Effect of shorting turns on a tapped air cored solenoid at RF #2

Effect of shorting turns on a tapped air cored solenoid at RF offers a simple model for estimating the effect of shorting turns on inductance (L) and Q.

A correspondent sent me a set of measurements he made of an air cored solenoid using a Q meter.

The coil was a 22t air solenoid of length 99.5mm and radius 31.56mm. Q of the whole coil (L12) was measured at 6MHz to be 475.

L and Q were estimated and measured with three different tapping points at one end of the coil.

Whilst the method described in the reference article does not attempt to estimate the effect of tapping where the unused turns are left open circuit, we might expect than when the unused section is a small part of the coil, that the effect is similar to that if the unused turns were not there.

A model as described in the reference article was constructed.

The notation is L1 is the used part of the coil, L2 is the unused part, L12 is the whole coil with no taps, Lms is measured L unused shorted, Lmo is measured L unused open, like wise for the Q subscripts.

Above, the model results There is quite good reconciliation with the predicted behaviour. Continue reading Effect of shorting turns on a tapped air cored solenoid at RF #2

IoT water tank telemetry project – part 3

Battery trials

The project continues, albeit slowly.

Some inexpensive DC-DC boost converters have been very slow from China, though multiply source, they have not yet arrived.

An older module which was on hand has enabled progress of reliability and battery trials.

Above is the current prototype. The module on the white plug in cable is a 4-20mA simulator set to 20mA for maximum drain during battery trials. The module at upper right of the pic is a TP4056 batter charger and 1S protection board for the 2000mAh LiPo. The PV array (partially obscured) is capable of 80mA of charge current in full sunlight. The prototype includes a red LED drawing 1mA, an additional 24mAh load per day. Continue reading IoT water tank telemetry project – part 3

Balancing a ceiling fan

I have a ceiling fan which has poor balance and is quite annoying on its high speed.

To solve the problem, I attached a flight controller board which I had on hand for this sort of purpose to the stationary spindle extension, and I have the associated configuration software installed for flying machines

Above, the OmnibusF4 v1 flight controller. Not a good flight controller for flying machines because of the silly pinout, but cheap (for that reason), about $20 on eBay. The flight controller contains a 3 axis gyro and accelerometer, the latter will be used here. Continue reading Balancing a ceiling fan

Hobbyking 2500mAh 18650 LiIon cells (9210000181-0) initial capacity test

This article is documentation of a capacity test of 5 x Hobbyking 2500mAh 18650 LiIon cells (9210000181-0).

The cells were purchased on 26/02/2018 (~$7 + shipping) and received at about 30% charge. They were each charged in a XTAR VC2 Plus charger at 0.5A until charged.

The cells are 65mm long, and do not claim to contain protection modules which are prudent in some applications.

Each cell was then discharged at 1A (0.4C) to 2.8V, the discharge was captured.
Continue reading Hobbyking 2500mAh 18650 LiIon cells (9210000181-0) initial capacity test

IoT water tank telemetry project – part 2

Calibration of the 4-20mA input

This article is a tutorial on calibrating the 4-20mA input which is designed for flexibility that is achieved through exploitation of the calibration.

The input device for this tutorial is a Pt100 RTD temperature sensor and inexpensive Chinese Pt100 – 4-20mA converter (loosely) calibrated for -50-150°. The Pt100, the converter, the load resistor, the divider resistors on the MCU board, and the MCu voltage reference all introduce error which is compensated in this end to end calibration procedure.

For this demonstration, two scenarios are measured:

  1. probe in still air whose temperature is captured with an accurate thermometer; and
  2. probe in boiling water whose temperature is calculated from known altitude and barometric pressure.

Another option would be to use a container of water filled with ice to obtain close to 0° for scenario 1… you don’t need a triple point cell for the end system stability and accuracy.

Temperature of boiling water

Using Calculate cooking time for soft boiled egg :

No, we are not boiling an egg, but the results include the temperature of the boiling water under current altitude and pressure. Continue reading IoT water tank telemetry project – part 2

IoT water tank telemetry project – part 1

This is the first in a series of articles describing a simple maker / DIY IoT water tank telemetry system.

