Such sensitivity is pretty impressive. Yet the microcomputer that runs the show need not be anything special. Indeed, one has a dizzying array of choices to pick from. But if you haven't a clue how to go about selecting and programming a microprocessor, don't worry: Greg developed his instrument with the novice in mind. He used the Atmel AT 89/90 Series flash Microcontroller evaluation kit, which includes a fully functional and extremely versatile microcomputer, one that links directly to a personal computer. This kit (model STK-200) includes everything you need to get going and costs less than $50 (see Amtel Corporation for a list of suppliers).

Unfortunately for Macintosh users, this system supports only IBM compatibles. In any case, you don't have to program everything from scratch, because Greg developed all the software needed to run the device, including instructions that show the weight in real time on a small liquid-crystal display (catalogue number 73-1058-ND from Digi-Key; 800-344-4539). You can download his code for free from the Web site of the Society for Amateur Scientists.

As with George's original design, almost any galvanometer plucked from a surplus bin will work. Just make sure that it measures small currents and that its needle tends to stay in place when the unit is rocked rapidly from side to side. Whereas George's prototype required the operator to squint at the needle, Greg's electrobalance senses the position of the needle electronically using a phototransistor and a light-emitting diode, which you can also purchase from Digi-Key (catalogue number QVA11334QT-ND comprises a single unit). Pierce a small piece of aluminum foil with a pin and center the hole on the phototransistor, as shown on page 90. With the foil covering most of the phototransistor, the signal will go from full on to full off very rapidly when the needle interrupts the light from the diode. Attach a sliver of balsa wood as shown to stop the needle exactly at that point.

If too little current is in the coil, the needle will rest on the bottom piece of balsa and block the light. Too much current lifts the needle completely out of the light path. Greg's software uses a sophisticated algorithm to keep the needle balanced between these two states. After the device has been properly calibrated and tared, this pulse width reflects the mass of the sample.

CONTINUOUS RECORDING of the changing weight of a one-centimeter length of moistened thread demonstrates the versatility of this inexpensive instrument.

The control circuit that helps accomplish all this magic is shown above. You will need to adjust the value of R1 to set the maximum current to something your meter can handle. The full-scale current might be indicated on the meter. Otherwise, use a variable resistor, a nine-volt battery and a current meter to measure it. Because Greg's galvanometer topped out at five milliamperes, he programmed the microcontroller to create a five-milliampere current by delivering a five-volt pulse across a one-kilohm resistor.

That current is not, however, directed through the coil. Rather it flows through a circuit called a current mirror, which forces an identical current to pass into the coil. This trick dramatically improves the long-term stability of the balance. Why? The resistance of the coil depends on its temperature, which rises whenever electrical energy is dissipated inside it. But the mirror circuit keeps the current constant no matter what the temperature of the coil is.

Of course, the resistance of R1 will itself vary somewhat with temperature, which could cause the calibration to drift. So you'll want to use a component with a low temperature coefficient. A 1 percent tolerance metal-film resistor, for instance, typically shifts a mere 50 parts per million for each degree Celsius. You will also need to keep the two transistors in the current mirror at the same temperature to prevent that circuit from drifting. It's best to use a set of matched transistors on a single silicon chip, like the CA3086 (48 cents from Circuit Specialists; 800-528-1417). Otherwise, wire two identical NPN switching transistors together with their casings touching as shown above.

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