This circuit can also be used to check forward voltage drop across diodes and LEDs at a pre-setted value of current. Here the maximum test current is near 12 mA with the 2.7K ballast resistor (R3) ...

The open circuit voltage tells you immediately, if this function can be used for testing higher voltage drop devices such as LEDs which require as much as 3V to turn on. Also, this DC voltage can be ...