CIRCUIT DESIGN

555 Timer Circuits

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Internal Workings

A simplified internal schematic of a 555 timer IC. Image from https://www.ti.com/lit/ds/symlink/sa555.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe.

A simplified internal schematic of a 555 timer IC. Image from https://www.ti.com/lit/ds/symlink/sa555.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe.

Modes Of Operation

Monostable Mode (Time Delay Mode)

Monostable mode is when the 555 timer is configured to output a single pulse after a fixed amount of time. It only outputs one pulse and then stops until it is externally reset. This mode is used for creating a time delay.

Schematic for putting the 555 timer into monostable mode. Image from https://www.ti.com/lit/ds/symlink/sa555.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe.

Schematic for putting the 555 timer into monostable mode. Image from https://www.ti.com/lit/ds/symlink/sa555.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe.

Astable Mode

Astable mode is when the 555 timer is configured to output a continuous waveform with a fixed frequency and duty cycle. It is similar to monostable mode, except that it continually resets itself after every pulse.

Astable mode is also called running the 555 timer as a multi-vibrator. The duty cycle of the output waveform cannot be reduced below 50%. If you want a duty cycle lower than that, you have to use an inverter on the output.

Schematic for putting the 555 timer into astable mode. Image from https://www.ti.com/lit/ds/symlink/sa555.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe.

Schematic for putting the 555 timer into astable mode. Image from https://www.ti.com/lit/ds/symlink/sa555.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe.

Equations:

\begin{align} t_H = 0.693 \cdot (R_1 + R_2) \cdot C \end{align}

\begin{align} t_L = 0.693 \cdot R_2 \cdot C \end{align}

\begin{align} T = 0.693 \cdot (R_1 + 2R_2) \cdot C \end{align}

\begin{align} f = \frac{1}{T} \end{align}

Astable 555 Timer Calculator


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