Laziness as an Embedded Engineering Virtue

After the Rover’s successful motor test, we noticed that assembling the device was a bit finicky.  Rather than fit everything onto the RCM5600W’s prototyping board, it would be easier to layout a printed circuit board.  We could also replace the 8 operational amplifiers with a single 8-pin buffer chip made by Texas Instruments called a 74HCT541.

Jimmy Buffer steps up to the Microphone

We have 8 digital outputs from the Rabbit’s parallel port A that we need to shift from 0-3 volts up to 0-5 volts and we have determined experimentally that we are too lazy to solder 8 Op-Amps.

First, the buffer is a digital device so it will either output 0 volts or 5 volts (Vcc).  According to the buffer chip’s datasheet, it will read anything above 2 volts as a logical 1.  The Rabbit’s output at 3.3 volts will get bumped up to 5 volts by the buffer and that means “bye-bye” to the operational amplifiers.

Swapping out the Op-Amps with an 8-pin buffer

Adding Features

With the circuit already drawn up in DipTrace, it wasn’t tough to update it and add a few new features.

• Add some 50-pin headers so we can plug directly into the RCM5600W’s demonstration board
• Replace the 8 Op-Amps with a single bus driver chip
• Add one more voltage regulator so we have access to 12 volt, 5 volt, and 3.3 volt supplies
• Add a socket for an XBee ZigBee modem

Check out Larry’s blog entry for more details and the files used to create the circuit board.

The New Schematic

Schematic Version 1.1

The last item is a big feature addition.  The Rabbit already has 802.11g Wi-Fi built into the RCM5600W and we’re planning on using the embedded server to deliver web pages for controls and other functionality.  However, Wi-Fi is short range and if we were to hypothetically toss the Rover out of a plane at 30,000 feet his odds of finding a Wi-Fi connection aren’t great as he plummets to the earth.  It might be useful to have a long range wireless connection for commands like “deploy parachute“.

Some of the ZigBee modems like the XBee can have much higher range because they operate at lower frequency.   A high gain ZigBee device operating at 900 Mhz with good line of sight can reach a range of up to 6 miles.  We won’t be sending data quickly, but remote operation becomes a real possibility.

The New Fangled Custom Printed Circuit Board (PCB)

The Rover PCB

The Joy of Lazy

While it might seem like a bad thing to be too lazy to solder down 8 Op-Amps, we have actually reduced the complexity of the design and the overall production cost.  In embedded engineering a simpler design is generally better and a little laziness up front will save us plenty of work later if we decide to build an army of Rovers to conquer the earth.