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LM555 and LM556 Timer Circuits

LM555 and LM556 Timer Circuits

This page presents general information and some tips when using the LM555 timer. Some of these circuits were developed just to see if the concept would work and have no intended purpose.
Calculation Value Notes

Data sheets for the 555 Timer use the value 1.44 and 0.693 as constants in the timing calculations depending on the way in which the equation was written. While these numbers are not exact reciprocals of one another they are close enough to be used without concern.

For ease of use, the calculators on this page have capacitor values entered in microfarads. This value is multiplied by the calculator to produce the correct result. (1uF = 0.000,001F = 1 X 10-6F)

“RESET” And “CONTROL” Input Terminal Notes

Most of the circuits at this web site that use the LM555 and LM556 timer chips do not show any connections for the “RESET” and “CONTROL” inputs for these devices. This was done in order to keep the schematics as simple as possible.

When the “RESET” terminal is not going to be used it is normal practice to connect this input to the supply voltage. This is especially true of the CMOS version of these timers as the inputs of these devices are very sensitive.


In many cases the “CONTROL” input does not require a bypass capacitor when a well regulated power supply is used. It is good practice however to place a 0.1 microfarad (C2) or larger capacitor at this terminal.

It is also good practice to place a 0.1uF – bypass capacitor (C1) across the power supply and as near to the timer as possible. This will reduce voltage spikes when the output transistors of the timer change states.

Note – If the period of the power supply variations is short when compared to the period of the timer, the overall effect of C2 is reduced.

For example; If the power supply – ripple voltage is 120 Hz and the oscillator frequency is 1000 Hz then C2 will have greater benifit than if the oscillator frequency is10 Hz.

Therefore, at low astable frequencies or long monostable lengths the effectiveness of a capacitor at the CONTROL input is less than at higher frequencies and short pulse times.

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