Following on from the simple CW touch keys in the previous page, this page shows a capacitive touch project which is the ideal touch sensitive key solution. The project described here is a iambic capacitive touch key which uses 2 AA size batteries for 3v power, draws <1mA current and has an ‘on’ switching resistance of <0.1Ω with a 3v supply.

The circuit diagram schematic is shown below.

Figure 1: Capacitive touch key circuit. Data sheets: AT42QT1011 | IRFML8244TRPbF

The circuit in figure 1 uses all surface mount (SMD) components. One reason for this is that the IC is only available in surface mount, but the reason for the use of SMD for the whole circuit follows:

The problem with using 3v is choosing a MOSFET with a low enough gate threshold voltage. Whilst searching for suitable MOSFET’s, it became clear there was no good candidate which was not SMD. Its a fact nowadays that SMD components offer much greater choice over their through-hole ancestors.

The MOSFET that I chose was the IRFML8244TRPbF which has a typical gate threshold voltage of 1.7v and a very low on resistance and comes in a SOT-23 SMD package. It was therefore decided to design the whole circuit around SMD parts. Many other logic MOSFET’s will work ok in this circuit, such as the IRL540, which comes in a TO-220 package and will switch up to 28A!

So, I had designed the circuit, tested the circuit, all that was left was to design the SMD circuit board, which I did with Eagle. The testing board I etched with photo resist PCB and developer. It worked fine and looked OK, but I decided as I had put the effort in to designing a board for this project, I would get some PCB’s professionally manufactured. The final design can be seen below. The PCB dimensions are 57mm x 37mm.

The completed board. 

If you would like to photo etch a board yourself, a mirrored PDF file can be downloaded here. I have no boards left for sale at the moment.

Below is a video of how to solder the board using solder flux and hot air.

Below are some photos from people who have made keys with this board…

Built by Chris, M5LRO

Will, PA3Q

Wil PA3Q and his Elecraft K1

Michael OZ9AEW built his from a HTC cellphone box!

Below is a video from Stefano, IK5XCT who built the key. An interesting story is that as he says on the YouTube page, he had a contact using the key with DK7FH, who was also using one! Thanks Stefano!

Stefano’s key

IK5WOB Fabrizio made an interesting looking key!

Frank DK7FH built it in a small steel container. He has some more QRP projects on his QRZ.com page.

Istvan HA5CLF made a nice small portable key

Istvan HA5CLF spent some money on his paddles! He used two coins for touch surface. They are 20 Ft each = 0.05 GBP!

If you decide to build any of the circuits listed on this page, I would be interested to know how you got on. Please drop me an email by going to the contact page. 73 de MØUKD.

Here are some simple CW touch keyer circuits for making your own CW touch paddle key. The first circuit in Figure 1 is very simple, just consisting of two NPN transistors! It works by using the 50Hz mains voltage that is induced into your body by nearby mains wiring to provide a small base current which switches the transistor on just enough to key some transceivers in some situations. I say some, because you will likely need to be indoors and your transceiver probably needs to be connected to a mains power supply for this to work. It’s not ideal, but it is super simple and if it works with your setup, great! This is what I am using in the video below.

I have tested this indoors with three Icom transceivers, the IC-703, IC-706MKIIG and the IC-7400, all of which worked fine. The video above shows the super simple key being used with an IC-7400. The Icom transceivers will key with a very high resistance, which is why this circuit works with them. Try it if you use an Icom indoors or scroll further down for more options!

Figure 1: Super simple touch key circuit

OK, so what if you have a Yaesu or some other transceiver? Well, using the circuit in Figure 2 will allow a much lower resistance to key the transceiver than that in Figure 1. It is essentially the same circuit, but to increase base current, you connect yourself to the positive of the same power supply that is feeding the transceiver (note: positive 13.8v NOT mains power!) and use your body as a base resistor. This means you dont need the 50Hz mains induced onto the body, so it can be used whilst operating portable. The downside of course is you need a wrist strap (such as an antistatic strap) connected to positive. This gives a keying resistance of less than 1kΩ, which Yaesu transceivers seem to be happy with. I tested this with the FT-817 and it worked fine. The wrist strap can go on either wrist, ankle, or any part of the body really! Another option would be to make the key in a metal enclosure and hold it whilst keying. Neither option is ideal, obviously!

Figure 2: Super simple touch key circuit with wrist strap

So, what if 1kΩ is not low enough for your transceiver? If you are going to have a wrist strap, you may as well build something that switches on with a very low resistance, so lets throw in a MOSFET after the transistor and include a 9v PP3 battery to assist in base current. The circuit in Figure 3 will provide a key resistance of 0.5Ω which will be low enough to key any transceiver, or even switch it on and off! This may be preferred to the circuit in figure 2 anyway, as it means you can plug the wrist strap into the box that the key is built into, rather than running a cable to the power supply. A simple bannana jack could be used. No power switch is required, as the circuit draws zero current when unkeyed and an incredibly low 8µA per circuit when keyed (8µA is 0.008mA). The power consumption of 8µA is because of the 1MΩ gate discharge resistor. This means that a typical 550mAh alkaline PP3 should last almost 8 years of continuous keying!

Figure 3: Simple touch key for all transceivers.

Components are not critical. Many different types of MOSFET can be used. The IRF510 is overkill for this requirement, but its cheap and has a very low on resistance of 0.5Ω. I also tried an IRF540 which has an extremely low 0.07Ω on resistance and can switch up to 28A at 100v!

Logic level MOSFET’s such as the IRL510 may also be used and these will happily switch on with only 3v gate voltage. If a logic MOSFET is used, you may if you wish, substitute the 9v PP3 for a 3v lithium cell such as the CR2032. Old motherboards provide a good source of logic level MOSFET’s, although SMD types. They can be easily removed with a small flame torch. Just google the part numbers!

A very cheap logic level MOSFET for this project is the 2N7000, which has an on resistance of 5.3Ω but you can buy 10 for around £1.

OK, so how do we make a CW touch keyer without an annoying wrist strap? Capacitive touch is the answer! The circuit below uses all surface mount parts, and this project has its own dedicated page!

Click the circuit below to view this complete project!

Capacitive Touch Key Schematic. Click to view project.