Hi everyone. I have been poking around a bit and I couldn't find the answer to my question, likely as I had no idea where to look. This is a practical problem and I don't really even understand the theory (physics was a LONG time ago)! Basic Problem: I am doing a project in my North American Car. The whole electrical system is based on 12v power, reality is of course around 13v while the car is running. I have a relay that reacts to an "input" and sends out a "12v" signal as its reaction. As long as the "input" is present it sends its 12v out. As soon as the "input" ceases the relay stops sending. I need to hook this 12v output into something that is designed to take around 9 or 10 volts maximum. What resistor would I have to put "in line" with the output of the relay to lower the voltage to 9 or 10 v? Full Circumstance: I have a car that did not come with factory navigation. The radio has an input for the factory navigation comprised of a MONO input and a mute sensor, such that when a 9 or 10 v current is applied to the mute sensor it turns off the stereo in the front speakers and plays what ever is coming in on the mono input through the front speakers. I have a TomTom navigation system hooked into a VOX relay via the headphone jack and a splitter. One goes to the VOX relay, the other to the stereo. When the VOX relay "hears" the TomTom output its sound it sends a 12v signal out. The radio in the car however was only designed to take 9 or 10 v in. What resistor would I have to put "in line" with the output of the VOX relay to lower the voltage to 9 or 10 v by the time it hits the stereo?