This circuit provides a low cost and reliable method of testing zener diodes. RVl can be calibrated in volts, so that when LED 1 just lights, the voltage on pins 2 and 3 are nearly equal. Hence, the zener voltage can be read directly from the setting of RVl. The supply need only be as high a value as the zener itself
For a more accurate measurement, a precision pot could be added and calibrated.