Stage 1 is the original circuit, Stage 3 is where I would like to go, if this circuit makes sense or can be improved.

Essentially the original circuit lights up the 2 LEDs individually. I wish to combine the voltage of these two outputs to control a transistor switch that when is in ON mode, will allow a brighter LED (LED C) to come on in its corresponding circuit. So if either one of the two original LEDs were ON, then the transistor switch is also ON. (sorry for the crude schematic)
Do I need diodes on each the + Vdc lines when intertwining them like this to insure current is flowing properly?

 

What is the voltage coming from the "CPU"? Is it a USB port?

 

The CPU is powered by a 6V adapter. Its not a USB port, its a small scoreboard circuit. LEDs light up when corresponding IR sensors are triggered.

 

The CPU is powered by a 6V adapter.

How many amps does the 6v adapter supply? Surely it would have enough to power the CPU and light an LED.

 

Diodes are definitely needed in the Vdc lines to protect the CPU.

If you are trying to get a different level of brightness out of the LED when one LED is on compared to when two LEDs are on, you will probably need a separate resistor in line with the diodes for each Vdc path.

If there is only one resistor, the voltage on the left side of the resistor will be 5 volts whether there are one or two outputs connected, therefore, no increased current into the transistor. (Not considering the diodes to be installed)

 

referring to this circuit, does the ground wire from the CPU even need to be connected to the secondary circuit as it has its own ground via the 9V battery that would be shared with the primary CPU circuit, because of their mutual connections via the transistor?

 

You use two diodes to make a simple OR gate, the inputs are somewhat isolated from each other. If you just tie two outputs together there is a good chance one will fry the other.

There is a kind of logic that is very common called open collector where this does not apply. When the outputs are tied together they form a NOR gate.

Yes, you need a common ground. How else will current flow through the BE of the transistor? While small, this current must be there. It doesn't have to be a common ground, but a current path is an absolute necessity.

 

You use two diodes to make a simple OR gate, the inputs are somewhat isolated from each other. If you just tie two outputs together there is a good chance one will fry the other.

There is a kind of logic that is very common called open collector where this does not apply. When the outputs are tied together they form a NOR gate.

Yes, you need a common ground. How else will current flow through the BE of the transistor? While small, this current must be there. It doesn't have to be a common ground, but a current path is an absolute necessity.

So then is the following schematic is valid? (I removed the ground connection from the CPU to the secondary circuit, since a current path will still exist for the small current at the base to flow into the negative terminal of the battery, rather than back into the ground of the primary CPU circuit...assume the two diodes will be in place, which are not shown)

 

Stage 2 is where I have the problem. There is a good potential for smoke of the bad kind.

The two negative leads are most likely ground (no guarantees). If so use something like this.

 

The positive voltage from the computer is only 6V with respect to its own reference or ground. The 9V from the battery is only 9V with respect to its negative terminal. To get them to work together they have to reference the same node or ground: the grounds need to be connected.

<edit> technically they don't have to be shorted together, but you do need a complete current loop from V+ to GND on the CPU side and a +ve to -ve current loop on the battery side (which you already have).

 

Note those aren't batteries, those are electronics (labeled CPU). You can't treat them like batteries. The current to transistor BE is the thing, voltage is not important.