16 May 2020

Honey, I shrunk the loop

Like most amateur radio antennas, magnetic loop do not receive positive looks from relatives, neighbors and by-passers. My 20m magloop on the balcony could be seen by us and very few other people, but would still hurt the eye when looking out in that direction.

So, I shrunk the loop, from 2.3 m circumference to 1 m. Of course it now operates on a different frequency: 50 MHz. It has been chosen on purpose, since May and June are the Sporadic-E ("ES") season. Thanks to the lockdown and work-from-home situation I stand more chances to be in the shack when ES hits, unpredictable as usual.

I changed the tuning capacitor from an air variable to a RG-58 coax stub, meant to withstand higher voltage, thus input power. It did work on the 20 m loop, but it doesn't on 6 m.

This is what happens. On the VNA the antenna is perfectly matched. On the transceiver the SWR meter is happy.  Then, if you keep the key down, SWR increases to well over 10:1. On the 20m loop the stub tip would produce a bright white spark when I raised the RF power above 30-40W, which was solved by making a bit more space between the braid and the inner conductor. On 6m there was no spark: the RG58 stub would heat up and change the impedance.

IMPORTANT. How to check if it heats? Either use a thermal camera or touch the stub once that both these conditions are met: you have produced the SWR increase AND you have have switched off the transceiver completely (so that it doesn't transmit while you sense the temperature with your fingers).

This is a sign that the voltage at the open loop side is high, so the match is good. Also, being the circumference close to lambda/4, the efficiency is higher and the same holds true for the voltage.

The maximum power the loop can withstand depends on the environment temperature, but the heat produced with 5-10W is negligible and has no effect. At least at 20 °C!

14 May 2020

NanoVNA as Signal Generator

If you are in a hurry and need a quick & square wave signal between 50 kHz and 300 MHz the NanoVNA can come to the rescue. Just set the centre frequency of the stimulus at the value you need and a span of 0 Hz. The signal is then available at port 1.

Be very careful not to overload the output or feed a voltage/signal to it, else the NanoVNA would be damaged! If the procedure and implications are unclear to you, do not try it.

Why does it not work all the way to 900 or 1500 MHz? Because the NanoVNA(-H) uses some clever tricks using harmonics at frequencies above 300 MHz, but you always get the basic signal 0-300 MHz at port 1.

28 April 2020

Longer delay modification for TDL-2023 PIR sensor

I have a TDL-2023 pass-through motion activated sensor that turns on a device. While I have already set the delay to the maximum value with the internal trimmer, the advertised 360 seconds were not enough.

Click to zoom.
On the small board there is a BIS50001 IC, whose datasheet can be found online. The delay is set with an RC combination and can be seen on some schematic diagrams that use the same BIS50001 chip. Starting from the trimmer pins I poked around and found a series of 1 Mohm (trimmer) and 56 kohm (fixed resistor marked R0). Since I need a longer delay I decided to replace the fixed resistor with a larger value, 1 Mohm being probably enough.

Click to zoom.
The smallest resistor I could find is 1/4W but it fits into the case if installed as seen on my picture. The delay is now over 8 minutes. Since I have not been able to locate the capacitor of the delay circuit I cannot confirm the datasheet formula, but the ON time "feels" like twice as much as before.

25 April 2020

An update to my KiCAD library for obsolete LED

I got a new batch of old LED displays and before doing something with them I needed a KiCAD symbol and footprint.

So I added TIL308 and TIL309 to my KiCAD libraries. Also fixed TIL306 and TIL307 that were missing their respective symbols.

My KiCAD library of obsolete displays (Nixie, VFD, LED) is published on a github repository.

Meanwhile I confirm that TIL311 symbol and footprints are correct since I got PCBs, built them and I am enjoying the result.

24 April 2020

Blue dot and leg glow in Z520M Nixie

I love Nixie tubes, and I especially love round top-view ones. Perhaps that is because the first ones I bought were of this kind.

Lately I got one Z520M with the red coating quite damaged (pictures seem to make it look even worse!). Since the red coating can be restored/replicated, I proceeded to test the tube. I was not surprised to find out that it needs to be driven a bit harder than newer Nixies, both because this one is quite old and probably it has been used a lot.

Digits 0 to 2 were behaving normally. When pulling low other digit cathodes only the leg was glowing. If left ON for few minutes the glow transferred to the digit from the area close to the pin. Unfortunately if the digit is switched off for a while the malfunction comes up again the next time the same digit is lit.

Now that I own a variable HV power supply I could experiment with the current through the tube, and saw that the "leg" does not glow if the current flow is lowered closer to the datasheet value. Even 0.1 mA makes a difference. But then the digit is faint or not completely lit (like "5" and "7").

Since the red coating is cracked I could see through the plain glass and note that the leg glow releases the infamous blue dot. It is caused by mercury and overcurrent. Since it does not happen on digits 0-1-2 it could be just a deposit on unused digits/wires and might be cured throughout hours of operation of the same symbol. Perhaps. I will not go further and keep the tube as-is in the collection. (Or make a test harness where I can leave it running for days out of the way in the lab?)

Can you see the infamous blue light through the crack in the coating?

16 April 2020

TTL integrated circuits "collection"

Not a real lockdown project, I tried to organize my "collection" of TTL integrated circuits. These have been piled up in numerical order to ease a future search for a specific part. The picture shows all of them and it is posted here for my own future reference.

I thought I had more laying around, actually. The 74(1)41 Nixie driver are not amongst them because they are the only 74-series ICs that I still use.

At least now I have a group picture in case I (or someone else) need one of these, or I really run out of interesting projects and start designing something out of them.

15 April 2020

Another HV DC power supply

DISCLAIMER. This "project" handles lethal voltages and currents. Do NOT replicate. I take no responsibility for any damage that might occur from replicating or imitating what is illustrated below.

Having lots of time to spend at home to do "whateveryouwant", I started to clean the lab: move stuff around, make room on the desk, prepare a trip to the recycling facility, see stuff under a different light.

I happen to have two 12V power supplies from 1980's, when heavy transformers were dominating the scene and guaranteed RF silence. The first purchase was 12V/2A, then upgraded to 12V/5A (not that I cared about RF noise back then in my CB years!).

Both power supplies, as of April 2020.
They took space in the lab, one partially broke, one was recently upgraded with a step-down converter in place of 7812 so, as the headline suggests, I made a high-voltage DC power supply out of them.

What was (left) inside. Dust included.

D-C rectifier and back2back trafo.
How? I wired the two transformers back-to-back and rectified the AC output. Bonus, since transformers were both centre-tapped, I can select two outputs just by switching the intermediate connection: 170V or 340V. And both transformers fit into one original case!

According to transformer ratings I should be able to draw about 100-120 mA at 170V and half of that at 340V. Then the voltage drops, transformers vibrate and overheat.

The transformer receiving the mains is the bulkier one, from the former 12V/5A PSU, so there is some room for a low voltage output with decent current.

Final look, with HV output yellow/black.
Needed improvements:
  • fuse the AC input
  • fuse the HV output
  • add a damping resistor on the HV output
  • attach the HV output selection switch to the case 
  • add a status LED/lamp
  • add the low voltage output circuit
Unloaded 340V.
Unloaded 170V.