Sturdy Attachment of a Lego Wheel to a Gearmotor
Nothing is more pathetic than a robot that has lost a wheel. Couplings based on the standard mating cross axle have a habit of sliding off. But, the peg holes present in larger wheels provide opportunities to securely attach the hub with screws. Using a lathe, milling machine, digital readout, magnets, PVC plastic, and a carefully-drafted layout can produce a rugged Lego coupler. Ah, an article on math and machining; what could be more universally appealing?

Replacement Battery Pack from AAA Cells
When a consumer device stops working because of a proprietary battery pack, you may be able to repair it by replacing the batteries with off-the-shelf rechargeables. If necessary, it is possible to solder together your own battery pack so long as you wear safety equipment, have a solder gun, and clean the terminals. Of course, this assumes you can find cells that match in size, voltage, and chemistry.

Afterthought Cake Robot
A pleasant robot that doesn't do much consists of a collection of interesting parts assembled together because they didn't fit any other robots. The robot is a successful experiment in making couplers shorter, employing escap rectangular motors, detecting distance moved with an interrupter infrared encoder, and a cube of multiple smaller PCBs. Powered by a single 180 mAh lithium polymer cell without a voltage regulator. Motors driven using low-voltage high-current Zetex bipolar transistors. Includes lots of big pictures and a descriptive demonstration movie.

Replacement for PNA4602M
The Panasonic PNA4602M 38-kHz infrared detector was a very popular electronic part for obstacle and wall detection. Unfortunately, the PNA4602M is discontinued and similar modules can't handle continuous signals. Good news! The new Vishay TSOP4038 is an excellent substitute. Tests compare detection distance, false pulses, and detection time. Also included is the relevant source code to variable-frequency duty-cycle measurement.

Serial Communication
A very popular method of sending and receiving data between devices and computers is good old asynchronous serial communication. See traces of characters being transmitted. Learn how far timing can be off before introducing errors. Includes a table of popular crystal frequencies and baud rates, as well as machining information and a source code snippet for an automatic bps detection tool.

Digital Indicator
Reuse is a good thing. In this case, a unique surplus item that appears to be a digital caliper is actually a digital indicator based on the caliper body and circuitry. The data port pinouts, mounting screw sizes, and example output analysis are posted, along with a cheap way of powering the tool from a 'AA' cell.

Repairing a Model Rocket
A rocket found in a tree had loose and missing fins, a broken parachute, and a lifeless shock cord. Replacement fins can be made from balsa wood, using a pen and a hobby knife. After sanding, they can be reattached with two kinds of glue. Simple repairs and replacements make the rocket flight capable again. After all that work, an overzealous choice of a rocket motor sealed the rocket's fate. Where do you think the rocket (and "Lego Major Tom") ended up?

Machining Magnesium for Elemental Density Comparison
To demonstrate the massive difference in densities of naturally-occurring elemental metals, a large magnesium ingot is machined down to a precise cylinder using a hacksaw, milling machine, and lathe.

Long Exposure Photography
LEDs create beautiful and informative light trails when a robot is photographed in a dark place with the shutter open. This approach captures the exact movements and behavior in a single photo, as opposed to replaying a movie over and over. Use a tripod, remote control, and experiment with lighting conditions for best results.

Capacitor Leakage
When power is disconnected from a circuit, the capacitors on the board are discharged by the electronic components until no voltage remains. That's fine. But, did you know that if you charge a capacitor on its own, it will slowly discharge through itself? More surprisingly, some capacitors become leakier with time, but can be healed through usage. I was shocked to discover how difficult self-discharge is to measure.

Industry-Standard LCD Modules
A blowout price on a 2x8 LCD module without specs leads to a bargain on industry-standard 14-pin alphanumeric LCDs. Demonstrates simple techniques for determining connector pins. Ultimately, the datasheet is found, and the contrast and backlighting pins are covered as well. Includes an example of writing custom bitmap characters.

Color Printed Circuit Boards
Green is so boring! Finally, your electronic devices can have beautiful red, black, and blue color circuit boards. The example shows an exposed-PCB Sandwich robot that starts with colored copper clad boards. The PCBs are etched at home with a peristaltic pump, after including an image of my dog and dealing with government-coded dots. The project is rounded out with a clear DIP socket and a shrouded battery cap.

Trimpots
A visual guide to breadboard-friendly trimpots (small variable resistors) that appear in nearly every hobbyist electronic project. The article features comparison pictures and part numbers of 3/8-inch and 1/4-inch square single-turn and multi-turn trimmers. Even more interesting is the cutaway close-up photographs of the inside gears and brushes of these tiny potentiometers.

Back-And-Forth Robot
A robot that drives forward to a wall and returns is a popular contest and student project. Surprisingly, this robot can be made with an off-the-shelf 8-pin motor chip, a couple of resistors, and a couple of chips. The article provides a complete walkthrough of the schematics, along with an animation of the changing states and a video.

Minifigure Multimeter
Have you ever had a pile of assorted resistors that you wish to quickly organize? Place each resistor in the hands of the electrified Lego minifigure to see and hear the nearest standard resistance. This practical meter includes both an ohmmeter for resistance measurement as well as a voltmeter for battery measurement.

Weatherproof Project Box
Modifying a Pelican micro-case for a dustproof, waterproof, and bugproof outdoor electronics container. Tips on punching a rubber liner and connecting external wires through a PVC pipe, or hacking a seal with poster tack.

Safe Sync for External Flash
Digital cameras have a relatively low voltage limit on their sync hot shoe, compared to mechanical cameras. This makes modern cameras susceptible to damage from higher-voltage external flash units, which consumers and hobbyists may attempt to reuse from previous generations. The circuit described in this article is a classic solid-state level-shifter often used to interface DC circuits to AC appliances.

Electrolysis Rust Removal
There's nothing more depressing than to see corrosion on your precision tools or antique hardware, particularly if the rust is in a hard to reach area. Fortunately, there's an electrical technique to reverse oxidation without harming steel or iron the way abrasives or harsh chemicals can. Includes results of testing a graphite electrode for the anode.

How to Measure Low Resistances
Occasionally you'll run across electronics such as relays, cables, igniters, switches, or current-sense resistors that have resistances less than one ohm. Few multimeters accurately measure less than 1 Ω, but most have a millivolt mode that can be combined with a 5 V source, a fixed resistor, and some simple math to determine milliohm resistances.

PCB Layout Tricks and Tips
A few extra holes, wider pads, and copper fills can greatly increase the versatility, solderability, and ease of testing of the printed circuit boards you design.

Solar Recharging
A small solar panel or series of solar cells can power an electronic device during the day, as well as recharge NiMH batteries to power the device at night. This simple recharger uses diodes for reverse-power protection and so that the project can measure the power source voltages. Includes schematics and graphs.

Multimeter Reviews
A digital meter is an essential tool for all robot builders and electronic hobbyists. If you're just starting out, can you buy the least expensive model and still get accurate results? How much do you need to spend to get a decent mid-range model? A dozen multimeters are tested and compared.