The letter-balance works like a standard mechanical letter-balance. The weight measurement is done by finding the angle of the lever arm. To do this without disturbing the measurement, a separate arm, equipped with the light sensor moves next to the lever arm of the scale. The angle where the light sensor shows the disturbance is measured and translated into the weight of the letter.

The program logic does not measure the actual weight, it only determines whether the weight of the letter (or the ball) is more or less than 20 grams. This simplifies calibration enormously.

If it is below 20 grams the NXT block shouts out "55 cent", the standard letter postage in our country, if it is above it shouts "90 cent". If the touch sensor is pressed, the postage for letters within Europe is given.

The acoustic files have been created using the wav2rso utility.

I was quite proud of the simplicity of the construction until I pointed this out to the family that promptly reminded me of the device we usually use for weighing letters. It can be seen in the last picture.

Hence I called the NXT device "deluxe" as it sings out the postage.