555 Timer Calculator (Astable timer)

For a recent (incomplete, ongoing) project, I wanted to create a circuit to trigger my ultrasonic rangefinder, so I bought some cheap 555 IC chips. Now there’s a lot of info out there about basic 555 timing circuits, and how to select your resistors and capacitors to generate square wave outputs of various frequencies and duties… However some of the calculators out there don’t tell you how they generate the numbers, and some of the sets of equations are simplified, or not quite correct.

The theory behind a 555 chip’s use is pretty simple. Here are the typical voltage levels for the chip:

  • Vcc is normally 5V, this is both the chip supply voltage and output voltage. This can vary, but that’s fairly standard.
  • threshold voltage is normally 2/3 of Vcc (e.g. 3.33V)
  • Trigger voltage is normally 1/3 of Vcc (e.g. 1.67V)

Here’s a summary of how the chip functions (see the referenced data sheet for a good explanation and tables showing input states versus output states):

  • Reset pin must be HIGH
  • When threshold pin voltage exceeds threshold voltage, the Discharge Switch goes open (connecting the Discharge Pin to ground), the output goes LOW
  • When Trigger pin voltage drops below the Trigger voltage, the Discharge Switch goes closed (thus isolating the Discharge Pin), the output goes HIGH

What this means is that through the use of a capacitor and a couple of resistors you can create a timer circuit to for a number of functions:

  • delay OFF button press (quick charge the capacitor with positive terminal connected to VCC through resistor, and using the button to connect the capacitor to ground) – also useful for debouncing a button press
  • Generate a set frequency  square wave signal (Astable operation), determined by the size of capacitor and the values of two resistors. This is what my calculator is for.
  • to generate an adjustable duty cycle square wave signal (PWM) using a potentiometer in an Astable circuit with the centre pin connected to the discharge, one outer pin connected to Vcc and the other connected to the trigger and threshold pins.

There’s plenty of information on these and other circuits, so I won’t go into it here, just have a look at the Reference pages below, or do a web search.

So, as I said, the calculators available on a lot of websites are a bit… not good. Some are “black box” calculations, and some are just over simplified. I put together some of the standard calcs available online for Astable circuits, and added my own calculations to solve for Diode Adjusted circuits (which allows you to achieve a full duty range from 0% to 100%. Here’s the excel calculation, which includes circuit diagrams, graphs, equations and instructions:

555 Calculator

Note: I pinched the pictures from a website, and I can’t remember the address, so… Yeah. As one of the older guys I work with would say “use your eyes and plagiarise”


example data sheet for 555 chip   –   http://www.ti.com/lit/ds/symlink/na555.pdf

example 555 circuits   –   http://electronicsclub.info/555timer.htm

example 555 circuits   –   http://www.555-timer-circuits.com/


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