Review of Banggood digital clock kit

The Banggood large screen digital clock kit is an inexpensive kit (A$13 inc case and post).

The clock uses a DS1302 clock chip and microcontroller for display. The microcontroller is a STC15F204EA, a Chinese 8051 microcontroller.

Clock01

The kit is supplied with Chinese instructions, but a Chenglish version is available online. The only reason I needed an English version is to understand the somewhat obscure setting procedure, but the Chenglish version was not clear about the so-called “timekeeping” settings. Continue reading Review of Banggood digital clock kit

VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

Peter, VK3YE, describes a small transmitting loop (STL) in his video at https://www.youtube.com/watch?v=Cv_RnLpZ9gw.

? 100 watt 7 MHz magnetic loop for units and apartments - YouTube - Mozilla Firefox firefox 04/08/2015 , 07:24:25

As far as I can glean from the video, it is made from a 3m length of copper tube 19mm diameter, and uses about 1.8m of RG213 to tune it, and appears to have its centre 0.7m above ‘ground’ .

Let us firstly look at a free space model of the antenna using Reg Edwards’ RJELoop1 tool.

Screenshot - 07_06_2015 , 09_53_12

This model has its limitations, but the calculated inductance is of interest. We can calculate the inductive reactance to be 118Ω. The capacitive stub of RG213 will need around 107Ω reactance, and solving for RG213, we find that 1.94m gives 0.19-j107Ω. The resistive component is important as it is ignored by the above model. The stub resistance is a loss resistance, and we need to recalculate the efficiency. Efficiency=Rrad/Rloss=0.005/(0.19+0.0351+0.005)=2.17% (-16.6dB). We can also calculate the Q as 107/(0.19+0.0351+0.005)=465 and half power bandwidth as 7100/497=15.3kHz. Continue reading VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

A test run of the generic heating / cooling controller with 10k NTC thermistor sensor

The generic heating / cooling controller (hcctl) is a flexible bang-bang thermostat controller based on an ATTiny25.

hcctl101

The test load is a pot containing 1l of water and a 1200W immersion element controlled by the SSR above (on-off control). The controller board is a ‘fully optioned’ test framework, hcctl is the left hand DIP8 and the other is a TC427 H bridge (not needed for this SSR which can be driven directly from the ATTiny25 output pin) for buffered output and alarm. Continue reading A test run of the generic heating / cooling controller with 10k NTC thermistor sensor

2.4:1 balun design failure

A lost soul searching for enlightenment on impedance transformation sought advice on a transformer at 2.4 : 1 BALUN.

Inevitably one of the forum experts counselled:

Assuming your quad is a single-band HF antenna, a conventional transformer using #2 powdered iron would be my choice for the balun function. The reactance of the secondary winding would need to be at least 600 ohms.

So, let’s put the forum expert’s advice to a practical test.

Fleshing out the proposed solution

I have at hand a T200-2 core, so lets calculate the secondary turns to satisfy the proposed solution.

Screenshot - 27_05_2015 , 07_05_11

Above is calculation from a popular online calculator. For 14MHz, the secondary should be at least 23.8t. We will use 24t. Continue reading 2.4:1 balun design failure

A test run of the generic heating / cooling controller with Pt100 sensor

The generic heating / cooling controller (hcctl) is a flexible bang-bang thermostat controller based on an ATTiny25.

hcctl101

The test load is a pot containing 1l of water and a 1200W immersion element controlled by the SSR above (on-off control). The controller board is a ‘fully optioned’ test framework, hcctl is the left hand DIP8 and the other is a TC427 H bridge (not needed for this SSR which can be driven directly from the ATTiny25 output pin) for buffered output and alarm. Continue reading A test run of the generic heating / cooling controller with Pt100 sensor

SPI input for the generic heating / cooling controller

The generic heating / cooling controller (hcctl) is a flexible bang-bang thermostat controller based on an ATTiny25.

The project has been expanded to accept a simple SPI temperature sensor. The test case uses a MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter for K type thermocouples. The MAX31855 is around US$5 at Digikey for singles, but the tests were conducted using Adafruit MAX31855.

MAX31855

Thermocouples bring two challenges for hcctl:

  • low noise amplification of very low sensor voltage;
  • compensation of the ‘cold junction’ temperature; and
  • high resolution ADC.

The MAX31855 provides a solution to all of these challenges in a single inexpensive chip. Continue reading SPI input for the generic heating / cooling controller

Adafruit MAX31855 checkout

Intending to enhance my generic heating / cooling controller to read SPI temperature sensors, I purchased an Adafruit MAX31855 module on eBay from a local supplier for about A$26 posted.

The module you might have guessed uses a MAX31855, a Cold-Junction Compensated Thermocouple-to-Digital Converter for K type thermocouple with an SPI interface. The Adafruit module includes a regulator and level translators to use it in a 5V system.

This article describes a simple checkout using a BusPirate V4. Conveniently, the MAX31855 module can be powered from the BusPirate. The thermocouple input is provided by a thermocouple calibrator.

TcCalTest

After a short wait to allow both devices to stabilise at ambient temperature, a test was run. The BusPirate session is as follows. Continue reading Adafruit MAX31855 checkout

Coupled coils – a challenge for hams!

One frequently sees discussions of coupled coils in ham fora, and the advice of the forum experts is commonly sadly lacking.

An example is the thread Impedance matching transformer where the OP is encouraged to make a transformer for 2:1 impedance transformation ratio based simply on turns ratio and a Rule Of Thumb for minimum number of turns.

Lets review a design where two windings of say 10µH and 20µH are wound on a toroidal core. With no flux leakage, the turns ratio would be 1:1.414. The model is a simple one of coupled coils and ignores self capacitance.

100% flux coupling

If there was no flux leakage, the mutual inductance is (10*20)^0.5=14.14µH, and we can build a three component model of the coupled coils along with the intended 100+j0Ω load.

Screenshot - 11_05_2015 , 08_36_22

Above the model for 100% flux coupling.

Screenshot - 11_05_2015 , 08_36_05And above, the response of the network. At 7MHz, the input impedance is 48.7+j8.7Ω, not perfect, but close (VSWR=1.2). Continue reading Coupled coils – a challenge for hams!