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Hardware hacking how-tos

Biasing That Transistor: The Common Emitter Amplifier

May 4, 2018 by 84 Comments

If you open up the perennial favourite electronics textbook The Art Of Electronics and turn to the section on transistors, you will see a little cartoon. A transistor is shown as a room in which “transistor man” stands watching a dial showing the base current, while adjusting a potentiometer that limits the collector current. If you apply a little more base current, he pushes up the collector a bit. If you wind back the base current, he drops it back. It’s a simple but effective way of explaining the basic operation of a transistor, but it stops short of some of the nuances of how a transistor works.

Of course the base-emitter junction is a diode and it is not a simple potentiometer that sits between collector and emitter. The “better” description of these aspects of the device fills the heads of first-year electronic engineering students until they never want to hear about an h-paramater or the Ebers-Moll model of transistor function again in their entire lives. Fortunately it is possible to work with transistors without such an in-depth understanding of their operation, but before selecting the components surrounding a device it is still necessary to go a little way beyond transistor man.

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Circuit VR: Sink Or Swim With Current Sources

May 3, 2018 by 22 Comments

If you got your start in electronics sometime after 1980 your first project might well have been to light up an LED. Microcontroller projects often light up an LED, too, and a blinking LED is something of the “hello world” program for embedded systems. If you tried lighting up your LED with a 9 V battery directly — not that you’d admit to it — you found it would light up. Once, anyway. The excess current blows up the LED which is why you need a current-limiting resistor. However, those current limiting resistors are really a poor excuse for a current source or sink. In many applications, you need a real current source and luckily, they aren’t hard to create.

As always with Circuit VR, we’ll be using LT Spice to examine the circuits. If you need a quick tutorial, start here and come back after that. If you use Linux, don’t be dismayed. I run LT Spice under WINE and it works great. You can find all the Spice files on GitHub.

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Directly Executing Chunks Of Memory: Function Pointers In C

May 2, 2018 by 75 Comments

In the first part of this series, we covered the basics of pointers in C, and went on to more complex arrangements and pointer arithmetic in the second part. Both times, we focused solely on pointers representing data in memory.

But data isn’t the only thing residing in memory. All the program code is accessible through either the RAM or some other executable type of memory, giving each function a specific address inside that memory as entry point. Once again, pointers are simply memory addresses, and to fully utilize this similarity, C provides the concept of function pointers. Function pointers provide us with ways to make conditional code execution faster, implement callbacks to make code more modular, and even provide a foothold into the running machine code itself for reverse engineering or exploitation. So read on!

Function Pointers

In general, function pointers aren’t any more mysterious than data pointers: the main difference is that one references variables and the other references functions. If you recall from last time how arrays decay into pointers to their first element, a function equally decays into a pointer to the address of its entry point, with the () operator executing whatever is at that address. As a result, we can declare a function pointer variable fptr and assign a function func() to it: fptr = func;. Calling fptr(); will then resolve to the entry point of function func() and execute it.

Admittedly, the idea of turning a function into a variable may seem strange at first and might require some getting used to, but it gets easier with time and it can be a very useful idiom. The same is true for the function pointer syntax, which can be intimidating and confusing in the beginning. But let’s have a look at that ourselves.

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Accessing Blockchain On ESP8266 Using The NodeMCU Board

May 2, 2018 by 24 Comments

Blockchains claim to be public, distributed, effectively immutable ledgers. Unfortunately, they also tend to get a little bit huge – presently the Bitcoin blockchain is 194GB and Ethereum weighs in at 444GB. That poses quite an inconvenience for me, as I was looking at making some fun ‘Ethereum blockchain aware’ gadgets and that’s several orders of magnitude too much data to deal with on a microcontroller, not to mention the bandwidth cost if using 3G.

Having imagined a thin device that I could integrate into my mobile phone cover (or perhaps… a wallet?) dealing with the whole blockchain was clearly not a possibility. I could use a VPS or router to efficiently download the necessary data and respond to queries, but even that seemed like a lot of overhead, so I investigated available APIs.

As it turns out, several blockchain explorers offer APIs that do what I want. My efforts get an ESP8266 involved with the blockchain began with two of the available APIs: Ethplorer and Etherscan.

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How To Put The ‘Pro’ In Prototype

April 29, 2018 by 5 Comments

It’s easy to get professional-quality finishes on your prints and prototypes if you take the right steps. In the final installment of his series about building with Bondo, product designer [Eric Strebel] shows us how it’s done no matter what the substrate.

How does he get such a smooth surface? A few key steps make all the difference. First, he always uses a sanding block of some kind, even if he’s just wrapping sandpaper around a tongue depressor. For instance, his phone holder has a round indent on each side. We love that [Eric] made a custom sanding block by making a negative of the indent with—you guessed it—more Bondo and a piece of PVC. The other key is spraying light coats of both primer and paint in focused, sweeping motions to allow the layers to build up.

If you need to get the kind of surface that rivals a baby’s behind, don’t expect to prime once, paint once, and be done with it. You must seek and destroy all imperfections. [Eric] likes to smooth them over with spot putty and then wet sand the piece back to smooth before applying more primer. Then it’s just rinse and repeat with higher grits until satisfied.

There’s more than one way to smooth a print, of course. Just a few weeks ago, our own [Donald Papp] went in-depth on the use of UV resin.

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Circuit VR: Oscillating Bridges

April 27, 2018 by 21 Comments

Circuit VR is where we talk about a circuit and examine how it works in simulation with LT Spice. This time we are looking at a common low-frequency oscillator known as the Wien bridge oscillator.

What makes an oscillator oscillate? A circuit with amplification that gets the same amount of the output signal fed back into its input, in phase, will oscillate. This is the Barkhausen criterion. Here, we’re going to look into what makes an oscillator work in simulation, and gain some insight into what happens when there’s too much feedback and too little.

In particular, we’ll look at the Wien bridge oscillator, a very simple design that originated as a way to measure impedance back in 1891. Modern versions add some additional features, but let’s start with the most simple implementation and work our way up.

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Is It On Yet? Sensing The World Around Us, Starting With Light

April 26, 2018 by 10 Comments

Arduino 101 is getting an LED to flash. From there you have a world of options for control, from MOSFETs to relays, solenoids and motors, all kinds of outputs. Here, we’re going to take a quick look at some inputs. While working on a recent project, I realized the variety of options in sensing something as simple as whether a light is on or off. This is a fundamental task for any system that reacts to the world; maybe a sensor that detects when the washer has finished and sends a text message, or an automated chicken coop that opens and closes with the sun, or a beam break that notifies when a sister has entered your sacred space. These are some of the tools you might use to sense light around you.

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