555 Timing Circuits

555 IC Timing and Pulse Generating Astable & Monostable Circuits

The 555 timer IC is a very versatile component which can be used to build many different timing and pulse generating circuits.

The 555 is used frequently in Astable mode to generate a continuous series of pulses, but it can also be connected to make a one-shot or monostable circuit. The 555 can source or sink up to 200 mA of output current, which gives it the capability of driving wide range of output devices.

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555 Astable circuit example

To calculate the time output is high (t1) use this formula:
Time High (in seconds) = 0.693 * (R1 + R2) * C

To calculate the time output is low (t2) use this formula:
Time Low (in seconds) = 0.693 * R2 * C

The output frequency can be calculated by the formula:
Frequency = 1.44 / ((R1 + R2 + R2) * C)

The duty cycle percentage is the relationship of the high time (t1) to the overall cycle time and is derived by the following formula:
DCP = (t1 / (t1 + t2)) * 100

Where resistance is in ohms and capacitance is in farads.

555 Monostable Circuit Example

Monostable means that once the circuit is switched on or triggered it will time only once. In order to start the timing process again it must be reset (or trigged) again.

The resistor value R and the capacitor value C are unspecified.  The values of R and C will determine the length of time the 555 output is in the high state, and they may be calculated using this formula

T = 1.1 * R * C

Where T is the time period in seconds, and R and C are the component values in Ohms and Farads.