Relays, transistors, and a buzzing sound

short version- I have a low voltage source which supplies a relay with enouh power to stay on, but not enough to keep it powered without making a buzzing noise (I'm assuming the contact is slapping of the coil rapidly). I don't know what to do because the same input also sees 12v at other times. long version- I'm new to using transistors, but I'm pretty sure I've got them figured out for a set voltage, though I have some important questions that searces haven't answered. I have installed LED interior lighting into a PT Cruiser. The power from the interior lights is used to turn on a relay which turns on the LED's (it is important that the trigger is the interior lights, and not the door) When you open the door the interior/LED lights work as one would expect, though the interior light's slowly dim off within a few seconds of closing the door, and while this happens, there is a momentary (less than a second) buzzing sound coming from the relay. I haven't installed it yet, but I believe I will be able to fix that problem with a pulse-constant relay setup using a capacitor, but I don't see how this would solve the second problem... The real issue is when you are done driving the car, and turn the car off. The interior lights are powered at a lower voltage (I'm assuming 5v, I haven't yet checked) until the door is opened, and then the power jumps up to 12v. As long as the lower voltage is being supplied, there is a constant buzz sound coming from the relay. I thought of a using a transistor, but the problem I see with that is when the 12v turns on, there will be issues because the resistor used with the transistor was designed for a 5v input.

Thanks for the fast reply. Sounds like I need to better understand transistors. I would be using a BJT, but how would I figure out what resistor to use for both 12v and 5v? More info about lights.... I'm using the power through the interior lights to trigger a relay which turns on some LED lights which I have added to the car. The LED's are meant to come on when the door is opened, or any other time when the interior lights come on. They are basically used to supplement the interior lights. The issue I have is that after driving the car and turning it off, the interior lights come on dim (the car does this on it's own), because they are supplied with a 5v power source, but, as soon as the door is opened, the interior lights receive full power at 12v. It seems that the 5v is only enough to partially activate the relay coil, causing the buzzing sound-I have no issues at 12v. If the source was always 5v, a transistor would solve all my problems, but, the 5v power source will be 12v sometimes. It seems to me I need to design a circuit that will operate a relay with a 5v input, but not be overloaded if 12v (or even 13.8) is applied, but I don't know how.

Much easier way. Just use a resitor to make sure the Transistor always sees 5v on the trigger wire.  Then use a ground output off the transistor to trigger the relay which is getting 12v from a constant source.if its jammed force it - if it breaks in the process it needed to be fixed anyway

Sorry for the delayed response, I just noticed the post. I'm confused on how this would work. I'm actually going to be working on the car this weekend. Thanks for your time.

robbie2883 wrote: Much easier way. Just use a resitor to make sure the Transistor always sees 5v on the trigger wire.  Then use a ground output off the transistor to trigger the relay which is getting 12v from a constant source.

robbie2883 wrote: Much easier way. Just use a resitor to make sure the Transistor always sees 5v on the trigger wire.  Then use a ground output off the transistor to trigger the relay which is getting 12v from a constant source.

? Im not sure I understand

Turns out I actually had a fading ground, and so I did some research, and found a different solution... I of course needed a PNP instead, and I decided to use a larger transistor without using a relay at all. I could find more details than this if needed, but I was hoping someone could overlook what I've done to be sure what I've done is ok. It's a TIP120 PNP transistor from Radio Shack, and cat# is 276-2028. Unfortunately the ratings aren't online, but I do remember a few... Max Power - 65w Max Voltage - 40v hfe - 20 max current - 10a I'm not completely sure about all the info, and I'm also not completely sure I knew what I was doing, but it is working. I wired the fading ground up to the base with (4) 390 ohm resistors in paralell (Not sure if I'm right, but I figured I needed around 100 ohms to ensure saturation and that's all I had). The collector is connected to 12v +, and the emitter is connected to the LED's which are grounded (roughly 600-750mA's for all LED's). The transistor does not get very warm, but the resistors get very very hot. I'm concerned that they may be getting too hot, or that I might be pushing the transistor too hard. I've had it on for at least 10 minutes without any problems, but I am concerned about the heat. Any thoughts/comments would be greatly appreciated. Thanks for all the help so far, Dan