You'll need to describe the circuit. How you do it depends entirely on what kind of circuit you're trying to control. Turning on a motor is different from lighting up an LED.
What microprocessor are you using? This question would be better placed in the Stamp or Prop forum (depending on which you're using) rather than Sensors.
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What microprocessor are you using? This question would be better placed in the Stamp or Prop forum (depending on which you're using) rather than Sensors.