A 555 timer           555 Timer

Astable Operation
Limits
Frequency Range
[0.1Hz, 100kHz]
Resistance
(R1 + 2*R2) in the set [1k-ohm, 10M-ohm]
Capacitance
C in the set [1nF, 100uF]; 
Control capacitor value should be about 10nF.

555 Component Calculator for Astable Operation at a Desired Frequency

Equations
  • Frequency = 1.44 / [C*(R1 + 2*R2)]
  • Duty Cycle = (R1 + R2) / (R1 + 2*R2) * 100%

*Note* For R1 << R2, Duty Cycle ~ 50%.  The linked program (executable jar and python versions are on the "Programs" page) will help you design a timer with a 50% duty cycle for any frequency.  The idea is to solve the frequency equation for R1 + 2*R2 and then assume R1 = 0. You then solve for R2 and round down to the nearest standard resistor value.  Once you have R2 you simply subtract its value from the idea or unrounded R2 value, which gives you the value of R1.

Monostable Operation

This circuit is good for debouncing switches and not so much for logic-triggered delays.  It can be difficult to trigger with 74 series logic because a pulse detector circuit built with these chips is too quick for the 555.  It is better, I think, to just use a 74121 or 74122 chip, depending on your needs. I tried to use the 555 as a monostable multivibrator in both Project 1 and 2 for Digital II, and each time they had to be scrapped for the 121 or a counter circuit.

Equations
  • Pulse Width =
    1.1 * R * C


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