This was featured on Hacked Gadgets on 26 January 2007

This is a fan controller for an audio/video cabinet.   It uses a PICAXE 08M and a DS18B20 temperature sensor.   The fan is turned on at 30 degrees C (~86 F) and off at 28 degrees C (~82 F).   The green LED is the activity indicator and also serves as a "ballpark" temperature indicator.   All off-board connections are made via screw terminals.   The left terminal block for the fan, the bottom block for power, the right terminal block for the DS18B20.

LED status indications
  • one second on - reading DS18B20
  • one second on plus two short flashes (fan off) - temperature rising between OFF and ON points
  • one second on plus fan on - at or above ON point
  • one second on plus two short flashes (fan on) - temperature falling between ON and OFF points
This project was initiated when we replaced a 12-year-old TV with a new 26" HD-capable LCD TV (happy 41st anniversary to us ;-)   This TV was the largest that would fit in the existing corner cabinet (which is the largest cabinet that will fit between the brick fireplace and the window on the side wall).   After moving the DVD player shelf up, there was still less than 2" (50mm) clearance above the new TV.   Initial testing (hand on the upper shelf) indicated this was not enough for adequate ventilation.   A thermometer verified this with a reading of just over 114 degrees F (~46 C) - a rise of 26 degrees F above room temperature.

The fan is a 12 volt, 11CFM unit running off the regulated 5.2 volt, 1.2 amp supply that also powers the PICAXE board.   The lower voltage reduces the fan speed (and noise) but the fan still delivers enough air flow to limit the temperature rise at the hottest point (directly above the TV) to 12 or 14 degrees F.   A maximum cabinet temperature of 92 degrees F (33 C) in a room that's at 78 degrees F (~25 C) is certainly acceptable.

This was installed on 24 September 2006.   After a week or so of testing, the circuit board will be placed in a case with a transparent or translucent cover (so the LED will remain visible).   Note that US/metric conversions are approximate - I'm not processing silicon wafers, just preventing overheating ;-)

Update 7 March 2020: The PICAXE is still merrily flashing the LED and turning the fan on and off but I did have to replace the fan.   It seems some things just don't last forever as the PICAXE does ;-)

protoboard (unboxed and propped on UPS cables for photo - the UPS ensures that my wife's soaps get taped)

draft copy of Circuit Diagram | program listing

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