A basic circuit for driving a light-emitting diode with constant 20mA current. A zener is usually used in place of the green LED, but this circuit shows how a LED's forward voltage is fairly constant over its current range. The circuit has the benefit of showing how although the current through the green LED changes considerably over an input voltage range of 6 to 12V, the current in the red LED is fairly constant. We can measure these values directly, but we can also tell visually that the green LED's brightness varies much more than that of the red LED.
