555 Timer. What are they and what do they do?. What do they do?. Many electronic systems require a timer to control their operation. The popular 555 timer IC can be used to produce two types of timed output. Monostable Astable. What’s inside?. That’s. 16 Resistors. 22 Transistors.
Trigger - This pin is the input which in turn causes the output to go high.
Threshold - Is one input which causes the output to go low.
Discharge - This pin is connected to the open collector of a NPN transistor (Q14), the emitter of which goes to ground.