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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Improved Automatic Exhaust Fan

The "Automatic Bathroom Exhaust Fan" in the May 2005 edition is easily modified to provide adjustable fan run time. Features of the original design include automatic detection of hot water usage via a pipe-mounted NTC thermistor, along with single pushbutton on/off control. Fan run time was fixed but using the details given here, you can add a potentiometer to give a useful adjustment range of 4-10 minutes.

Click for larger image

Several modifications to the orig-inal circuit are also required. We’ve reproduced the circuit from May 2004 for convenience and highlighted the alterations in red. In summary, the changes are as follows:

  • Replace the PICAXE-08 with the PICAXE-08M version.
  • Move the piezo transducer to output0 (pin 7).
  • Add a 10kΩ potentiometer (VR1) and 270kΩ resistor.
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