DJ5IL radio topics: CapKey+ 1

CapKey+An improved

PIC-based capacitivesensor-paddle for electronic

Morse code keyers

the sensor-paddle evolutionEarly sensor-paddles used by radio amateurs

worked with the skin resistance, which is a widelyvarying parameter and makes two electrodes neces-sary - in the form of split or meandering electrodes orsolid electrodes and a conductive hand pad. Theywere notoriously unreliable and no pleasure to oper-ate, and that's why they gained a bad reputation.

Being a passionate high-speed CW operator, in thelate 1990s I had the idea for a sensor-paddle basedon the self capacitance of the human body to virtualground, which has a much more settled value about100 - 200 pF. Imagine a 40 x 30 mm conducting sensorplate covered with a protective 0.1 mm self-ahesiveplasitic foil. It has a typical capacitance of less than 5pF to ground which increases by about 2 pF when itis lightly touched with a finger. It follows that irre-spective of its principle of operation a sensor-paddlemust be able to reliably detect a capacitance changeof merely 2 pF, which is not a trivial task.

So I developed a simple circuit around two CMOSICs 4093 / 4013 with an 18 KHz square wave chargingand discharging the sensor capacitance through a

variable series resistor for sensitivity adjustment.Touching the sensor increases its capacitance and sothe time constant of this series RC network. The resultis an increased rise / fall time of of the voltage at thesensor capacitance, and if it lags the edges of thesquare wave by a certain amount a touch of the sensoris detected.

This initial circuit proved to be reliable and precisein operation, and furthermore one single electrodeper sensor now did the job - no slits or meanders, noconductive hand pad was necessary and the sensorscould even be varnished or covered with plastic foil.Since then all my mechanical paddles collect dustbecause I do not use them any more. However, itturned out that it worked without problems only inmy preferred keying mode, which is plain iambicwithout dot/dash-memory, but regardless of the keyerused it did not allow modes with dot/dash-memory(iambic type A = Curtis-keyer / type B = Accu-Keyerand ultimatic). The reason is that in these modes thepaddle is polled while the transmitter is keyed, andeven very low levels of radiated HF coupled into thecircuit simulate a touched sensor already when afinger is brought only close to it.

My second circuit appeared in 2004. It was basedon a design by Milt Cram, W8NUE, which used theLTC1043 (a quite expensive switched-capacitor in-strumentation IC) together with a dual op-amp LMC6462 and worked with the capacitance between twoelectrodes which increases when they are touched 1.I modified Milt's design so that it works with the hu-man body's self capacitance to ground and thereforeneeds only one single elctrode per sensor, exactly asmy initial circuit did. However, its principle of oper-ation is totally different: the sensor capacitance CS isalternately switched at a rate f0 of about 50 KHz bet-ween the supply voltage V and the parallel RC networkof a 22 nF reservoir capacitor CR and 100 KΩ resistorRR so that the circuit works as a switched-capacitorcharge pump. 50,000 times per second the very smallswitched (or "flying") sensor capacitance is at first fullycharged to the supply voltage and then it dumps itsabsorbed charge into the large 22 nF reservoircapacitor, which is like filling a large water basin witha small bucket.

The switched sensor capacitance may be consid-ered to be equivalent to a resistor RS with a value of1 / (f0 CS) Ω which together with the 100 KΩ reservoirresistor RR works as a resistive voltage divider, so thatthe voltage across the parallel RC network settles at avalue of V x (RR / (RR + RS)). It takes many cycles untilthis voltage is reached, but after that initial chargingphase it follows small changes of the sensor capaci-tance quite rapidly, so that by comparing it with anadjustable reference voltage the touch of a sensor

Karl Fischer, DJ5IL, Friedenstr. 42, 75173 Pforzheim,Germany, DJ5IL@cq-cq.eu, www.cq-cq.eu

can be reliably detected. And because fast time-varying signals are effectively absorbed by the largereservoir capacitorCR , this charge pump approach hasa high intrinsic immunity to radiated and conductedHF, so that it can be used in any keying mode.

Both circuits soon became quite popular amongGerman CW operators, printed circuit boards ap-peared on the scene and even completely assembledkeyers were manufactured and offered by radio ama-teurs. Capacitive sensor-paddles gained popularityalso abroad and a number of articles were publishedin amateur radio magazines. Simultaneously indus-trial sensor applications became widespread withspecialized ICs appearing on the market. One popularexample is the QT113 , a charge-transfer touch sensorIC made by Quantum / Atmel. However, the responsetime of this sensor is typically in the order of tens ofmilliseconds and it also exhibits a threshold hysteresisbetween "make" and "break" which makes it well suit-ed for controlling electric devices but less for serioushigh-speed CW operation.

