my recent reads..

LEAP#176 nRF24 Breakout Board

So recently I've been intrigued by small-batch PCB fab services, like OSH Park. Like the ready supply of components from various online sellers, it's amazing how cheap you can get boards made over the net these days. How do they do it? As I learned in this hardware hangout, there are some neat tricks done behind the scenes to aggregate small orders into full panels sent to a fab.

But to get PCBs made, you need to be able to drive an EDA tool like KiCad. KiCad's a beautiful open source tool, but it is quite idiosyncratic. In some spare time, I dived into the KiCad like a Pro course and found it an excellent leg-up on the process.

The nRF24 breakout board (below) is just the first trivial exercise from the course. But it will come in handy - I have a batch of nRF24's lying around that I've been planning to experiment with, and the breakout board will be quite handy for that.

I just received my boards from OSH Park (1 month from order to delivery in Singapore - not bad, especially for $4.80 all-in). Very impressed with the board quality. Not so much with the board design! A few things I can see could be easily improved, but that's my fault (see my notes). But they do work!

As always, all notes, schematics and code (including the KiCad project) are in the Little Arduino Projects repo on GitHub.

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LEAP#186 Asynchronous Counters with JK Flip-Flops

A quick demonstration using four JK flip-flops set up in "toggle" mode to implement an asynchronous 4-bit binary counter. This is the classic circuit, straight out of a text book. It's implemented with:

  • two 74LS73 for the flip-flops
  • a 555 Timer astable oscillator providing the clock pulse
As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.

Why "asynchronous"? This refers to the fact that the output of each flip-flip cascades to trigger the next in series. Although instantaneous to the eye, there is none-the-less a propagation delay as the changes ripple through the flip flops. It's clearly seen in an analog scope trace. Here is the falling edge of 0xF transitioning to 0x0, which ripples over about a 42ns period:

Here's a quick video of the circuit in action ... though definitely not the most exciting thing you'll see on YouTube today:

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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.

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LEAP#184 Adjustable Pulse Generator

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.

Frequency for all values of R1, R2 (C=1µF)

Duty Cycle for all values of R1, R2 (C=1µF)

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