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.



31 January 2021

QYT KT-8900 12V on the mic socket and more noise

Alright. I found a spot to get 12V inside the QYT KT-8900, which is pin 5 of the audio amplifier IC. It is located near the entrance of the DC cord. I bought it to pin 1 of the MIC socket, which is unused in this radio: that's where you can push the audio output for Packet/APRS (the documented mods mentioned in the previous post).

The result is that now the buzz is present both on transmit AND receive.

The oscilloscope explained everything. The wireless dongle generates digital spikes on its power supply line. Since it is designed to operate from the internal battery, nobody cares if there are 100mV 500Hz spikes.

Solution: add 47-100uF electrolytic capacitor on the power supply line of the dongle.

Thirty seconds after finding the solution I burnt the receiving dongle. (Hint: capacitors retain the charge.) Actually the LDO inside went short circuit. I removed and bypassed it, but while 3.3V to the dongle do power it up, it does not pair with the headset and shuts down after 10 seconds.

Considering that I am not operating /M on a daily basis as in pre-pandemic times I am now thinking of possible evolutions or restoring the same setup.

30 January 2021

Better 8V on QYT KT-8900 microphone socket

I want to simplify my mobile radio operations so that I do not have to switch on the wireless microphone receiver. In the car I use a [insert_your_adjective(s)_here] QYT KT-8900 VHF/UHF mobile, that I have described in the past. It outputs 8V on the microphone socket, so I wanted to use it to power the microphone receiver dongle bypassing the embedded battery.

I built a simple adapter with a 78L05 + the required capacitors + dropping diodes, but the voltage drop was too high when the dongle was powered through it. Then I checked online and a mod is suggested in order to get real 8V on pin 2 of the mike socket, which translates in adding a 10 ohm resistor between a point in the front panel board and the socket pin, like this:

How it is supposed to be done. But adds noise!

Well, I do get stable 8V out of the radio body, but now the transmitted audio has a strong buzz. The dongle drains about 35 mA at 4V.

I think the transceiver does not like when someone messes with the 8V line, as I had discovered when tracing the buzz on the received audio (see the first link above). Next move is to bring 12V out through the unused "pin 1" on the microphone socket. Fingers crossed (that nothing melts in the process!).

25 January 2021

Getting ready for some freeform electronics

Three sizes of brass wire.

Through hack-a-day stories and events I got to know Mohit Bhoite's freeform electronic projects (link goes to his Twitter page). He builds working circuits "in the air" as a form of visual art. He is not the only one on Earth publishing freeform electronics circuits, but he has a tendency to build "squared" shapes that better suit my current artistic abilities.

In order to get started I needed the wires that act both as conductors and supports. Mohit uses brass wires in 20 and 22 AWG sizes. That translates to 0.8 mm and 1.0 mm diameter in metric units. While I was at it I also bought 1.5 mm diameter brass, just in case I will need to support something heavier that a couple of ICs and a dozen of LEDs.

My first project will be a solar-powered super-capacitor LED flasher that promises to recharge even during short Winter days (already gone-by for Winter 2020-21).