In some circumstances, it may be necessary to select LEDs with closely matched characteristics. This design makes the job a whole lot easier. It uses two tracking current sources to allow the comparison of the two LEDs under test. LED current is adjustable by potentiometer P1 giving a range from 1 to 50 mA. Zener diodes D1 and D2 ensure that the voltage across the LEDs cannot rise above 4.7 V. This prevents the LEDs from being destroyed if they are accidentally connected to the tester the wrong way round.
Each of the two opamps together with a transistor builds a voltage controlled current source (more accurately a current sink). Each of the 100 Ω emitter resistors acts as a current sense, the voltage developed across them is proportional to the LED current. A voltage of 100 mV per mA of LED current can be measured across the emitter resistor using either a DVM or panel meter. This allows precise control and display of the LED current.
Current through both LEDs track together with very good accuracy and makes it a simple job to identify matching LEDs.