In my car I have two sources of 12v , one from foot well light and other from when car is started . I want to connect a light when either of these voltages are available.
How can I build a circuit that will give me 12v when either of these voltages are present?

I found this post on forum and I want to use the two diode solution but its not showing what resistor value to use

https://forum.allaboutcircuits.com/threads/12v-2-input-or-gate-circuit.139270/

 

Use two diodes, one from each source. Connect the cathodes together and to your light and then connect the anode from each to the corresponding source. This prevents the two sources from driving each other, but lets the light pull power from whichever source has the higher voltage.

 

Use two diodes, one from each source. Connect the cathodes together and to your light and then connect the anode from each to the corresponding source. This prevents the two sources from driving each other, but lets the light pull power from whichever source has the higher voltage.

if you look at the post they are using a resistor at the output , its not needed? without a ground reference the outputs of diodes will be floating I think.

 

if you look at the post they are using a resistor at the output , its not needed?

No.
The resistor is just to pull the output to zero if both inputs are zero for signal purposes, but you are using the diodes just to isolate the two batteries, so no resistor is needed.

 

No.
The resistor is just to pull the output to zero if both inputs are zero for signal purposes, but you are using the diodes just to isolate the two batteries, so no resistor is needed.

in this case if both inputs are zero the output will be zero also?

 

in this case if both inputs are zero the output will be zero also?

If your load has a capacitor with no bleeder or leakage current, then the capacitor voltage will be present.

 

If your load can conduct current to ground you will not need the resistor.

The output will be about 11.3 volts if input 1 OR input 2 had 12 volts on the anode. This is often referred to as a ”Diode OR” connection.

 

if you look at the post they are using a resistor at the output , its not needed? without a ground reference the outputs of diodes will be floating I think.

Your lamp is the resistor in your case.

 

in this case if both inputs are zero the output will be zero also?

How could it be anything else? Unless your lamp is somehow sustaining a voltage across itself....

 

In my car I have two sources of 12v , one from foot well light and other from when car is started .

So you want the light on whenever the car is running OR when the door is open. That's what your statement suggests.

Often automotive interior lighting is connected to a constant source voltage (the battery). It only comes on when a ground is provided. If you tie into the foot-light circuit, which is likely constant powered but only on when grounded by a switch then the diode approach won't work. It IS the right approach for how you describe the lighting circuitry, but the best I can see you hoping for is the additional light is powered by a switched 12V source, and the added light will be on because it's constantly grounded.

I see a potential issue: If the car is on - the light is on - and you open the door, the foot light goes on because of a ground - you short 12V directly to ground and blow out a diode.

Before you attempt this project it's advisable you first understand how the foot-lights are powered. And there may even be a BCM (Body Control Module) that detects the door open and activates the foot-lights.