Choosing a ferrite mix for a 160m unun rationally

One often sees people ask for help in choosing a ferrite mix for a particular application. A recent thread on social media asks for help designing a unun for the 1.8MHz amateur band, and it has provided the opportunity for participation, even if the content was not good.

An important early step in designing a ferrite cored transformer is to find a combination of ferrite material, core geometry, and number of turns to deliver acceptable core loss at the lowest desired frequency.

Design of a transformer to cover just the 1.8MHz ham band is a relatively simple exercise. Continue reading Choosing a ferrite mix for a 160m unun rationally

Chinese batteries – replacements for Makita 1220 battery pack

Buying Chinese batteries is a minefield. Experience has been that they rarely meet specified capacity, and less that 10% of specified capacity is not that unusual.

So, with that background I searched eBay for replacement batteries for a Makita drill / driver. The current pair of batteries are Chinese source, purchased about 5 years ago, label rating is 2.0Ah, and measured capacity now is 1.3-1.4Ah… which is not too bad, they were 1.7-1.8Ah when new (yes, below spec).

There were very many sellers selling a lot of two packs rated at 3.6Ah for around $34 including delivery. That is very cheap, too good to believe? Let’s rely on eBay Buyer Protection to put them to the test.

The two batteries arrived quickly and were put through charge / discharge cycles to condition them.

Above are the C/5 discharge curves for both batteries on the third cycle, capacity is 1.3Ah, just 36% of the label rating. Continue reading Chinese batteries – replacements for Makita 1220 battery pack

Yet another ferrite toroid calculator – but is it any good?

In a recent online thread, a ‘new’ online calculator was touted:  https://miguelvaca.github.io/vk3cpu/toroid.html .

References without any qualification surely imply a recommendation.

In the same thread, Roger Need compared his measurement of a FT50-43 with Calculate ferrite cored inductor (from Al) (one of a set of related calculators), and Ferrite permeability interpolations.

Above, his calculation reconciles well with measurement at 3.6MHz. Continue reading Yet another ferrite toroid calculator – but is it any good?

Review of inexpensive Chinese thermostat – DST1020

The DST1020 targets the market for inexpensive digital thermostats, the most popular being the STC1000. There are two novel features to the DST1020:

  • uses DS1820B digital temperature sensor (well probably a Chinese clone);
  • two line display shows PV and SV simultaneously.

The DS1820B should be considerably more accurate and overcome the significant error in the conversion of NTC resistance to temperature in the other thermostats (experience is that the approximation used for the NTC characteristic is simple and inaccurate).

It is sold with brief and inadequate / incorrect user instructions.

Above, the front of the DST1020. Continue reading Review of inexpensive Chinese thermostat – DST1020

JDY-31 Bluetooth SPP module

A friend referred me to a ‘replacement’ for the HC-05 Bluetooth module, a JDY-31.

Above is a JDY-31 bluetooth module with header pins fitted. The physical design is poor, the header pins can be fitted from only one side (not plated through holes, no pads on the other side, probably to suit the base board below), and the black plastic part obscures the board labelling of the pins. Continue reading JDY-31 Bluetooth SPP module

IoT water tank telemetry project – 4-20mA / 1MPa pressure transducer trial

This is a new project derived from IoT water tank telemetry project – part 1 , but using an inexpensive 1MPa 4-20mA pressure transducer.

Above is the electronics and 4MPa pressure transducer fitted with a Nitto male connector for a trial. Continue reading IoT water tank telemetry project – 4-20mA / 1MPa pressure transducer trial

IoT water tank telemetry project – HC-SR04 – second trial

IoT water tank telemetry project – ultrasonic sensor – #1 described an inexpensive ultrasonic ranging sensor for trial.

Above is the HC-SR04 ultrasonic ranging sensor, it was purchased for around $6 from a local eBay seller and delivered within days. Note that there are somewhat similar looking things with a second board on the back and a different interface, the basic HC-SR04 as pictured suits this project.

In this second trial, the battery saving feature has been activated. It powers the HC-SR04 on for each measurement, waiting 600ms for the HC-SR04 to stabilise. Continue reading IoT water tank telemetry project – HC-SR04 – second trial

IoT water tank telemetry project – JSN-SR04T-3.0 – first trial

IoT water tank telemetry project – HC-SR04 – first trial mentioned an inexpensive ultrasonic ranging sensor for trial.

Above is the JSN-SR04T-3.0, a waterproof transducer on a cable and the electronics board. The protocol is HC-SR04 PWM. No specifications or datasheet were found (other than the seller’s brief description). Continue reading IoT water tank telemetry project – JSN-SR04T-3.0 – first trial