LM 555 Timer IC Pinout

Ground - 1
Trigger - 2
Output - 3
Reset - 4

8 - +Vcc
7 - Discharge
6 - Threshold
5 - Control

Output is ON when ..

The Trigger has a Reference Voltage of less than 1/3 the Supply Voltage.

Output is OFF when ..

The Threshold has a Reference Voltage of more than 2/3 Supply Voltage.