Tilt Sensor: Cheap, Easy, Nontoxic

make a simple tilt sensor for your electronics project. this sensor will allow your robot or whatever to know when it has been inverted (or tilted to some angle such that the tilt sensor is inverted (i.e. needle side towards the center of the earth)

you'll need:
mechanical pencil
ball bearing with a smaller diameter than the pencil
two sewing needles
lighter or matches.

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1) take apart the mechanical pencil so that you have a hollow shaft, you don't need the other parts of the pencil

2) solder lead wires to the dull ends of the sewing needles.

3) heat up the sharp end of the needles with a lighter.
while still hot stick the needles through one end of the pencil shaft as seen in the diagram. they should be side by side with ~2mm space between them. (i.e. wide enough apart so that the ball bearing can sit on them comfortably.

4) put the ball bearing into the shaft.
stick something (cotton ball, balled up rubber band, tape, whatever) into the open shaft end to keep the ball bearing in there.

you're done.