Convert 600T fan controller to volume controller - (electronics help request!)

As most 600T owners know the built-in fan controller is pretty pants, so I thought I'd mod mine and turn it into a volume control, but not just wired into the line out but rather to actually adjust the windows master volume control just as you would using the system tray speaker icon.

All my fans are run off separate controllers, so the 'built-in' one has nothing connected to it.

I figured I would take one of the outputs from the controller and feed it into a little circuit that will generate pulses which can be fed back into the un-used 'case-fan' connector on the motherboard. The frequency of the pulses is directly controlled by the voltage output of the fan controller. Then using a bit of software I will read the 'fan speed' and adjust the volume accordingly.

Well, the software bit is done and if I connect an actual fan to the mb and press my finger on the fan to adjust the speed my volume goes up and down accordingly.

Technically I don't need any additional circuitry, I could connect the fans +v to the controller output and use the fan to generate the pulses, but thats a bit yuck.

So I need a little help with the electronics. What I'm after is a 'voltage controlled pulse generator' that generates a pulse on the with a frequency proportional to the voltage supplied by the fan controller. This is not a PWM generator.

Whilst I envisage some circuits may work better if they were connected directly across the fan controller pot rather than taking the variable 12v output, I would rather keep the controller intact, but am open to all ideas.

Any pointers, suggestions, volunteers greatly appreciated.

Chris

Thanks for the speedy reply.

I measured the voltage range on the fan controller and it goes from 8.9 to 11.8 which is less than what I was expecting and I'm not sure whether that range will give enough resolution. I was thinking of simulating 1000 to 2000 RPM and I'm assuming there is 1-pulse-per-revolution so thats a pretty low frequency range (16Hz to 32Hz).


Couple of questions..

Looking at the circuit sample you posted, would it be ok with taking +12v off the power supply and I'm pretty sure one of the ATX pins as -12v on it which I could tap into or would I be better using something like a DCP010515DB DC/DC converter which will give a +/-15v from a 5v supply? Maybe there are other similar chips I haven't come across.

Secondly, in the shown circuit what controls the relationship between input voltage and output frequency? Is there any info on what freq range that example gives and what would you tweak to adjust the range?

Thanks

Chris

Thanks again for the reply.

So I dug out my box of bits but couldn't find a one 555 timer so I ordered a few bits of ebay to try out some 555 based circuits and decided to bite the bullet and get a 'used' scope to help check my outputs.

I'm beginning to think that it might be simpler to just use a basic 555 setup connected directly to the pot rather than doing what I was initially planning by taking the output voltage from the pot, sure I can google a few circuits but if you have any suggestions it would be appreciated.

Whilst looking thru my bits I did find some old PIC bits so quickly knocked up an app and bread-boarded it to generate a signal and pump back into the mb, it worked a treat so thats another option since it can also handle an analog input.

Chris