19 September 2015

Transforming 7-segment LED clock into IV-6 VFD clock

Take a cheap 4-digit 7-segment LED clock kit (like Bangood SKU142210, about 6.5 USD [red, without case]), design an adapter board and replace the display with four IV-6 VFD Russian tubes. That's what I have been working on during the last three weeks.

According to the schematic, the original clock uses common anode displays and multiplexes all four digits: IV-6 VFD satisfy this requisite, even though I have no idea how fast the multiplex is and whether tubes can react that fast.

IV-6 on veroboard, note the 3rd tube leads.
These VFDs require a grid and anode drive of 12-30 V or more, while LED displays and the clock run at 5 V. After few tests I opted to keep the 5 V input voltage (ubiquitous USB...) and insert a step-up module to obtain 12 to 35 V, which also controls luminosity.

The clock microprocessor outputs a low logic level to turn on segments, while VFD requires a high "logic" level, so the adapter board must both adapt voltage levels and invert the signal. Well, the ULN2003 darlington transistor array is fit for this purpose. Since I need to control 7 (segments) + 1 (digital point) + 4 (tubes) I need 12 lines, two ULN2003 chips (total of 7 + 7 = 14 transistors).

For the sake of simplicity I opted for wiring in series the four VFD filaments that operate at 1 Vmax and add a voltage-drop/current-limting resistor on the cold end: this ensures that the anode voltage is below the grid potential, so the segment turns completely off (otherwise it could still be visible in complete darkness).

Wiring up the boards requires a lot of concentration.

In order to limit the current through ULN2003 transistors when they are "ON", meaning a segment is "OFF", I needed to choose a suitable pull-up resistor value: too high and the current will not be enough to switch off, too low and the overall current consumption increases as well as unnecessary heat dissipation. 3k3 ohm is fine but pretty low, 9 mA apparently not worry much, but they mean 270 mW if I run anodes and grids at 30 V, on a single resistor. 51 kohm with their 51 mW (@ 30V) are too much and Darlingtons don't turn off properly. At least at a first test, but I want to add decoupling capacitors on the high-voltage side because moving wires around seem to fix the problem.

31 August 2015

QYT KT8900 - digital noise on audio

One of the most noticeable defects of QYT KT8900 is a digital buzz that can be heard when the stock microphone is plugged in. This problem has been tracked down to the digital logic on the microphone communicating with the main radio board.

The picture shows how the loudspeaker signal looks like on an oscilloscope:


Spikes are about 25 mV peak, one every 3.5 ms or so. I couldn't get a stable trigger on the signal and the picture shows my best capture (100 MHz analog Tek, 100 MHz probe at 1x).

Any idea for a simple fix?

30 August 2015

AA to C cell adapter

[In Italian: "Adattatore da stilo a mezza torcia"]

So, your kid just received a new toys that requires batteries. Not plain simple AA cells, but C size. "Batteries not included" the box says. After the fist moments of unspoken words, this is a good chance to prove your audience your homebrew ability with a macgyverism!

Grab two ubiquitous AA cells and wrap them with cardboard until their diameter fits comfortably into the battery compartment. Fix the wrapping with a bit of adhesive tape. That's it!

AA on the left was partially extracted from the adapter.

How is it possible? AA and C cells share the same height. A C cell is an AA put on steroids. Theoretically it holds 2-3 times the capacity, but some people reported fake C batteries that were wrapped up AA's.

Have fun with the new toy!

26 August 2015

QYT KT8900 current consumption

Is it me, or the QYT KT8900 manual does not specify the current consumption? I measured it. The unit I got my hands on, performs as follows:

RX: 0.27 A (backlight on, stand-by)


Readings taken at 12.5 V out from the PSU.

I have no idea what the RF power output is. But I noticed that it will not transmit below 10 Vdc, while the current drain is constant between 10 and 13 Vdc supply.

24 August 2015

QYT KT8900 arrived

The parcel containing a QYT KT8900 has arrived today. It took less than 20 days and it was shipped through Germany to Italy. I haven't opened it yet...

22 August 2015

QYT KT8900 transceiver

Online reviews do not rate it at the top of the category, but QYT KT8900 VHF/UHF mobile transceiver has worthy features for less than 100 USD. And it is small, very small.

I have ordered one, that will come once it completes its long journey from CN/SG/HK. I am looking for two info that current reviews do not cover:
  • spectral purity of the transmitter
  • microphone sensitivity

My own tests will tell me how it performs. The challenge will be to build a probe that turns 25W RF into something acceptable by the spectrum analyzer...

14 August 2015

Insulated element support for ultralight VHF/UHF Yagi-Uda beams

Once upon a time, 10 years ago, I used to build VHF beams for high mountain contesting that could be carried in the backpack. My best result was a 5 element Yagi of DK7ZB design whose boom was a telescopic fiberglass fishing pole, elements attached with clothes pegs.

The system could not grow, though: longer poles have larger diameter, while there are no large clothes pegs.

I had built a 4 metre long aluminium square boom cut in two sections, that is OK for backpack transportation where there is no vegetation around. I broke it before the first contest.

Fast forward 6-7 years and I still haven't found a solution to the larger clothes pegs. But while hanging around "special offers" from Chinese online retailers I came across this product: Adhesive cable ties in packs of 6. I couldn't pass on them at the incredible price of 0.8€/pack, including shipping. Their original purpose is to keep cables (like USB) from falling off (behind) furniture.

But their base size matches my aluminium boom (6 or 8 mm per side) and the cable hole is perpendicular to the longest side: do you see a better, lightweight and cheap alternative to large clothes pegs?

The clip sitting on my old boom.

8 mm element into the clip.
The hole diameter is about 7.5 mm. An 8 mm dia. element fits perfectly with the right friction. A 6 mm dia. element needs to be embossed at the junction point with some rubber pipe.

While I could stick them to the fishing rod once fully extended, I will rebuild the Yagi beam with a square boom and element supports already in place.

Side view of the element inserted into the support.
Too bad I got only one color!