Design criteria

  • capture water depth, temperature and relative humidity;
  • IoT connectivity;
  • solar / battery powered;
  • wireless connection;
  • use existing inexpensive electronic modules.

Design choices made initially:

  • 4-20mA water pressure sensor for depth measurement;
  • ESP8266 Wemos D1 mini pro for the MCU and wireless elements;
  • NodeMCU / Lua software environment;
  • external antenna for improved WiFi range;
  • 6V 100mA PV array;
  • module with TP4056 batter charger and cell protection chip;
  • 2500mAh 18650 cell;
  • AM2320 temperature and humidity sensor;
  • bipolar transistor switch for boost converter;
  • Thingspeak RESTful interface for data accumulation and presentation.

Block diagram

Above is a block diagram showing the major system components. Almost all of the electronics is on easily obtained low cost electronic modules source from eBay, assembled on a Veroboard backplane. Continue reading IoT water tank telemetry project – part 1

ARRL guidance on design of ferrite cored inductors

The ARRL handbook for radio communications (Ward 2011) gives guidance on designing with ferrite cored inductors:

Ferrite cores are often unpainted, unlike powdered-iron toroids. Ferrite toroids and rods often have sharp edges, while powdered-iron toroids usually have rounded edges.
Because of their higher permeabilities, the formulas for calculating inductance and turns require slight modification. Manufacturers list ferrite AL values in mH per 1000 turnssquared. Thus, to calculate inductance, the formula is



L = the inductance in mH
AL = the inductance index in mH per 1000 turns-squared, and
N = the number of turns.

Example: What is the inductance of a 60-turn inductor on a core with an AL of 523? (See the chapter Component Data and References for more detailed data on the range of available cores.)


Lets follow the example through. Continue reading ARRL guidance on design of ferrite cored inductors

Review of inexpensive Chinese LAN cable tester

I bought an inexpensive LAN cable tester to give to my daughter.

Above is the sellers pic, the specifications states that it checks data wires 1-8 and the shield / ground connection of STP cables.

On test, it failed to show the shield connection on an STP cable, the LED did not light on either the master or the slave unit.

I tore it apart to see if it was worth getting a replacement. Continue reading Review of inexpensive Chinese LAN cable tester

Speaker tick generator (for polarity testing)

A recent purchase of an inexpensive ($6) speaker polarity tester prompted a need for a stand alone driver for speakers.

Above, the tester has a microphone that senses the polarity of the pressure wave and indicates with one of two LEDs.

The tester comes with a CD containing a file that can be used to provide the test signal on a complete system with CD player, but there is a need for a stand alone driver for testing bare speakers or speaker units. Continue reading Speaker tick generator (for polarity testing)

Common mode choke for DSL line

Having decided to sack iiNet broadband because of recurrent under performance, I need to change VDSL2 modem as the one they supplied was locked to their SIP server (despite their assurances that there was no equipment lock-in).

I replaced it with a TP-Link Archer VR200v which seems to work ok except it is susceptible to disruption when I transmit on HF. The disruption is severe, it causes the VDSL2 modem to disconnect, and it takes around 5 minutes to reconnect.

Several different common mode chokes were tried, all of measured performance, and they all worked in that they eliminated the disconnection problem though they all resulted in small but acceptable uncorrected upstream errors. (Upstream errors are interesting since the upstream modem is 1000m distant.)

The ‘final’ design was chose as the core was just large enough to wind ordinary 4W modular cable through it. So the choke comprises a 2m length of 4W flat modular cable, one end wound 6 turns through a Fair-rite 2643102002 (FB43-1020) suppression sleeve, and RJ12 connectors crunched on in straight through pin wiring (ie reverse the plugs). I found the line jack in the modem would not accept RJ11 (4P4C) plugs readily (common with RJ45 sockets), it required an RJ12 plug. Continue reading Common mode choke for DSL line