EXPERIMENT 3
Try building this circuit and see what happens when you turn the input
switches on and off. Note how the diodes allow the input signals to
flow from the switches to the output but not back the other way.
This is a simple form of logical "or" gate. It has a number of drawbacks, though:
* It uses a lot of power, because current is flowing in the resistor whenever any of the inputs is high.
*
It's not very fast. You may have noticed that the output takes some
time to go from high to low. That's because the charge
that's built up in the output connection has to 
leak away through
the resistor. This time can be reduced by using a lower value resistor,
but then the circuit uses even more power.
* All the power to
drive the output comes from the inputs. In a complicated circuit with
lots of gates this can be a serious problem, because often the output
of one gate needs to drive the inputs of many others, and if the gates
are all like this, it mightn't have enough power to do so.
In the next few lessons, we'll learn about ways to solve all of these problems.