This is a device I built a long time ago for testing and comparing LEDs side by side. I used it mainly for testing and matching LEDs for use in Television and film props. The quality of LEDs is not consistent between batches even from the same supplier, so this device lets you do a visible test on two similar LEDs by pointing them at a surface and switching back and forth between them. You can instantly see intensity and beam quality differences. As an added bonus the unit also trickles 5uA through an LED when its socket is turned off to help identify defective Gallium Nitride layers. A good LED will generally glow new out the packet, but a defective one will leak current at low level and not light. Sometimes they will work after being run at full current, but it's an indication that the LED may fail early.
The device is simple. It uses a common PP3 9V battery with appropriate connecting clip to power the LED sockets via a 330 ohm resistor. The resistor feeds the middle pin of a single pole changeover switch (SPDT Single Pole Double Throw) to alternately feed the positive side of each Molex style connector. A 100K resistor across the switches two output terminals provides the leakage test current. The negative terminal from the battery goes straight to both the LED negative connections.
The Molex style connectors are 0.1" (2.54mm) pitch and have individual crimp-on spring terminal inserts. If you can't justify buying the proper crimping tool then you can solder to them too, taking care not to let solder run into the contact area. The sockets are just friction fitted into two holes in the end of the case.