22 January 2015

BITX, I'm in

Ten years six months after joining the BITX transceiver project mailing list I decided to actually build one. A decade ago I had even matched the XTALs for the IF filter, but other interests hijacked my time.

Ten years later there are (several) PCB designs available, and I could not resist a sale price of 10 USD@2015 for BITX version 3. I don't mind if it is not the latest version with frequency counter, DDS or other amenities. I want a QRP SSB RTX I can carry around without fear of breaking it but, most of all, I am after the fun of building it.

The PCB set is on its way from India in a Registered letter. India Post tracking system seems a bit slow with database updates since 14 days after shipping there is still no mention of my parcel. Meanwhile I study the documentation and gather components.

13 January 2015

A variable PSU with LM2596 adjustable DC-DC step-down regulator?

Now that I have a lab PSU with an adjustable regulator inside, how about bringing the voltage control to the front panel?

The voltage regulator module is based on LM2596-ADJ and sells for ~3 USD@2015. Output voltage is controlled via a 10 turns 10kohm trimpot.

The datasheet says that the adjusting resistor must be connected as close as possible to the LM2596 (and away from the inductor). Moreover it is completely floating off GND, so any modification is out of question.

It is better to look for a different step-down adjustable circuit, or use a long shaft to bring the control to the outside. Nevertheless, multiturn trimpots are not meant for daily usage, but rather for a one-time set-and-forget application.

12 January 2015

(27 y.o.) PSU update

The aforementioned LED strip in the shack needed a 12V power supply. Since I had wanted to turn one of my fixed PSU into variable (or multi-output), I jumped in and applied the mod.

The chosen unit is my "rms K132" linear PSU:

It contained a 7812 regulator produced in mid 1987. Since it never needed a repair, the unit should be ~27 years old! That's when I started on 27 MHz Citizen Band. I remember the price was a linear function of amperage: 2A = 20'000 (good old) Lire; 5A = 50'000 Lire. By the way, 20000 L@1988 ~= 20 €@2015.

The idea: replace the 2A 7812 linear regulator with an adjustable switchng board of equal continous current capacity. I used a board based on LM2596-ADJ and it fits comfortably given the large PCB and the amount of free volume inside the PSU.

One more change was to replace the red LED with a yellow one, as a warning sign that the PSU is NOT 12V anymore.

For the LED strip I tuned it down to 10V, and I probably go even lower. The FT817 receiver does not complain of this switching PSU since there is no added noise vs my usual RTX PSU.

DISCLAIMER. This operation was made inside an high-voltage AC unit. ALWAYS disconnect the power cable from the AC socket and dischage the large capacitor before touching anything inside it. I will not take responsibility for injuries or damage to anything/anyone if you decide to replicate or reproduce what I have shown above.

11 January 2015

LED strip, a little discovery

Last Friday I received my first roll of LED strip. 5 metres of warm white 5050 LEDs, out of which I need only 1.10 m. When preparing my strip for deployment I noticed that it is not made of single, long, uninterrupted material: it is a sum of 1 m long sections.

At 12 Vdc the strip is pretty bright and does produce a decent amount of heat. No wonder people complain these LEDs loose luminosity! Heatsink is required for a long LED life.

Since I do not need that much light I reduced the supply voltage to 10.0 V and the result is still too much for night work.at the laptop.

04 January 2015

Nativity scene light effects with Arduino - the video

It is hard to film something almost totally dark, so this recording was done with the room light on.

The short film has been cut to remove the 20 seconds of full daylight duration (according to the Arduino code published in the previous post), so you will not see the full cycle. Just the day to night transition.