555 timer integrated circuit IC voltages – NE555 – LMC555

The 555 timer has a number of voltages that it is looking at (input) or outputting at any given time. Understanding how these voltages influence the 555 timer will help a lot in understanding the 555 and other integrated circuits.

Other voltage topics to check out:


  • NE555 – A typical 555. Will work in most 555 timer circuits, as will most other ICs labeled as being 555.
  • LMC555 – A special CMOS version of the 555. Outputs rail to rail voltages unlike most 555 timers, but only at low current levels. 15V max. Typical output voltage with 5V suppy is 4.7V @ 2mA.

Affiliate link ad. That’s where I got LMC555, which cost a lot more than most 555 timers and can’t provide near as much power so only get them if they will work out better, or for experimenting/fun.


  • GND supply pins 1 (-/ground) and  supply pin 8 (+/Vcc), powers the IC and the output. Plus it sets the 1/3 and 2/3 threshold voltages.
  • Trigger pin 2 – Voltage Input – 1/3 or less supply V sets the output high and stops discharge pin from discharging.
  • Output pin 3- Powers a load with either a high (as close to Vcc as the 555 outputs) or low (pretty much a direct connection to 0V/ground).
  • Reset pin 4 – Voltage input
  • Control pin 5 – Helps stabilize the supply voltage with a small capacitor, and can be used to adjust threshold voltages (not covered here)
  • Threshold pin 6 – Input Sets output high and sets the discharge pin to discharge.
  • Discharge pin 7 – Connects to ground/0/- when output is set low and basically switches of (doesn’t conduct at all) when the output is high.
  • Supply pin 8 – Included with supply pin 1 at the top of this list.


Voltage of 555 timer output comparing NE555 to the CMOS LMC555 as digital inverter schmitt trigger

Astable pin 6 to 7

Voltages across 555 timer astable mode pin 6 to 7 timing resistor oscilloscope measured

Voltages across light dependent timing resistor for astable 555 oscilloscope measurements

Info not guaranteed to be accurate. Always verify through testing, datasheet review and other research!