In 2011 I developed CapKey, a capacitive sensor-paddle based entirely on software logic utilizing onesingle 8-pin PIC 12F683 microcontroller instead ofdiscrete and specialized ICs. Being self-calibrating itneeded no adjustments and responded about 100times faster than the QT113 without any hysteresis.The sensor capacitance is cyclically at first quicklydischarged and then charged through a large 1 MΩresistor while a charge counter is incremented in stepsof 0.5 µs until the voltage at the sensor equals abouthalf of the supply voltage. And if the charge counterexceeds the calibration counter of the untouchedsensor by a certain amount, a touch of the sensor isdetected.

This principle of operation is able to detect verysmall changes of capacitance very quickly, but it hasa drawback: the positive halfwaves of even very lowlevels of radiated HF coupled into the circuit interrupteach charging cycle at its very beginning and thusinhibit the detection of a touched sensor. A largecapacitor in parallel with the sensor which could ab-sorb fast time-varying signals is not feasible, becausecontrary to the charge pump approach it must becyclically charged through the 1 MΩ resistor (of whichthe value can not be reduced without degrading thesensitivity), and therefore the time constant and withit the response time would vastly increase. So againlike my very first CMOS circuit it worked without prob-lems in plain iambic mode but did not allow modeswith dot/dash-memory.

What follows is the description of the latest evolu-tionary step, its improved successor CapKey+ whichnow emulates the charge pump approach based onthe mofified design of W8NUE. One cheap 14-pin PIC16F684 microcontroller replaces the 18-pin LTC1043and 8-pin LMC6462 in a very simple circuit with onlyfew discrete components. And it has the same high

intrinsic immunity to radiated and conducted HF, sothat it can be used in any keying mode.

circuit detailsThe circuit is shown in the above figure, program-

med PICs 16F684 are available from the author andthe firmware hex-file can be downloaded 2 for perso-nal non-commercial use. The supply-voltage has arange of 3.0 to 5.5 V, I recommend 3 AA batteriesgiving 4.5 V with a current drain of approx. 1 mA whichis very low but still makes a switch necessary. DuracellMN1500 batteries for example should provide morethan 1000 service hours. The transistors are general-purpose NPN types. Only if the controlled keyer ispowered by the same voltage source, the twotransistors together with their 10 KΩ base resistorscan be omitted and pins # 2 and 13 connected directlyto the associated keyer inputs with only one 10 nFbypass capacitor from each pin to ground. The firm-ware detects pulled-up keyer inputs and deliversactive low signals to pull them down instead of activehigh signals to make the transistors conduct. Thesensitivity trimpot is a multi-turn type for higher ad-justment resolution, its value is not critical but shouldbe in the range 50 KΩ to 1 MΩ. The two 4.7 nF reser-voir capacitors must be high quality film / foil caps,do not use cheap ceramic capacitors instead. The 10KΩ resistors to the sensors were added to furtherimprove HF immunity by forming an RC low-pass filtertogether with the large reservoir capacitors.

In order to work reliably, the circuit must be ableto make use of the human body capacitance to groundor at least to a substantial conductive structure. There-fore paddle-ground must be connected to station-ground which is usually provided by the shield of thekeying line.

construction notesBecause the circuit is so simple and construction

straightforward, no detailed description and no circuitboard layout are presented here. Instead of an etchedboard you can use a small piece of perfboard, or usesingle-sided circuit board material and mill out thecopper traces with a dremel tool as I did with my pro-

DJ5IL radio topics: CapKey+ 2

DJ5IL radio topics: CapKey+ 3

totype shown on the above pictures - my preferredconstruction method for simple sircuits. But do nottry to use a solderless plug-in bread-board (plug-board), because the significant stray capacitance andhence coupling between its connection strips makesit impossible for the circuit to work reliably.

The sensors plates can be made of any conductivematerial, for example aluminium stock or single-sidedcircuit board material. In the latter case, the coppershould be varnished or covered with self-adhesive

plastic foil with max. 0.1 mm thickness, otherwise thecopper will be weared off after only a few months ofintensive use. Varnish or plastic foil reduces thesensitivity, but the paddle still works perfectly well.The sensor plates can be of any shape, but becausethe capacitance between them should not be higherthan approx. 1 pF the following relations betweensensor-area Sa and sensor-clearance Sc must be satis-fied:

Sa [mm²] <= Sc [mm] x 113 mmSc [mm] >= Sa [mm²] / 113 mm

For example, if each sensor has an area of 40 x 25mm = 1000 mm² as in my prototype shown on thepictures, their clearance should be at least 1000 mm²/ 113 mm = 9 mm. If both dot and dash are triggeredwhen only one sensor is touched, the sensor area Samust be decreased or the clearance Sc increased.

