Build A 3D printed Telegraph Key

Some one asked me to share  the files for my  3D printed telegraph key. So I am going to do that.

Here is a link to the STL file for the key parts.

There are 3 files that print the base, key and upper bracket separately. All of the holes are included but they are undersized. Some you will drill larger and others are tapped.  There is a horizontal hole in the key to run a wire back from the top key contact and the same is true for the base. I printed at 100% fill using ABS. HIPS would work well also. The magnetic damping instead of a spring works really well. The 4-40 contact screws need to be filed flat after being nipped to size.

Required Hardware:

2″ of brass 1/8″ rod  (for hinge point)

2     3/8 6-32 screws  (front legs)

2      1/2 6-32 screws (mounting upper bracket)

3     3/4 6-32 screws (Key adjustment and back legs)

7     6-32 nuts

2     6-32 knurled nuts

2     1″ 4-40 screws (for key contacts and holding magnets…will be trimmed)

2     4-40 nuts

misc  4-40 washers for spacing adjustment

2    .6″ diameter rare earth disc magnets with center hole

24  gauge scrap wire


bottom view

Bottom View

top bracket

Top Bracket

contact view

View of Magnets and Contacts

code key side

Side View

code key back

Back View


25 Watt Hybrid EL84 Tube Amp

This is my new  hybrid guitar tube amp which utilizes a solid-state input stage, DSP reverb, and solid-state phase splitter. Only the push pull, class AB output stage utilizes tubes, namely two EL84’s run at 390 volts with cathode bias. The bias uses two 15 volt zeners which creates a bias current of about 26mA. This requires almost 30 volts of swing on the grids to drive the amp to saturation. This is accomplished with a little switch mode boost converter that generates 29 volts to drive the phase splitter opamps. All of the solid-state circuitry runs off the AC filament supply for the tubes. The solid state portion is basically my stomp amp design( also on this blog) minus the final power amp, which is replaced with the phase splitter.

A couple notes about the design: Using zeners works great, but they can fail(haven’t had a problem  yet) and typically they fail by shorting(very bad for the tubes!) it may be prudent to parallel with 1k ohm  resistors and .1 uF caps to make them more tolerate of current or voltage spikes. I use 5 watt zeners and have yet to have one blow on me with numerous amp designs.

Also the gain distribution is not ideal. This is because of the low headroom of the FV-1 reverb IC which runs at 3.3v. This requires that there be lower gain in the first two stages than is possible – degrading noise figure somewhat. Despite this, the amp is very quiet – even with noisy un-bypassed zeners in the final bias circuit.




completed amp head
Front View of Amp Head
back of amp
Back View
completed amp circuit
Complete Circuit
hybrid amp circuit board
Solid State Circuit Board


Amp Demo:

Coming Soon! New Hybrid guitar amp, New version of my 1/4 portable antenna, Completion of my Antenna Analyzer

Well I am a little behind schedule! I started a new company: Global Technology Integrators…and I am swamped! But later this summer things will settle down and I am going to get to more projects! I feel bad because I like to crank out ideas and experiments as fast as I can! I am working on some really neat stuff and I am excited to eventually I will inject some fresh DNA into the site. I intend to finish my antenna analyzer, I am also working on a 20-30 watt hybrid guitar tube amp with el84’s and an LND150 fet front end. Finally, I am doing more antenna experiments that look very promising.

Forget the Bridge! Determine SWR with a Resistive Divider.

A resistive bridge type SWR meter doesn’t actually measure SWR – it measures impedance mismatch or impedance(however you want to look at it). SWR is calculated by the ratio of two volatges on the bridge. The SWR and impedance only relate correctly if the bridge impedance and feedline impedance are the same(50 ohms typically).  A resistive bridge works fine but you can do the same thing with  a simple resistive divider. When using a divider, you can use any value resistor you want(50 ohms in my design) and as long as you know the characteristic impedance of your feedline, you can correctly calculate the results with any feedline impedance (not just 50 ohm coax).By measuring three voltages across the divider(Vs, Vr and Vz), one can easily derive: total Impedance, resistive impedance, reactive impedance, SWR, and return loss. A resistive bridge is most accurate when mismatched, but the resistive divider is most accurate when match is optimal. So ask yourself would rather know more accurately what’s happening between SWRs of 2:1 or when it’s 3:1 or greater? The only drawback of this method is that you cannot determine the sign of reactance(capacitive or inductive) directly, but you can derive all of the magnitudes. This is because when you convert the voltages to DC values by means of diode detectors, all phase information is lost.

