# LED Tester Schematic Circuit Diagram

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 sensor, 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 both LEDs track together with very good accuracy and making it a simple job to identify matching LEDs.

A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. A potentiometer is a type of position sensor. They are used to measure displacement in any direction. Linear potentiometers linearly measure displacement and rotary potentiometers measure rotational displacement. Potentiometers work by varying the position of a sliding contact across a uniform resistance.
It is resistance, usually expressed in ohms. Depending on how you connect its three terminals, you can get either variable ohms (zero to some maximum value) or two variable values of ohms, the sum of which is always a fixed value For a primary circuit of the potentiometer the potential gradient remains the same. The potential gradient is calculated as K = V/L, where V is the voltage across the potentiometer wire and the L is the length of the wire in the potentiometer. So the unit of the potential gradient is volts/meter.

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