28 February 2021

Checking a static discharge trick with the NanoVNA at UHF

Today I received a question about a post from 8 years ago: how to tame the static build-up in a GP or collinear antenna. Back then I had done some research and concluded that a high value non-inductive resistor could be used, especially at low TX power or RX only.

Now that I own a NanoVNA I can confirm that the impedance is not affected. I quickly reused the LoRA antenna from last Summer, which turned out to be resonating at 750 MHz (if we trust the NanoVNA). This is the curve on a 300 MHz span (well, it stops at 900 MHz on the right):

Just the "open" GP antenna.

Then I added a 10 kohm 1/4W resistor across radiator and radials and the result is almost identical (the marker has moved):

After adding the 10kR across. See next picture.

The very (quick && dirty) antenna under test.

So, if we trust the NanoVNA at the upper limit of its working range, this trick does not compromise the impedance. I might also have been lucky on the single experiment I've made, so do your measurements first.

09 February 2021

Sorting a box of vacuum tubes

So, there comes the time you become the owner of a shoebox full of almost-no-value one-hundredish vacuum tubes and you need to sort them out. I do want to use (some of) them, not immediately. I also have no facilities to keep them sorted in a logical way, so I had to do it efficiently.

First of all I separated the magic eyes, since they can be easily recognised and tested.

Then ... I simply transferred the tubes one-by-one from one box to another and typed the part number in a spreadsheet, when still visible. Those tubes without a usable marking have been compared physically & visually to others and some got a supposed part number with a marker pen. A dozen were left anonymous, separated from the rest, for other projects.

Once all the names are in a spreadsheet just create a pivot table to see how many different types you've got, and how many each.

Now go back online and look for projects with those tubes :)

05 February 2021

Freeform two transistor LED blinker completed

My first freeform electronics sculpture is finished! Visually it is a replica of Mohit's work with a change in the diagram (one resistor less). The LED blinks approx once every second, and blinks become more frequent as the available voltage decreases towards LED's forward voltage.

The 1uF capacitor that sets the frequency is charged though the leakage current passing through the LED. In ultrabright clear-case LEDs you see a faint emission in this phase (dark room implied). Then you are blinded by the following pulse :)

I also added a 1F 5.5V supercapacitor that keep the blinker running for more than 12 hours with a white LED. If a red LED is used, it lasts 24 hours.

If a solar panel is added in parallel to this circuit to top up the supercap during daylight (add a diode to prevent discharge back to the panel), basically it lasts forever.

The stand-by current is fractions of microamperes [uA] and the average lies around 30 uA (calculated). Not easy to measure, huh!

The hardest parts are to get straight wires and component leads and to keep everything in place when soldering when both your hands are needed to hold the iron and the solder. Then you need to make clean solder joints.

While is may seem a useless exercise, freeform electronics forces you to think out of the box even if you are reproducing someone else's work. Give it a try.

PS: working circuits impress 10x fold the audience.

03 February 2021

Freeform two transistor LED blinker

Two transistor LED blinker, construction in progress.
Intermediate result.
As announced, I wanted to try Freeform Electronics Art. My last PCB designs for clocks were meant to be artistic so that they could be displayed to the public even without a case. Freeform Electronics uses no PCB: it's all self-standing and built "in the air".

I chose to replicate Mohit Bhoite's two-transistor LED flasher because of its simplicity and I had all parts already at home.

I deliberately chose to ignore some of his planning and construction techniques (shared in videos featuring him and his art) so that I would learn more of and during the process. Moreover this blinker is very simple, forgiving and lightweight and almost any building technique should work.

Two transistor LED blinker, construction in progress.
Another view, next to an SD/TF card for size comparison.

Pictures show an intermediate result with 2N5401, BC347C and three resistors. I am half-way to the end, component-count-wise.