Switch question without voltage drop

[superflux]

I have attached an image.

I have 2 voltage sources coming in the circuit (X and Y). If voltage is present on X, then Y is not needed. X takes priority in this circuit, so Y is only used if no voltage is present at X.

Currently, I am using a mechanical switch as in the top diagram. I want to get away from this; however, I need 5v at Z whether using X or Y.

If I use a transistor, such as a PNP, in the lower diagram, which would tell me if X is off, then I would have to use a Diode on X to prevent the voltage from Y altering the state of the transistor. When I add the diode, I will get a minimum of a .6 voltage drop which I cannot have. I must have 5 volts at Z.

I'm looking for solutions.


Thanks!


**broken link removed**

 

[Nigel Goodwin]

There's far too little information there, you need to provide EXACTLY what everything is, and how it's connected.

 

[superflux]

Hopefully this will help a bit:

I'm using a PIC 16F88 to read voltages from an automotive sensor. The "Load" would be the ADC input (PORTA,4). I have a 1k resistor before the ADC port to reduce the current.

I haven't calculated the "base" resistor for the PNP transistor, but it shouldn't take much current to saturate the transistor.

 

[Diver300]

It seems the easiest thing to do is to run one more ADC channel on the PIC. It has 7 ADC inputs.

Then just use software to decide which reading to use.

 

[Nigel Goodwin]

It seems the easiest thing to do is to run one more ADC channel on the PIC. It has 7 ADC inputs.

Yes, by FAR the best way - particularly as he must have a spare pin as he's using to switch inputs.

 

[superflux]

ALright guys, I'll think through what you've given me and work up a plan.

Thanks!

 

[Nigel Goodwin]

ALright guys, I'll think through what you've given me and work up a plan.

What's to work out?, everything you need is already in the chip - check my PIC tutorial which uses two channels of the A2D.