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Circuit Help Request.

Objective:
Have additional lighting source (foot well lighting or under carriage lighting for example) that is controlled by dome light activation, retains dome light dimming effect, and also has a kill switch in line.
Simple solution: One power supply
So, of course, one simple solution to this problem would be to power the additional lighting from the same source as the dome and wiring an inline rocker switch. However, my concern is if you run enough lighting this way the circuit may become stressed by adding more resistance from wire and draw than originally intended. Also, I’m concerned brightness might be compromised. (are these valid concerns?)
My question:
If I wanted to power the additional lighting separately, and retain the dimming effect would I want to use a SPST relay? My thought being main power would be delivered on the switched load, and the dome light would be extended to the control power on the relay.
My concern with this idea is that I don’t think an SPST relay can replicate the dimming effect (I think it’s either on or off). I’m hoping someone out there that sells products like these might have a suggestion for how to wire this to achieve this.
Diagram of my thought attached (although Im not sure this is ‘right’)

Short story:
I’m thinking of adding undercarriage lighting to my car that already has LEDs from the foot wells and dome that are linked. I have never installed them before, but I assume that I don’t want to daisy chain their power on the same circuit with the dome light and foot well lights, which are already piggybacked off the dome light

try to put something on the dome light power so when the coil collapses a spike doesn't go into the dome light power system. Say Capacitor or a clamping diode.

i believe a mosfet would allow for an alternate power supply, and still make the new lighting act in-line with the oem dimming

Quote:

Originally Posted by soundman98 (Post 1588960)

i believe a mosfet would allow for an alternate power supply, and still make the new lighting act in-line with the oem dimming

Doh. A Transistor.. Yes this makes sense to me

Kind of depends on how the manufacturer of the undercarriage lights says their illumination level should be controlled (assuming it can).
Meanwhile, as a simple on/off switch, your schematic will work, but personally I'd move the switch to between the Dome Light Power and the relay coil. Use the switch to control the relay coil, which then controls the lights.
I'm pretty sure the amount of energy discharged by the coil in this case is minimal. There should be no need for a capacitor to absorb the spike, but like the old lady with the chicken soup said, "it couldn't hurt."

Quote:

Originally Posted by Chimera (Post 1589924)

Doh. A Transistor.. Yes this makes sense to me

2 different types-- a transistor is just a semiconductor on/off switch, where a mosfet is a 'booster', so it would carry over the dimming feature

Still working on this..
so the LED I used in the dome light of my car dims. after doing some research and thinking I think this means that the power delivered to the dome light must be Pulse Width Modulated (PWM). (PWM is a power deliver scheme used with motors typically, it represents a power deliver scheme of high and low on a duty cycle vs a voltage supply somewhere in the middle, this is done very rapidly so it seems to be dimmed).

Does anyone know where I might be able to find a data sheet on the dome circuit that could confirm this?

And the truly tricky question:
Assuming the OUTPUT of the dome circuit is indeed PWM, can I use something like this:
http://www.ebay.com/itm/like/191092024531?lpid=82
To trim the power delivered to the end accessory safely?

I don't think that will do it directly, but something that will allow me to impose PWM on a pass thru signal.

Thoughts please

Edit:
Right now I'm thinking a MOSFET that's wired into the ground of the separated circuit that would have the power from the dome applied to the gate of the mosfet, there by applying PWM to the ground of the otherwise unaltered lighting circuit. Only problem I see with this is that solutions that use a control box might be adversely effected.

Today I learned that's there's a lot to know about electricity..
Here's my current mindset about this:

use one of these:
http://www.adafruit.com/products/355
and a generic 10k resistor to control current (but keep above the 2.5v gateway threshold) (also not entirely sure this is necessary..?)

see diagram: