I have the following question about a cascade 555 timer circuit. The thing what I do not understand in this schematic how the second input trigger (2) is triggered for low condition. I know that the capacitor (see red arrow) will be discharged after the output (3) of the first timer goes low. But how can trigger (2) of the second timer see that output (3) of the first timer goes low, because it is connected to the negative (lower) side of this capacitor.
The first 555 chip is triggered when SW1 is depressed, taking pin 2 to ground. This action takes the output on pin 3 to high, which lights LED1. Notice, however, that pin 3 of the first 555 is connected through a small capacitor to the trigger input of the second 555. As soon as the time interval expires on the first 555, its output goes low, which turns off LED1 and at the same time triggers the second 555, which in turn lights up LED2. LED2 stays lit until C2 charges, and then it goes out. The circuit then waits to be triggered again by a press of the switch.