pwm cpu fan speed to leds

[PaulMRo]

Hy 2 all this is my first post :smile:

I have 2 x 12v pc case "neon" with leds , one red one blue and i want them to light up from pwm cpu fan speed .. blue one at low speed and red when on high speed .. the fan i will setup from bios to try to achieve a certain temperature so bios will control the fan speed from the cpu temp .. the power will be from a molex plug .. is it possible ? Any idee what can i do ?

 

[kam1787]

do you want both LEDs off if the fan is off?

do you mind if both LEDs are on [at high speed]?



I'd suggest an averaging circuit [simple RC might do) to get the average DC value, and use that to either feed two transistors to control the LEDs or a voltage comparator.

 

[PaulMRo]

The fan will never be off (only when pc is off ) it will be ideal to have separate lighting according to cpu fan speed with the 2 led strips on at the same time only when fan is at 50~60% speed

 

[schmitt trigger]

"I'd suggest an averaging circuit [simple RC might do) to get the average DC value, and use that to either feed two transistors to control the LEDs or a voltage comparator."

I second this suggestion. As the PWM value changes, so will the average DC value. You can compare this DC value with a dual comparator to enable the colour light strips. You'll have to experiment a little with the threshold voltages.

 

[PaulMRo]

im a total noob with electronics but i know with end to grab a solder gun , this is what i dig on google from the suggested solution from you guys (much appreciated btw)



is it ok like this ? And can i power the led strips directly from a molex 12v+ ? and only the ground from the comparator circuit ?

 

[kam1787]

that's the basic idea, though the top of RV would be connected to DC OUT

one thing you might not like is that the red led would be on and stay on once Vin rises above the point it turns on

 

[frozenfire_981]

i suggest that u may use a small microcontroller and read the PWM signal and then u can control the LEDs.
u can set any threshold voltage level for each LED separately.

 

[kam1787]

i suggest that u may use a small microcontroller and read the PWM signal and then u can control the LEDs.
u can set any threshold voltage level for each LED separately.

true but a bit expensive and complicated