I decided to build a  small and reasonably simple antenna analyzer for 1 -30Mhz. I wanted it to provide accurate measurement of SWR, Total Z, R and X of the ANT.  Simplicity in operation and  moderate power consumption were also design goals. The schematic below is what I came up with.

Preliminary Schematic(not built yet)

Antenna Analyzer

I have etched a circuit board and will build the analyzer in early 2016. I will update the post as I make progress. Expect some values in the schematic to change. Below is a discussion of the math required to derive all of the analyzer measurements. Vr requires a floating measurement, Vs and Vz are ground referenced.

Diagrams for Analysis

example pic

Voltage dia

Using the Voltages Vs, Vr and Vz, one can create a triangle related to the classic power factor triangle where the hypotenuse(Vz) can be  seen to be shared and the cos(ang) allows one to find the reactive and real components of the impedance. One is required to find the cos(ang) using only the lengths of the sides of the Vz, Vr, Vs triangle. These values are the measured voltages from the diode peak detectors shown in the schematic. The law of cosines provides the solution.

cos(ang)  = (Vs²+Vr² – Vz²)/ (2* Vs* Vr)

Using node voltage equations, the following relationships can be derived from the simple divider shown above. Note that the triangle legs:Vs, Vr, VZ , VZr and VZx are synonymous with Zs, R, Z, Zr and Zx respectively

The total impedance seen across the signal source (Zs):  Zs = R*Vs/Vr

The absolute value of the complex impedance seen across the load connections of the resistive divider(Z): Z = R*Vz/Vr

The real part of the load impedance (Zr): Zr = Zs*cos(ang) – R

The complex part of the  load impedance (Zx): Zx = SQRT(Z² – Zr²)

With these equations we now have total load impedance Z, the resistive component Zr, and the magnitude(but not sign) of the reactive component Zx.

If we know  Zr and Zx, we can calculate the SWR and return loss as well:

First we calculate  Γ:  Γ = SQRT( (Zr-R)²+Zx²)/SQRT( (Zr+R)²+Zx²)

Now  SWR: SWR = 1+Γ/1-Γ   and return loss: return loss = -20 log Γ

Note: R used in Γ calculation is the characteristic impedance of the feedline not the R used in the divider (in my case they will be the same: 50 ohms).

QRP On Vacation

I used my new XCVR and my short portable dipole down in Ft Pierce Fl, over Thanksgiving.  The rig, antenna, feedline and tools all fit  in a camera bag and it was easy to setup. I made some 800- 1000 mile contacts on 1 watt and drank a few beers and smoked a stogie while doing it!

Short Dipole strung between two Palm Trees


QRP While Lounging Outside



40 meter XCVR size of an Business card with 1 Watt output, 7.000 to 7.150 MHz coverage

I tweaked my previous XCVR design to use push button tuning and made the board layout extremely compact. I improved the side tone injection to be absolutely  perfect – not too loud, full break in and no clicks. I went ahead and integrated my switch cap 8 pole CW filter. The schematic is fairly simple with 65 parts or so.. and the whole design fits into a 3D printed enclosure the size of an business card. The only penalty in the design is  signal on both side of zero beat(direct conversion). There is no AM leakage though and the filter rolloff is very good. The circuit is easy to build as it required no adjustment, or alignment, but uses some fine pitch surface mount parts requiring a very precise soldering station. When I got my boards in I literally just built it – applied power and starting using it. In this one I used a ferrite toroid for the Sorta-Balun, but I have used scrap bobbins from HF power inductor for this with great success . The push button tuning works well, it has freq (up/down) buttons which shift through at 10 Hz increments for 5 sec then goes to 1 Khz. It can run on a 10 to 24 volts supply. The 8 pole audio filter is adjustable to taste, by changing one capacitor(C27), and the side tone injection level with one resistor(R12). The choice of band only requires a firmware change and 3 inductors and three capacitors – which make up the RF matching filter. Its really a unique little transceiver. Its very small, reliable and has excellent performance. The TX frequency is offset 700Hz above receive frequency, but this could be made adjustable if desired(using the push buttons and the Key) – I just havn’t done it yet.



The built XCVR

QRP UltraLite     QRP Ultralite2

I am going to provide a link to all the design files in the next few days including the 3D print files for the enclosure. I am also going to sell a couple units at cost(get rid my extra boards!) as partial kits( I will put most of the surface mount components on) and provide the switches and connectors that fit the enclosure (which I will also include). Will probably be around $70 for everything for a complete unit(except wire).