Simple 4n25 optoisolator question

Newbie alert, but what does pin 6 do?

I have pins 4 and 5 hooked up to a test LED so when I apply power to 1 and 2, the test LED lights up. What does pin 6 do? I've heard mention that it will amplify the signal, but when I connect it to ground or power, my test LED just goes out.

If it will amplify a signal, I'd like to know, because the signals I'll be receiving are potentially low.

Pin 6 is the base of the transistor. Ordinairily pin 6 is not used but since all the terminals of the transistor are available, you can use the transistor like any other. If you are looking for more sensitivity from the LED input to transistor output, use an opto with darlington transistor.

All opto datasheets show the coupling-ratio %. Connection the base with resistor to GND (or to emitter) will reduce the sensitivity, in some case need this for increasing the noise-immunity.

Thanks, guys.