Then knowing he needed 4 mA to power each LED for backlighting he used some 1978 vintage current limiting diodes designed to pass 2mA each regardless of voltage source, within limits of course.

Figure 1 Current through a zener diode creates a fixed voltage across the device. The red LED provides visual indication of the current flowing through the resistor-zener-diode circuit. The higher the ...

The test current will therefore flow through the diode being tested instead of through the bypassed LED on the unit. The series diodes (D3 and D4) ensure that the voltage drop of D1 and D2 are ...