Twin touch paddle with capacitive sensing, version 3 with integrated keyer

Good keyer paddles are often master pieces of mechanical engineering. One needs also good

machinery and tools + good skills if you want to make your own.

As mechanical engineering is not my strength, I have tried to go around those mechanical

challenges and replace them with electronics. Over the years I have tried many such solutions, including infrared sensing.

The results have been at best only moderate. So I have continued using commercial mechanical paddle even my

preference is home brew.

I have seen some articles in the net about capacitive keyer paddles. Those have been done with charge transfer devices QT113

manufactured by Quantum Research . One such article was written by KG4JJF and published in QST 3-2007.

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

with CMOS logic circuits. The results were so good that I want to share my solution here.

My core component is CMOS dual NAND-gate with Schmitt-trigger inputs. It is used as an oscillator, monostable

multivibrator and NAND gate, even two functions at the same time.

The schematics explain it all. X1 and related components make a square wave oscillator. Its frequency is about 13kHz and .

pulse ratio 4 - 6 at point "In". During the shorter, negative going pulse paddle capacitance is charged via R0.

Depending on paddle capacitance value, voltage "Out1" at gate X2 may reach logic 0 or 1.

"Out1" is read at the end of charging period. Timing pulse "Out2" for that is generated from up going edge of oscillator signal "In".

If paddle capacitance is low, less than 20pF, Voltage "Out1" is logic 0 and gate X2 gives logic 1 during reading. That do not

activate the monostable X3 and no signal to keyer is generated.

If paddle capacitance is high, more than 30pF, Voltage "Out1" is logic 1 and gate X2 gives logic 0 during reading.

Logic 0 at X2 output triggers monostable multivibrator made around X3, which gives a positive pulse, 110uS long.

This pulse is 1.5 times the cycle length of the oscillator, which means that continuous logic 1 is maintained as long

as paddle capacitance has its high enough value. The multivibrator drives MOSFET X6, which conducts and drives the normal

keyer.

In twin paddle similar channels are for dot and dash keying. This paddle works with all keyers which assume grounding inputs.

Proto 1 electronics was build ugly style. That helps to keep stray capacitances low, as we are here measuring

capacitances in 20-40pF range.

One shall understand that the capacitance of our body we are observing here, is against ground. So the paddle ground shall be

somehow connected to real ground, at least via large capacitance (>300pF). It can be via transmitter antennas or mains grounding.

If you try to transmit CW with battery operated VHF rig with whip antenna, this paddle may not work. There will not be

enough capacitance from paddle electrodes via your body to your VHF rig, which is floating and isolated from the ground.

I can use this paddle with linear amplifier without any problems. My paddle is very rugged and pleasant to use, even ugly.

I would say that it compares very well with commercial products like Bencher, which I also use.

24.10.2008

Pekka OH1TV

5.11.2008

version 1.1

10.11.2008

16.8.2010