I cannot claim a shred of credit for this invaluable piece of Javascript. It was written by Robert Davis of Goldsmith's College. See his page here. I'm not in the habit of ripping off other people's stuff*, but this is just TOO useful to risk him taking his page off line.

555 Calculator

This calculator evaluates the performance of the generic bipolar 555 timer chip in both monostable and astable modes. Please enter the component values you require in the spaces provided, then click on the button and the results will be displayed.

As a rule of thumb, it is best to choose resistor values above 1K and below 1Meg. Large electrolytics should be avoided in favour of large value R's to reduce power consumption and timing inaccuracies due to leakage.

monequation.gif (432 bytes)

R:
C:

T Output High (Seconds) =

astequation.gif (1551 bytes)

R1:
R2:
C:

T1 Output High (Seconds) =
T2 Output Low (Seconds) =
T Total Period (Seconds) =
Frequency (Hz) =
Duty (%High) =

Note: It is not possible to have the low period longer than the high period with this basic circuit. Fortunately the 555 is able to sink as well as source current, or alternatively a PNP output transistor may used to give the necessary inversion.

*I will of course remove this material at Mr Davis' request.