So…you are truly curious about this little piece of components.
The above pictures show what it looks like on the board and the schematic diagram for the circuit. This circuit is called a logic testing circuit. This can test if the outputs of IC’s or digital outputs are high (1) or low (0).
This circuit uses the fundamental concept of diodes and electric current. Electricity flows from high potential to lower potential like a river, but you may be asking, how does the potential change?
The potential of the circuit drops after every component. So, at one end of a resistor, for example, will have a higher potential then at the other side. This drop is called a voltage drop and is caused by the resistor’s characteristics and is found through Ohm’s law.
Ohm’s law: Voltage = Current x Resistance
The diodes also have a voltage drop across them which drops the voltage further as you go along the circuit. This goes on until you hit the ground symbol this represents zero potential or zero voltage.
Now the question, how does this circuit test for a logic high (1) or a logic low (0)?
Well, when we connect whatever logic output to the point in-between the two LED’s it puts a voltage potential at that point. Using the fundamentals of diodes because LED’s are Light Emitting Diodes and follow the same principles, diodes only allow current to flow in one direction. That is why when you wire up the LEDs in reverse they will not turn on.
The effect of this point in-between the two LEDs causes this characteristic to happen. When the point is a logic high (1), a 5 volt potential is placed at that point and since the voltage potential before the RED LED is lower than the potential at the test point then the RED LED will not turn on. However, the GREEN LED will turn on. This will show that whatever you are testing is at a logic high(1).
And vice versa, when the test point is at a logic low (0) there will be zero voltage potential at the test point. This will only allow the RED LED to turn on, showing that whatever point you are trying to test is at a logic low.
And that’s all there is to it! Pretty simple right?