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555 Timer Calculator
Use DigiKey’s 555 timer calculator to calculate the output duration or frequency for a 555-timer circuit
output signal as determined by the resistance and capacitance values of the circuit.
This 555-timer circuit calculator will determine the output characteristics of square waveform output from a
555-timer circuit for both Monostable mode (one shot mode) and Astable mode (free running mode).
In Monostable mode also called One Shot mode, the output will be a single, short, positive pulse generated
for a single input event, the duration of which will be determined by the resistance and capacitance values
of the circuit.
In Astable mode also called Free Running mode, the output of the 555-timer will be a continuous rectangular
wave output signal, the frequency and high and low times can be calculated based on the resistance and
capacitance values of the circuit.
CHOOSE CONFIGURATION
R1 RESISTOR VALUE
C1 CAPACITANCE VALUE
FORMULA
OUTPUT PULSE DURATION
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555 Timer Explained
The 555-Timer IC (Integrated Circuit) is one of the most popular and widely spread ICs available
with uses for nearly everything from clock timing, signal delay, pulse generation and signal
oscillation applications.
In Monostable mode the 555-timer IC forms a Resistance-Capacitance circuit with the external
resistor and capacitor. When the input signal is applied to trigger pin, the internal workings of
the IC start to charge the capacitor. When the voltage across the capacitor is equal to 2/3 the
supply voltage of the IC the capacitor stops charging and the output signal of the circuit returns
to the low state ready for the next input signal.
The rectangular output waveform duration can be shortened or extended by adjusting the value of the
resistor and capacitor, changing the rate that the capacitor charges.
In Astable mode, the 555-timer forms a continuous output of rectangular waveform with a specific
frequency with a fixed portions of the output signal in a high and low state with two resistors and
one capacitor. When the 555-timer in astable mode is power on for the first time, the capacitor
starts to charge with voltage, driving the output signal high. As the capacitor charges until it
equals 2/3 the IC supply voltage. At that point, the capacitor starts to discharge, driving the
output signal low. When the voltage across the capacitor drops to 1/3 of the IC supply voltage, it
starts to charge up again driving the output signal high again and the process repeats again.
As the capacitor charges and discharges voltage through the 555-timer IC, it switches between the
two output states, High and Low. How much time is spent in those states and how quickly the cycle
repeats is a function of both resistors and capacitor values.
Examples
Monostable
The signal duration of the output can be modeled as Time in
seconds (T)
equal to the constant 1.1 multiplied by the resistance R measured in ohms (Ω) multiplied by the
capacitance value C measured in Farads (F).
So, for a circuit with 10,000 (10k) ohms (Ω) of a resistance and a capacitance value of 1,000
microfarads (1000 µF), the total length of the output signal would be as follows
Astable
The continuous output signal in Astable mode allows you to determine both the frequency in Hertz
(Hz), how long it takes the signal to repeat and the duration of time the signal is in the high and
low states. All three values that would define the output signal can be found using the values of R1
and R2 measured in ohms (Ω) and the capacitance value C1 measured in Farads (F).
So, for a circuit with resistors of 10,000 (10k) and 15,000 (15k) ohms (Ω) of a resistance and a
capacitance value of 10 microfarads (10 µF), the output signal would have the following
characteristics
Time High
Time Low
Frequency
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