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