So what your saying is by taking electron flow from negative to positive the power is coming through the pot to the base of Q1; and through the opto-isolator to the collector and all of this heads towards +5V through the emitter?

 

So what your saying is by taking electron flow from negative to positive the power is coming through the pot to the base of Q1; and through the opto-isolator to the collector and all of this heads towards +5V through the emitter?

The simple answer to that is yes but please read on.

Call me old fashioned, but I believe that electronics should be learned from the ground up, not the other way around. If I drew a series-parallel circuit employing a DC voltage source and resistors only, would you be able to solve for the following?

The current through each resistor.
The voltage drop across each resistor.
The total circuit current.
The total circuit resistance.

I ask this because basic Ohms Law and Kirchhoff's Law should be understood and the student should feel comfortable with it before graduating to solid state theory and design. There will be little point in further explaining the Current Sensor circuit if you can't answer yes.

The typical structure of electronics courses look like this and usually in this order:

Understanding Static Electricity. (Covers relationship between Electrons, Protons & Neutrons)
Basic Series Circuits. (Includes Ohms Law)
Basic Parallel Circuits. (Includes Ohms Law)
Basic Series-Parallel Circuits. (Includes Ohms Law)
Basic AC Theory & Design. (Includes Reactance & Ohms Law For AC)
Introduction To Solid State Devices.
Introduction To Integrated Circuits.
And so forth.

 

CDRIVE, I do know the basics as even mechanical engineers are required to learn resistors, capacitors, and inductors. When the classes get to semiconductors I guess the consensus is that is beyond the need-to-know so they just skim it. But on the plus side I read all of the BJT sections of the textbooks on this site and I actually get them now! Quite a feat considering I've browsed countless sites trying to understand. And the ground sensing circuit works exactly as it should. I just want to be sure of one thing: the 1.5k resistor is there so that the current has to be above .5mA between +5V and ground before the potential difference across that resistor is great enough to overcome the diode junction of Q1 right?

 

OK, we can work with that.

As an engineer I think you'll appreciate the curves I just ran, as you will get more out of them than a textual description. You can clearly see how the Base-Emitter resistor effects the sensitivity of a 2N2907 PNP. The curve showing R3=10Meg can be regarded by the transistor as not including R3 at all. Increasing R3 to infinity won't change things much.

Edit: I should have mentioned that the meter labeled Base Cur is actually measuring the sum of the current from the Emitter-Base junction and the current through R3. Also, though the schematic shows a Base circuit voltage source of -5V, it's actually being swept from 0 to -5V by my spice program.

 

Having R3 = 1K increases the current flow at low input voltages but it delays the flow of current through the collector until about 1.3 volts input. And just like in the ground sensor there is no flow through the collector until the base current reaches about .6mA. It all makes sense now and seems so simple. How accurate are spice simulations when you have designed an entire circuit? Reasonable to trust your design is good if the simulation is good?

 

They are surprisingly accurate and they've made me very lazy! That said, there's no substitute for real world conditions, but even some of those can be plugged into the simulator.

Time to hit the sack... It's getting late!