Electronics Fundamentals/Integrated circuit/Timer

Multivibrator 555Edit

The 555 timer IC provides a very versatile, effective, and easy-to-use astable mode. By changing component values, one can get frequencies from a maximum of about 300kHz to as low as limited by the leakage of the capacitors used in the circuit. It can also have the mark-space ratio altered almost limitlessly.

 #str="kB wireless "
  1. print string 1fst and 4th character
  2. print string 5th 6th 3rd and 7th 8th 9th 10th character
       // by k show

Variable mark-space ratioEdit

This variation allow much easier control of the mark-space ratio without too much impact on the frequency. The only difference is a diode petween Pins 6 and 7, with the cathode at Pin 6. The effect of this is to shunt resistor R2 when charging C1. This means that C1 charges though R1 only. This removes R2 from the "high-time" equation, and make calculating the mark-space ratio easier and prevents the frequency from being altered excessively by changes in the mark-space ratio ( although this still happens to a certain extent).

Design equationsEdit

This is different to the design equation for the standard 555 Astable. (R2 instead of 2R2)


This astable uses fewer components than the basic astable, and requires only the IC, one resistor and one capacitor, plus the Pin 5 capacitor for final versions.

This astable has a MSR of approximately 1:1, but this tends to increase under heavy current load, as the output does not swing fully to the power supply. This astable is not a reliable source of frequency, and should never be used where an accurate, stable clock is needed.

Design equationsEdit


Pin 4 EnableEdit

Pin 4 is normally held high, and in this state it does nothing. However, when Pin 4 is brought low, the 555 is disabled, and the output is held low. This can be used to turn an astable on when desired. An example of this is the double-astable bleeper, which uses the output from one astable to enable another, faster astable.

Pin 8 should not be used as the enable pin, as this will draw all the current needed to run the device from the triggering circuit.

Other astable circuitsEdit

Units of Design EquationsEdit

The units that the component values and frequency are measured in are the standard base units: ohms, farads and hertz. This is often inconvenient, as the values for components are usually several orders of magnitude away from the base unit. It is possible to use more convenient units to calculate the frequency, and the common combinations are given below:

Resistance Capacitance Period Frequency
Ω F s Hz
μF s Hz
μF ms kHz

See AlsoEdit