Hi!
I'm about to design a circuit that will convert a PWM signal to a bar with 20 LED's. The PWM signal is locked to 300hz and 12Vpp with an offset at 12v. The signal is also inverted (100% duty when the level is at 0V.)

I want a LED bar with the resolution of 20 LED's to represent the duty cycle. I.E 50% equals 10 LED to be lit and 60% would result in 12 LED.. and so on.

My first thought is to use a second order low pass filter and a LM3914N (dot/bar display driver) but i have some trouble finding out which components and values to use.

I'm from Sweden but I hope I made my self understandable.
Thank you in advance!

 

My first thought is to use a second order low pass filter and a LM3914N (dot/bar display driver) but i have some trouble finding out which components and values to use.

I agree with that approach. A simple L-C filter will convert the pulse train into it's DC value (which is proportional to duty cycle). To fine tune component values, build it on a proto board. At 300 Hz, you won't have any problems using a flying leads breadboard.

100% duty when the level is at 0V.

That makes it more complicated, you will have to invert that if you want the LED string to read proportional to input duty cycle.