Or saturation.
Is approximately equal to 0.2 volts.
Now you can see that it only takes a small change in Vin to move
the transistor from it's cut off region here to it's saturation region here.
And another thing to notice about the voltage between collector and
emitter, is that it has an inverting characteristic.
When Vin is small, the voltage output, or the voltage at the collector, is large.
But when Vin is large, the collector voltage is small.
So, if we thought of this as a circuit where the output is
taken at the collector and the input is here then we can consider this circuit to
be an inverting circuit or a BJT inverter.
So I wanted to show you an example circuit where we use an LED load on the switch and
we're going to determine the proper resistor values to have this func,
this circuit function correctly.
So, our input voltage, you could consider this to be an output from say,
a microcontroller, where the input to the microcontroller is a temperature sensor.
So say, the temperature is greater than some threshold.
Then we'd have the microcontroller put out a high voltage of 5 volts,
which would cause this BJT to turn on.
Which would cause current to flow in this loop, and have the LED illuminate.
The light emitting diode.
But if the temperature is less than the threshold determined by our sensor and
micro-controller combination, Vin would be equal to 0 volts,
the BJT would be off and the LED would not be lit.
Now, from the LED data sheet we can find that for
a diode current of about 20 milliamps.