Need help on driving relay with PWM output

Thread Starter


Joined Jun 5, 2009
Here is the background....
My car using the Hi-beam as both Hi-beam and DRL, I would like to make the DRL function to a seperate LED circuit, So the Hi-beam bulb is only on when Hi-beam is switched on, and a set of LED will be on for DRL.

I used a DMM to measure the voltage on the Hi-beam wires, and it gave me reading of about 12v when Hi-beam is on, and 7.6v when DRL on. So I was thought I can use a SPDT relay with 8v pull-in voltage to seperate the circuit for Hi-beam and DRL, then use another SPST relay with 6v pull-in voltage to switch on a 12v with the 7.6v output to drive the LED set.

Looks good when Hi-beam is on, but the when DRL on, the LED is not turned on, and I hear some buzzing sound come from the relay. I guess the 7.6v for DRL may be actually achieved with PWM and thats why the relay didn't switch to on. So, is there any solution for this? Will adding a capacitor like the following works?