The RS-455-3671 sensor used in the Automatic Rear Bicycle Light project published in the July/August 2010 edition can be replaced by a motion sensor that costs nothing instead of a fiver or thereabouts. The replacement is a homemade device, built from components easily found in the workshop of any electronics enthusiast. Effectively it works as a variable resistor, depending on the acceleration force to which the device is submitted. A prototype presented a resistance of 200 kΩ when not moving, and 190 kΩ when dropping about 1 cm. Constructing is easy. Cut off a piece of about 10 mm of copper tubing. Take a piece of conductive foam, the kind used to protect integrated circuits. Cut a rectangular piece of 10 x 50 mm. Roll up firmly until it can be push-fitted securely into the copper cylinder. Then insert a conductive wire through the centre of the cylinder, bend it and (optionally) add protective plastic sleeving to each side. This is the first contact. Finally, solder a thin wire...