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 center of the cylinder, bend it and (optionally) add protective plastic sleeving to each side. This is the first contact. Finally, solder a thin wire to the copper cylinder. This is the second contact. The foam resistance is pressure dependent. Consequently, when the device moves due to an external force, the inertia of the cylinder causes varying pressure in the foam, resulting in a small change of resistance between the inner conductor and the cylinder. Because of that, it’s important to ensure the cylinder vibration is not restricted in any way by the connecting wire or the PCB.
The comparator circuit shown here is capable of resolving the resistance change of the proposed foam/wire/copper sensor, allowing it to detect the motion of a vehicle for alarm or other purposes.