22 January 2021

DMM Input Impedance

I am working on a very low power LED blinker and the need of measuring currents in the range of microamps (uA) has opened me new challenges in the world of lab instruments.

Once you can measure milliVolts, just let the uA current flow into a large-ish resistor (100k, 1M) and measure the voltage across it and apply Ohm's law. Then you remember that your instrument has a finite impedance and it might influence your readings. So, how to estimate a DMM/DVM input impedance? Or how to confirm what is written in the accompanying leaflet? 

In this experiment I have been using an ANENG AN8002 DMM, because it is small and uses 2xAAA cells instead of a PP3 9V battery. It should have 10 Mohm input impedance in DC Volts range.

In order to confirm or calculate the desired value, build a resistive voltage divider say, with a 10 Mohm resistor like this:

+V ------- 10 Mohm -------- DVM -------- GND

Let +V be a known voltage value (measure it in advance using the same DVM) and read what the DVM display says. The inverse formula says:

Rdvm = Vdvm*10 / (V - Vdvm)  [Mohm]

Using my values I get 11 Mohm of input impedance in the >2V range. I do have measured the 10 Mohm resistor with several instruments and they all agree on its value.

Since the AN8002 is an autoranging instrument, the input impedance may change with the auto-set range. And it does indeed, but it is always around 10 Mohm.

Do I need a microAmp meter? Maybe not yet.

Oh, by the way, the blinker takes 0.4 uA (that's 400 nA) when OFF. I have repeated the math several times and taken several measurements in different ways, but they do agree! It runs 24 hours on a 1F 5V capacitor ... if you don't mess with it with a DMM :)

31 December 2020

20m magloop update

Considering the success with a magnetic loop for 20 metres, now I wanted to be able to use a bit more power than 2.5W (at 5W it heated the RG-58 coaxial capacitor, at 10W the tip produced sparks).

I got a thicker coaxial cable that serves both as main loop and coaxial capacitor stub. That's about RG-213 size, with 3 mm solid copper inner conductor: the main loop is stronger, but the stub gets more on the way. Also soldering the whole thing together was not easy, let alone cutting the capacitor to the right length!

The current upgrade is shown in the picture at the right. The loop is slightly longer than Spring 2020 edition, while I have not modified the small coupling loop: this results in SWR of 3:1 at resonance point. Still, reports on the reversebeacon.net are a bit better than with the previous version.

First task for 2021 will be to fix the coupling loop to get a match as close a possible to 1:1. And do have some CQ QSOs!

29 December 2020

FNIRSI DC-6006L Power Supply and a bug

During "Black Friday|Week" 2020 I bought the DC power adapter FNIRSI DC-6006L. I would not call it a "power supply" since it requires an AC-DC adapter (like an old laptop PSU). Provided there is enough "juice" provided by the external PSU, the DC-6006L is capable of 0.1-60V output up to 6A: that's 360W in a aluminum case lighter than a smartphone and smaller than a sandwich! You can find a good description of the product (in decent English language) on their website or on the usual Far East sources that sell it.

Check out the size comparison with the Nissei SPS-250A:

FNIRSI DC-6006L vs Nissei SPS-250A.

First things first. The Fnirsi unit is not very RF silent, so it will increase the background noise if used to power an HF receiver. But it works OK as a lab power adapter.

I have found one dangerous bug. When DC is applied to the input of the DC-6006L, there appears voltage on the output even though it is configured to start in OFF position at "cold boot". It might last 100-200 milliseconds, enough to burn whatever is connected on the output! Looking at the output on the oscilloscope, the voltage reaches 6-7V regardless of the output voltage set. I did this test both with 19Vdc input on the barrel jack and with 14Vdc input on banana sockets. I have an analogue oscilloscope so I cannot do proper measurement of amplitude and duration. Just consider that an Arduino based circuit springs to life. This happens before the boot screen appears on the DC-6006L display.

On annoying thing is that the beep cannot be disabled via software. While it is useful when over-voltage/current protection triggers, you might not want to hear it for each key press or knob rotation. And it is quite loud too.

I tried serial communication without the FNIRSI software, but all I get is a string that changes in sync with operator's actions through the front panel controls.

