Here is a circuit diagram for a simple logic probe in my digital electronics book I bought to learn electronics on my own.

It says in book when switch is High or Low, only that LED will light, but I'm having difficulty visualizing this. Why can't current flow straight down on right side from +5v to GND at all times lighting both LEDs?

 

Current comes about from a difference in voltage.

When the input is high, the top diode (plus resistor) is between 5V and 5V.
There is no voltage difference therefore there is no current in the top diode.
The bottom diode is between 5V and 0, so there will be current in the bottom diode.

When the input is low, the top diode is between 5V and 0. The bottom diode is between 0 and 0.

If the input wasn't connected both LEDs would be on.

 

Current comes about from a difference in voltage.

When the input is high, the top diode (plus resistor) is between 5V and 5V.
There is no voltage difference therefore there is no current in the top diode.
The bottom diode is between 5V and 0, so there will be current in the bottom diode.

When the input is low, the top diode is between 5V and 0. The bottom diode is between 0 and 0.

If the input wasn't connected both LEDs would be on.

Ok I'm still confused. Isn't the path from top to bottom 5v to 0v regardless of the input?

 

Ok I'm still confused. Isn't the path from top to bottom 5v to 0v regardless of the input?

Well yes, the very top is 5 and the very bottom is 0.

The middle node, output, is equal to the voltage given from the input.

So when input is high, you have:

+5V
Top diode + resistor
+5V
Bottom diode + resistor
0

When the input is low, you have:

+5V
Top diode + resistor
0
Bottom diode + resistor
0

Only elements between a voltage difference will conduct current.

 

Why can't current flow straight down on right side from +5v to GND at all times lighting both LEDs?

Yes, it certainly can. That's how it shows you the "undefined logic level".

 

Hello,

You could also take a look at my TTL Logic probe:
http://forum.allaboutcircuits.com/showthread.php?t=22076
It is using the correct high and low levels, and will also show pulses.

Bertus