Laptop Cooler

This project was an experimental one learning about PICAXE programming.  Using a microproces-
sor is a bit of an overkill but the cooler is quite useful.  My laptop does get quite warm underneath 
and the fans seems to stabilise it at around 23 Celcius, 73.4 F.

What does it do?

It is a base for a laptop, a bit like a docking station, with fans underneath which blow air under-
neath the laptop to keep it cool.  This one is made with persplex and features some LEDs to 
brighten things up.  

It features:
    * Two low noise Evercool fans
    * PWM fan speed adjustment to minimize the noise
    * Temperature sensor (between the fans and a little lower)
    * Flashing LEDs and a piezo buzzer to warn of over-temperature
    * Power sourced from USB socket on laptop - there is an additional USB socket on the side 
      of the cooler to make up for the USB socket taken up plugging in the cooler
    * Pushbutton to change mode of operation (see below)
    * Surface mount PICAXE-18X SMD chip control for easy programming

On power up the fan's starts at low speed (about 30%, I found that the fan's wouldn't operate much 
below about 25%) and is virtually silent.  The LEDs flash randomly.  The temperature is sampled 
every 10 seconds, and once the temperature rises above 21 C, 69.8 F, the fan speed increases 
and the LEDs flash a little faster.  This happens linearly up to 30 C, 86 F, at which full fan speed 
is reached and the Red LEDs blink on and off and the buzzer beeps.  Above about 60% the fan 
noise becomes noticable.  

The pushbutton allows for several modes of operation:
    * standard operation as outlined above
    * as above, but no LEDs flashing
    * fan at full speed, Red LEDs flashing (but no beeping)
    * at power-up if the pushbutton is ON, there is a 30 second demo mode.  The fan and LEDs 
      cycle through 30%, 50%, 75% and 100% modes of operation, the normal operation com-
      mences.  This is just to demonstrate operation - otherwise a hairdryer or some other heat 
      source is needed to show how temperature changes mode of operation.

The main board is shown on the main page.  The surface mount PICAXE chip is on the underneath of the board, with a few other surface mount components. 

Circuit

I wanted to run the cooler off the laptop USB port, which are 5 Volt and rated to 500 mA.  Unfortunately I couldn't get a larger fan than 45 or 50 mm.  Otherwise I would have used a 
single 80 mm fan, but would have needed a 5 V - 12 Volt converter.  I believe laptop USB 
port's are protected with a PTC, but I included a resettable 500 mA fuse (this does have a 
small resistance causing a small but measurable voltage drop).  Better than destroying an expensive laptop port whilst experimenting.  The mini USB socket is extremely hard to work 
with - I used this because I had several USB cables of this type.  Working with the standard 
size sockets is much easier.  I included one on the board but others are easily added 
for expansion purposes.  The fan speed control is via PWM.  I tried several different methods, 
and at low speed (100 Hz, 4 kHz) the fans make a terrible noise.  In the end I used the PICAXE 
pwmout command with 60 microsecond duty cycle (16.6 kHz - above my hearing).  The fans 
need to be run at full speed for a few seconds, then will reliably run down to about 25% speed - 
below that and they stop.  I connected the RPM monitoring lead (these are 3 lead fans) back to 
PICAXE inputs, but when speed controlling via PWM it is hard to monitor the speed this way 
(the pwmout would need to be temporarily set to full speed to allow the pulse on the RPM moni-
tor lead to be read reliably).  I purchased 100 high brightness LEDs off Trademe so these work 
out at around 28 cents each, and plenty of colours to choose from.  I am running the LEDs at 
10 mA.  The fans draw about 170 mA each full speed, so even with all LEDs on we are still 
below the 500 mA max. for the USB port.  The FETs to switch the LEDS ON/OFF were about 
30 cents each from Farnell.  No problems using the surface mount PICAXE which I got from 
Sicom.

What would I have done differently?

I used perspex so the LEDs would be clearly visible.  In hindsight I believe it would have been 
better to make the top opaque and just use perspex for the sides.  A single larger fan may 
have been better, but would need a 5 V to 12 V converter, or an external power supply.  The 
ON/OFF switch could be replaced by another FET and auto-poweroff if running at low temp 
for a while (using the pushbutton to bring it back to life).  LED flashing could be more 
interesting.