I have drawn 45W on a resistive load without ill effects.

I paid about 30€ and it is worth it, especially if you already have a fixed DC source.

27 December 2020

Eight TIL308 displays in a clock

I would not say that the long lockdown and alike has allowed me to reach the bottom of the to-do bucket, but it slowed down the consumption of projects in the waiting list.

When I spend the whole work-day at the computer, writing firmware in the remaining hours is not so pleasant. In the Xmas break I finished the code for the clock that uses eight Texas Instruments TIL308 vintage LED+logic displays.

Final firmware onboard! I can solder the remaining capacitors to complete symmetry.

Hardware design files and software source code have released to github.

This clock is self-standing if the RTC module is inserted perpendicular to the display board. Adjustments are accomplished through a linear potentiometer and a push-button. With a software trick ;) the same analog input is used both for reading the light-dependent resistor to adjust display luminosity and the push-button. I had to adjust resistor values and add a couple of them outboard.

I like these clocks to do something unexpected, so every 60" it might display a word built using available letters (or lookalike) and numbers:

// Available letters are:   ABCEFIOSG-
// Mapped to these symbols: A8CEF1059B  (mind the mix of letters and numbers!)
// Plus space ("D") and all the numbers are at your disposal!

TIL308 is not a full hexadecimal display because it maps "B" to "dash" and "D" to whitespace. This reduces the possibilities of long words and exercises your fantasy a bit further to find meaningful text to display randomly. Don't forget you can use multiple languages for your words!

A nice addition would be to be able to add words at runtime (and store them in EEPROM), rather than at compile time. That means more firmware to write ...

13 November 2020

New banner picture with 1972 Toshiba logo

After a long time I have changed the banner picture of this blog.

I was looking inside a 1972 CB handheld from Midland and noticed these metal case active elements with a very cute Toshiba signature. Too cute to stay inside the transceiver and not be seen by newer generations. So, here it is, the new banner.

The new banner as of 2020-Nov-13.

And here is a slight zoom-out on the circuit with one of these Toshiba components:

 Really, really nice.


28 October 2020

ICOM BP-84 battery pack

At a slow but steady pace I am working on a 10 GHz WBFM  receive chain not based on RTLSDR. The path requires having a receiver capable of WBFM at 600-700 MHz, which is covered by most "scanners".

Recently I bought an ICOM IC-R1 without battery packs, so I sourced one large enough to contain an energy source both for the receiver and the LNB.

I bet on a BP-84 hoping it would be tall enough to contain 18650 LiXx cell and it does indeed.

LtoR: BP-84 shell, AA for reference, 6x custom NiCd cells, 2x 18650.

Since the project around the IC-R1 scanner has been put aside in favour of another scanner, I will write only about "how to remove the original content of BP-84?"

First of all, the bottom cover must be removed. My BP-84 came with the cover already off: saw it off carefully since you will not split in two parts the shell. Once the bottom of the cells is exposed you need to cut off three metallic tabs (visible in the picture) which also hold in place the 6 NiCd cells. Once the three tabs are loose, the 6-cells pack can be pulled out with slight force.

A 18650 Li-xx battery is perhaps 1 mm taller than those custom size NiCd's. But given that the BP-84 bottom is now lost, you can handle the extra length when building a new bottom.

Three 18650 in [v^v] shape (zig-zag) should fit in there. That gives 12.6V to 10.5V. Consider adding a fuse and a Li-xx protection circuit. The 12V line can go out to a power injector towards the LNB so everything is self-powered. Enjoy!

13 October 2020

Kit Oscilloscope Clock 8SJ31J review - 3

Since I have written a review of this kit I have been asked to share pictures of my build. I am unsure how useful they can be, maybe just reassuring.

IMPORTANT. DO NOT RELY on these pictures to decide which component goes where. In order to build that kit you need to be able to use an ohmmeter to sort resistors, read IC markings and understand capacitor values. Also, the kit seller (that is NOT me) may change components depending on his sources, and some values too.

Click on pictures to get the larger version.

Direct all your questions and problem to the seller, not me.

All pictures are watermarked and cannot be used elsewhere.

How I stacked the two boards, size is about 20x8x7 cm.