PWM with temp sensor on 08?

[joe of loath]

hi again;
i have an antiquitated computer that does not have 3 pin fans (ie no control like on newer computers) and i want to make it quieter (by PWMing [pulse width modulating] the fans to keep the CPUs at about 40c). i have thought about it and decided an 08 with a temp sensor would be the best bet. firstly, can an 08 run off 12 volts (at about 200ma), does the 08 support PWM, and would i be able to dynamically control the fan by PWM, with constant input from the temp sensor?

thanks,
Joe

 

[Matthewriley]

It can't run of 12V directly, you would need soemething like an 7805 regulator (100mA version)or a zener shunt regulator (not as complex as it sounds). between that and the 08. All PICAXE chips need a voltage roughly between 3-6volts (where is 3v is low and 6v is pushing it). There is a fairy stable 5V provided by the computer PSU as you know.

 

[Cruiser]

Joe,

I think this has been done before so do a search of the forum and you'll probably find all you need.

But to answer a couple of questions...

1. To run a PICAXE off 12 volts you need to add a 5 volt regulator such as a 7805 chip. However, your computer will already have regulated 5 volts all over the place. It would be easier to tap into that to run the PICAXE.

2. It is possible to get PWM out of an 08 chip but it would be better to get an 08M which can do it in background while your program does other stuff.

3. You will need some sort of output driver between the PICAXE and the fan motor. Either a darlington or MOSFET. This is to switch the 12 volts required by the fan and also boost the current.

Chris.

 

[moxhamj]

This will end up being quite a neat and simple circuit - the 5V can come from anywhere on the motherboard, and it will be a 08M, temp sensor, 0.1/33uF filter caps, one resistor and one driving transistor.

You said the PC is old. Is it running Win 95 or 98? If so, there is a very clever free program called Rain which sends Halt commands to the CPU when it is not in use. The CPU actually runs cold. (WinXp has this inbuilt so Rain has become obsolete.) The program is getting hard to find on the internet so if you want a copy post your email and I'll send a copy.

 

[joe of loath]

well, i'm running 2000 as it is multiprocessor (pentium 2 ) and i can find rain-like programs on majorgeeks.com. i think the floppy runs off 5v, but all the 4 pin connectors have it anyway, so i could just power it off one of them. currently it runs at about 5c above ambient, with the fan on. i also looked on the search, and found some similar topics, but not many. i would like to try it on a strip board thingy. i guess it is possible on one of them?

joe

 

[premelec]

You can modulate the sense pin on an LM317T type regulator with PWM - it wants to 'see'
1.3 volts or so on that pin so a PICAXE could drive it whilst it was connected to 12 volt input to output... there might be some oscillations... capacitor should fix that.

 

[moxhamj]

All the 4 pin power connectors have 5V and 12V. There are various ways to get into them - maybe nibble the side of a wire till the copper is exposed and solder onto that. Or cannibalise a mating plug off a dead hard drive (I did this once - it took 10 seconds work with an angle grinder!). At least with sockets/plugs the project can be moved to other computers or back onto the bench for debugging.