CIRCUIT DESIGN

# 555 Timer Circuits

## 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.

### 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.

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}