Please note that the circuit traces and wires whichare in contact with the two sensors are also touch-sensitive and increase their capacitance, which whenuntouched should be as low as possible and differ bythe least possible amount in order to get about thesame sensitivity for both. Therefore the surface areaof these traces and wires should be as small as pos-sible and they should have a clearance of at least 5mm to conductive cabinet parts. Furthermore, thelayout of the traces and wires connecting pins # 5and 9 with the sensors should be as symmetrical aspossible. If wires are used to connect the sensors withthe circuit, they should be stiff, as short as possibleand at least 5 mm away from each other and fromother conductive parts.

Sensitivity adjustment should be done with thekeyer connected to the transmitter, so that paddleground is connected to station ground by the shieldof the keying line. Set the keyer to iambic type A or Bmode. To check the sensor capacitance symmetryleave both sensors untouched and adjust the trimpotso that only dots or only dashes are just starting to begenerated, then advance the trimpot so that both dotsand dashes are just starting to be generated. The smal-ler the difference between these two setpoints thebetter the symmetry of the sensors. Finally adjust thetrimpot so that dots and dashes are reliably generatedas long as the associated sensor is lightly touched.

The oscilloscope screenshots on the next pagevisualize the operation of the circuit and can help youto troubleshoot it. A unique PIC-based multi-modeMorse code keyer that works perfectly together withthis capacitive sensor-paddle is described in my article"Keyrama" 3.

Prototype shown from rear top (above) and rear bottom (below).Two single-sided PCBs are solder-joined forming a "T", one holdsthe sensors and the other one holds the circuit. The sensors aremade of anodized aluminium angle stock. The copper traceswere milled with a dremel tool.

DJ5IL radio topics: CapKey+ 4

references1. Cram, Milt, W8NUE, "The NUE Key - An Electronic Touch SensorPaddlle", QST, July 2004, page 28.2. http://cq-cq.eu/capkey+.hex (click URL to download)3. http://cq-cq.eu/DJ5IL_rt008.pdf (click URL to open)

Probe connected to a 4.7 nF reservoir capacitor which is chargedby an untouched sensor plate. The voltage of the square wavealternates between a high value of +5.0 V (supply voltage) whilethe sensor capacitance is charged and a low value of +2.0 Vwhen it has dumped its absorbed charge into the reservoircapacitor.

Now the sensor plate is touched and its increased capacitancecauses a higher voltage of +3.2 V when it has dumped its ab-sorbed charge into the reservoir capacitor. The time constantof the touched sensor increased significantly so that chargingtakes more time, but it can still be charged to the full supplyvoltage of +5.0 V.

Now the 4.7 nF reservoir capacitor is charged by a touched sen-sor plate and its increased capacitance causes a higher voltageof +3.2 V when it has dumped its absorbed charge into thereservoir capacitor.

Two probes connected to a 4.7 nF reservoir capacitor (magenta)and to the asscociated pulled-up keyer input (yellow). Note thatthe scale is now 1 ms / div. so that what looks like two magentatraces is actually only one single trace of a square wave alter-nating between its low and high value like on the two previoussreenshots. About 5 ms from the left edge of the screen theassociated sensor is touched and the low value of +2.0 V startsto rise, after 2 ms ("make" response time) the touch is detectedat a threashold of +2.5 V and the keyer input goes down.

Probe connected directly to an untouched sensor plate, thevoltage varies between +5.0 V (supply voltage) when the sensorcapacitance is fully charged and +2.0 V (voltage at the 4.7 nFreservoir capacitor measured through the 10 KΩΩΩΩΩ resistor) whenit has dumped its absorbed charge into the reservoir capacitor.The sensor capacitance is charged / discharged through the 10KΩΩΩΩΩ resistor and the time constant of this series RC networkcauses the deviation from an ideal square wave. The horizontalscale is 2 µµµµµs / div. so that one complete cycle takes 10 µµµµµs whichgives a rate of 100 KHz.

At the left edge of the screen the associated sensor is releasedand the low value of +3.2 V starts to fall, after 7 ms ("break"response time) the release is detected at the same threasholdof +2.5 V and the keyer input is pulled up again.

DJ5IL_rt011.pdfOriginal version: 16.6.2020Revisions: