The first version of my Pressure paddle (written up here) was released in early 2019. In using this version I noticed something interesting – holding both paddles continuously (iambic sending) kept working indefinitely, even though the storage capacitor would discharge and they should have stopped after around 5 seconds. This caused me to examine this further and I came to the realisation that the paddle could be greatly simplified.

There is an interesting property of MOSFETs: if the gate and drain of an enhancement mode mosfet are connected together, then the voltage across the drain-source junction will equilibrate to the threshold voltage at the current being passed through the device.

This mechanism allows an extremely simple circuit to be created to achieve reliable switching of key lines. Here’s the new schematic:

How does it work?

In this circuit, S1/S2 are the pressure sensors as used in the original design. They have an extremely non-linear pressure-resistance characteristics. With no pressure, their resistance is > 10 Mohms. With even light pressure, the resistance reduces to less than 20 kohm.

If we look at the non-activated state, the sensor is essentially an open circuit and the gate of the mosfet will be pulled to ground through R1/R2, switching off the mosfet and the key line will float high by the action of the pull-up resistors inside the transceiver.

When the sensor is lightly pressed, the resistance drops to say 20 kohms. This will raise the gate voltage of the mosfet to greater than 95% of the drain voltage forcing the mosfet to conduct. As the mosfet switches on, the decreasing resistance of the mosfet will drop the drain voltage towards zero. However, as it reaches the threshold voltage of the mosfet, the resistance will start to rise until an equilibrium is reached close to the mosfet threshold voltage. As long as this equilibrium voltage is below the Low threshold of the key sense line, then it will be “read” by the microprocessor as key down.

Most processors operate between 3 and 5V Vdd and for CMOS logic, the low threshold is typically 1/3 of Vdd. Taking the worst case of 3V logic, we need a mosfet threshold voltage of below 1V for reliable operation. The chosen mosfet (BSS806N) has a threshold voltage of 0.5-0.9V which is low enough to operate reliably with 3V logic. Note that there are many similar mosfets that would work just as well (e.g RYC002N05, DMN2024U, etc.)

It is interesting to note that the operation of this circuit is almost independent of the size of the pull-up resistor used for the key lines. I’ve tested it with values between 4.7k and 100k with voltages between 3 and 5 volts and it works reliably for all. However, do not try and use this for vintage rigs with high voltages on the key line. 6V is the maximum key line voltage that this design is specified for.

The only other component in the circuit is C1/C2 which is there to bypass any RF that may find its way to the gate of the mosfet. This capacitor combined with R1/R2 sets the release time of the circuit to around 170us which is short enough not to be a problem at even high keying speeds.

Construction

Similar to the first version, I’ve designed two versions of the PCB to accomodate individual preferences as to board length. Due to the many fewer components in this design, I have been able to include mounting holes without the need for separate mounting flanges for anyone who wants to attach it to something.

As previously, the simplest packaging is to simply use a couple of layers of heat shrink tubing to protect the components and provide a grip. If you prefer something a little more substantial, you can either mount the board to a wood or plastic carrier of some type, or sandwich the boad between two pieces of closed cell foam and hold it together with heatshrink. Have a look at VK3PF’s blog to see this style of packaging.

The PCB is designed to have a 3 core cable attached directly and held in place by a small cable tie through the provided holes.

The whole paddle and cable weighs in at just 14g (small) and 19g (large), so is probably about as light as you can get for a portable CW paddle.

I have provided build data below if anyone wants to try it out.

Appendix – Build data

PCB gerber files:

There are two versions of the PCB available. There are two lengths available: 63mm and 100mm. Both are 22mm wide.

Here are the links to the gerber files:

• V2.0A – 63mm long
• V2.0B – 100mm long

For PCB manufacture I use AllPCB.com or JLC PCB which will both produce 5 of these boards for less than US$5 plus shipping – they are located in China and provide fast turn around and excellent quality in my experience. Note that when ordering, the board sizes are:

• V2.0A: 22 x 63mm
• V2.0B: 22 x 100mm

Bill of materials:

The following table contains all the components together with stock numbers for Digikey. Note that if stock shortages result in some components being temporarily unavailable, it is fine to substitute components with similar specifications. As noted above, the key specification for the MOSFETs is the threshold voltage which should be below 1V.

Part	Qty	Value	Case	Desription	Manufacturer	Manufacturer part no.	Digikey number
Q1,Q2	2		SOT23	N-Channel MOSFET, 20 V	Infineon	BSS806N	BSS806NH6327XTSA1CT-ND
C1,C2	2	10n	805	Ceramic NPO capacitors	Würth Elektronik	885012007009	732-7579-1-ND
R1,R2	2	470k	805	1% resistor	Yageo	RC0805FR-07470KL	311-470KCRCT-ND
S1,S2	2			Force sensor	DFRobot	RP-C18.3-ST	1738-SEN0294-ND

Pressure sensors:

The sensors used in this project are now available from Digikey, but may also be found on Amazon, Aliexpress and Ebay. I suggest you Google: RP-C18.3-ST

Construction hints:

• All components are located on one side of the board except for the dash pressure sensor
• The component placement is clearly marked on the PCB, so placing components should be straight forward
• Suggest fitting the surface mount components first using solder paste, a hot plate and hot air gun – the component order is not important
• Given the simplicity of this board, you should be able to successfully build it with a fine tipped soldering iron by tinning one pad of a component, placing it and re-melting the solder, then soldering the remaining pins
• Before you fit the pressure sensors, check their resistance with no pressure applied. It should be very high (>10 Mohm). I have occasionally come across faulty sensors with much lower resistance.
• Fit the sensors – they have adhesive already on the sensor covered by a protective paper. Remove the protective paper and align them carefully to each side of the board with the contacts centred on the corresponding pads and solder.
• Attach the cable last with whatever plug your transceiver requires. Hold the cable in place with a small cable tie (zip tie) through the holes provided