Timers And Multivibrators

Multivibrators are 2-state electronic circuits used to make simple waveforms, oscillators and timers.

There are three main types of multivibrator:

• Astable multivibrator: A circuit which is not stable (hence astable) in any state, it continuously switches from one state to the other.
• Monostable multivibrator: One state is stable, while the other is not. A trigger input puts this circuit into the unstable state, and at some time after this, the circuit will automatically switch back to the stable state. This is also called a one shot.
• Bistable multivibrator: The circuit is stable in both states. A trigger input is needed to switch from one state to the other. This circuit is much more commonly known as a flip-flop.

History

The first recorded multivibrator was the Abraham-Bloch oscillator in 1919, which functioned as an astable multivibrator. The circuit was called a multivibrator because the square wave output contained a large proportion of harmonics.¹ Instead of being unwanted, these harmonics were actually useful in the calibration of RF devices.

Basic BJT Astable Multivibrator

A basic astable multivibrator can be made from two BJT NPN transistors and a handful of passive components, as shown below:

A basic (and somewhat of a classic) BJT astable multivibrator.

Note

The capacitors do not need to be polarized — they are just shown as polarized as is traditional for this circuit, and it helps you understand how the circuit works by showing the voltage polarity across each.

As a throwback to the past, Dick Smith’s Fun Way Into Electronics (first printed in 1979) contains a “flasher” which is exactly this circuit. This book is arguably what sparked my interest in Electronics!

’The flasher’ circuit here is a BJT astable multivibrator. The diode protects the circuit in case the 9V battery gets hooked up the wrong way, and the 390R resistor limits the LED current. Image from Dick Smith’s Fun Way Into Electronics, Eighth Printing, page 24.

How It Works

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The 555 Timer IC

Due to its popularity, the 555 timer is sometimes referred to as the “IC Time Machine”.²

Internal Workings

A simplified internal schematic of a 555 timer IC. Image from the TI SA555 datasheet, https://www.ti.com/lit/ds/symlink/sa555.pdf.

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. The schematic below shows a basic monostable 555 circuit.

A circuit showing how to configure a 555 timer into monostable mode.

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 multivibrator. 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 the TI SA555 datasheet, https://www.ti.com/lit/ds/symlink/sa555.pdf.

Equations (where $R_1$ and $R_2$ are labelled $R_A$ and $R_B$ respectively in the schematic above):

$$\begin{align*} t_H = 0.693 \cdot (R_1 + R_2) \cdot C \end{align*}$$ $$\begin{align*} t_L = 0.693 \cdot R_2 \cdot C \end{align*}$$ $$\begin{align*} T = 0.693 \cdot (R_1 + 2R_2) \cdot C \end{align*}$$ $$\begin{align*} f = \frac{1}{T} \end{align*}$$

Astable 555 Timer Calculator

Use the calculator below to find the two timing resistors ($R_1$, $R_2$) and the on/off times needed to run a 555 in astable mode at a given output frequency, duty cycle and timing capacitor. The duty cycle must be greater than 50% — to get a lower duty cycle, run an inverter on the output.

Enter the desired output frequency, duty cycle and timing capacitor to get the period, on/off times, and the two timing resistors (R₁, R₂) for a 555 in astable mode. Duty cycle must be greater than 50% — use an inverter on the output for D < 50%.

f

Hz

The desired frequency of the 555 output waveform.

Duty

%

The desired duty cycle. Because of the design of the 555, this must be greater than 50%. Use an inverter on the output for lower duty cycles.

C

F

The capacitance of the timing capacitor connected to the 555.

T

100 µs

The period of the output waveform, T = 1/f.

t_H

60 µs

The time the output is high in each period.

t_L

40 µs

The time the output is low in each period.

R₁

2.885 kΩ

The top timing resistor (between V_cc and the discharge pin).

R₂

5.771 kΩ

The middle timing resistor (between the discharge pin and the threshold pin).

Footnotes

1. Abraham, H.; Bloch, E. (1919). Mesure en valeur absolue des périodes des oscillations électriques de haute fréquence [journal article]. Annales de Physique. Retrieved 2026-07-04, from https://doi.org/10.1051/anphys/191909120237. ↩

2. Maniktala, Sanjaya (2012). Voltage-Mode, Current-Mode (and Hysteretic Control) [technical article]. Microsemi. Retrieved 2021-08-22, from https://www.microsemi.com/document-portal/doc_view/124786-voltage-mode-current-mode-and-hysteretic-control. ↩