BJT H-Bridge.

Suppose you needed for current to be able to flow two directions through a component. Then you need 4
 switches around the component to form an H-bridge. You can do this with 4 transistors as well, 2 NPN or 2 PNP. You could also use FET's in a similar arrangement.

I recently realized that you could control both switches in the pair if you hooked it up right. Play with it on the Circuit Simulator.

I plan on using this arrangement to drive a two way latching a relay. A Brief pulse on one side sets the relay, and a brief pulse on the other side resets it.

ARM Development Board.

I built this ARM development board:

It includes everything you need to start programming the board. The board can be programmed via a serial bootloader, or via JTAG.

I'm programming it with completely custom firmware, so I figured out how to write a linker script, how to build a cross compiler for it, and how to setup the firmware so it starts executing.

I've done some simple exercises with it to make sure I understand how to use the devices.
I know how to use and setup the clocks, the dma controller, the general purpose IO's, the timers, the external interrupt controller. Next I'd like to get it to have the usart working so I can have debug printf's and status messages.

My motivation for building it was twofold. I wanted to learn to do surface mount soldering (check the full size image), and I wanted to learn to do micro-controller programming. Super powered arm chips like this one start at $2.00, so for many things where you would have used individual transistors or logic gates in the past, there's not many reasons left not to just use a micro controller.

Next up: Switch debouncing.