555 Timer cheatsheet

Here's is a quick visual guide to illustrate how the trigger and threshold pins can affect the output of this timer IC chip.

1. Trigger active

   Here the signal sent to the trigger is less than 1/3 of the supply voltage (VCC). The output is 1.

   The discharge transistor is disabled.

   

2. Threshold active

   Here the signal sent to the threshold is more than 2/3 of the supply voltage (VCC). The output is zero.

   The discharge transistor is enabled. Commonly this is used to discharge a timing capacitor to ground.