### LEAP#185 Building the Böhm Stirling-Technik HB13 Small Bonsai

What has this to do with electronics? Well, nothing (yet), but there's ample scope later;-)

A Stirling engine is a closed-cycle regenerative heat engine with a permanently gaseous working fluid. They are named after Robert Stirling who invented the first practical example in 1816.

Böhm is a small(?) specialist manufacturer from Germany that's taken up a side-line in producing Stirling engine kits. When I first discovered them, I couldn't resist. So here's my build of the Small Bonsai (HB-13).

As always, all my notes and schematics are in the Little Arduino Projects repo on GitHub.

Yet another variation on the basic 555 timer astable oscillator to allow a wide range of frequency and duty cycle adjustments. It's similar to the circuit used in kits like this.

As you can see from the schematic, it's the combination of pot and capacitor selector that produce the wide range of oscillator control:

But the challenge with the classic 555 astable circuit is trying to hold frequency or duty cycle constant while adjusting the other. This circuit doesn't solve that problem, but for something different, I plotted the functions with WolframAlpha. If you correlate the two graphs below, you can see the severe penalty you pay in terms of duty cycle when attempting to push for maximum frequency. It definitely is a case of trying to find the best compromise for your application.

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub, including live links to WolframAlpha to reproduce these plots.

### LEAP#183 Polarity Testing

A simple polarity test for uses a series of inverters. The input signal is pumped into two parallel inverter chains:

• a single inverter
• a series of two inverters
So, regardless of input polarity, one chain output will be high and the other low.

I used a venerable CD4069 for a quick test, although any inverter (matched to the voltage of the signal) will do. The result of the polarity test is displayed with a pair of LEDs reversed in parallel.

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

### LEAP#182 Building a Bench Power Supply

I've wanted a variable mains-powered power supply for a while, so when I found this kit for a reasonable price I decided to give it a go. Some things that attracted me:

• 220V/110V mains-powered
• isolated output
• nice acrylic case
• built-in LED voltmeter

The kit and PCB comes with a few "valued-added features" unrelated to the power supply function (CD4069 square-wave generator, externally-triggered piezo buzzer, externally-triggered polarity tester), but I decided to leave those out of the build.

I also enjoyed investigating and old-school transformer-based power supply. These are getting rare .. it's hard to even find a 220/12 transformer for less than the price of the kit these days, and then they are mostly used/refurbished.

How does it perform? Nice! No smoke on power-up, but a few things to note and/or improve. And I'm sure it wouldn't pass a safety certification, so don't go building one like this for friends.

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.