Im trying to used a hacked optical mouse to drive a relay. The tracking LED in the mouse is lit when the device is on and the LED is off when the unit is in sleep mode (the USB port has power in both modes).

The tracking LED is pulsed around 4Khz. I want to smooth the 4Khz signal so that I can drive a relay (ie LED lit - even +/- 4Khz the relay triggers, LED off the relay is off).

I was fooling around with some RC filters but couldn't get the desired result.

Some help would be appreciated.

 

If you want to convert the pulsed signal a steady DC, you can rectify and filter it with a diode and capacitor as shown below:

 

Thanks for the help. the diode was the one thing I didnt try. Worked perfectly.