the normal |
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keyer. |
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In twin paddle similar channels are
for dot and dash keying. This paddle works with all keyers which assume
grounding inputs. |
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Proto 1 electronics was build ugly
style. That helps to keep stray capacitances low, as we are here measuring |
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capacitances in 20-40pF range. |
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One shall understand that the
capacitance of our body we are observing here, is against ground. So the
paddle ground shall be |
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somehow connected to real ground,
at least via large capacitance (>300pF). It can be via transmitter
antennas or mains grounding. |
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If you try to transmit CW with
battery operated VHF rig with whip antenna, this paddle may not work. There
will not be |
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enough capacitance from paddle
electrodes via your body to your VHF rig, which is floating and isolated from
the ground. |
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I can use this paddle with linear
amplifier without any problems. My paddle is very rugged and pleasant to use,
even ugly. |
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I would say that it compares very
well with commercial products like Bencher, which I also use. |
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24.10.2008 |
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Pekka OH1TV |
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Schematics |
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5.11.2008 |
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version 1.1 |
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Waveforms, no mark |
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Waveforms, mark on |
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Pictures of first proto |
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Pictures of second proto |
10.11.2008 |
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v 3 includes CMOS keyer |
16.8.2010 |
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Back to projects page |
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