Zener Diode Tester circuit

In this schematic circuit is presented a very simple Zener Diode Tester circuit that require few external components to  be build .

As you can see in the schematic circuit , the Zener Tester  project is based on a common 741 operational amplifier .The supply need only be as high a value as the zener itself .
The variable resistor P can be calibrated ( in volts ) , so when the LED 1 lights the voltage on pins 2 and 3 of the 741 operational amplifier is nearly equal .