Electronics Handbook/Components/Multivibrator

Multivibrator 555

The 555 timer IC is a very versatile and useful component and can be used for many things, from giving clock pulses to switch debouncing and as an output transducer. The basic 555 IC is housed in an 8-pin carrier, and these come in DIL and SOIC formats. They run from 3-15V, and are generally quite hardy devices.

The 555 has 8 pins. The layout of these pins is shown to the right and the function of these pins in given in the table below.

Pin	Abbrv.	Name	Function
1	GND	Ground Supply	This is the 0V Power Supply pin for the IC
2	TR	Trigger	A low voltage (less than 1/3 Vcc) starts the timer
3	Q	Output	This is where the output signal comes from
4	RST	Reset	A low state on this pin resets the timer, which will not be enabled until the Pin goes high again.
5	CV	Control Voltage	This is used to adject the trigger voltage. Usually, it is unused, and is connected to ground to avoid noise in the circuit.
6	THR	Threshold	This pin stops the timer when the voltage reaches 2/3 Vcc.
7	DIS	Discharge	This pin is connected to ground when the output goes low. This is used to control timing in certain applications.
8	+V	Positive Supply	The positive voltage supply to the chip. Must be between 3 and 15V.

Basic Circuit

The basic 555 astable circuit is set up like this.

At higher frequencies and loads, it is advisable to put a 100nF capacitor between Pin 5 and 0V.

The frequency and mark-space ratio are defined by the values of R1, R2 and C1 only. For a roughly even (1:1) mark-spce ratio, R1 is usually set at 1K, and the other resistor (R2) and capacitor are changed to alter the frequency. If better control over the mark-space ratio is required, use the next circuit.

Design equations

f={\frac {1.44}{\left(R_{1}+2R_{2}\right)\times C}}

.

{\mbox{High time}}=0.69\times \left(R_{1}+R_{2}\right)\times C

{\mbox{Low time}}=0.69\times R_{2}\times C

{\mbox{Mark-space ratio}}={\frac {R_{1}+R_{2}}{R_{2}}}

There is an online calculator here that will give the frequency, high time and low time for a given set of components.

Variable mark-space ratio

This variation allow much easier control of the mark-space ratio without too much impact on the frequency. The only difference is a diode petween Pins 6 and 7, with the cathode at Pin 6. The effect of this is to shunt resistor R2 when charging C1. This means that C1 charges though R1 only. This removes R2 from the "high-time" equation, and make calculating the mark-space ratio easier and prevents the frequency from being altered excessively by changes in the mark-space ratio ( although this still happens to a certain extent).

Design equations

f={\frac {1.44}{\left(R_{1}+R_{2}\right)\times C}}

.

This is different to the design equation for the standard 555 Astable. (R₂ instead of 2R₂)

{\mbox{High time}}=0.69\times R_{1}\times C

{\mbox{Low time}}=0.69\times R_{2}\times C

{\mbox{Mark-space ratio}}={\frac {R_{1}}{R_{2}}}

Mini-astable

This astable uses fewer components than the basic astable, and requires only the IC, one resistor and one capacitor, plus the Pin 5 capacitor for final versions.

This astable has a MSR of approximately 1:1, but this tends to increase under heavy current load, as the output does not swing fully to the power supply. This astable is not a reliable source of frequency, and should never be used where an accurate, stable clock is needed.

Design equations

f={\frac {0.72}{R_{1}\times C}}

.

{\mbox{High time}}={\mbox{Low time}}=0.69\times R_{1}\times C

Pin 4 Enable

Pin 4 is normally held high, and in this state it does nothing. However, when Pin 4 is brought low, the 555 is disabled, and the output is held low. This can be used to turn an astable on when desired. An example of this is the double-astable bleeper, which uses the output from one astable to enable another, faster astable.

Pin 8 should not be used as the enable pin, as this will draw all the current needed to run the device from the triggering circuit.

Other astable circuits

Units of Design Equations

The units that the components values and frequency are measure in are the standard base units: ohms, farads and hertz. This are often inconvenient, as the value for componets are usually several orders of magnitude away from the base unit. It is possible to use more conveninet units to calculate the frequency, and the common combinations are given below:

Resistance	Capacitance	Period	Frequency
Ω	F	s	Hz
MΩ	μF	s	Hz
kΩ	μF	ms	kHz

This plugin consists of a single 555 Astable. This plugin is very versatile, and can be used for any number of things, but it often is used to give a clock to run a circuit. The plugin below gives a pulse of about 2 Hz with a very low mark-space ratio. The circuit diagram is given below.

This circuit obeys the following formulae with regards to its frequency and mark space ratio.

f={\frac {1.44}{\left(R_{1}+R_{2}\right)\times C}}

.

This is different to the design equation for the standard 555 Astable. (R₂ instead of 2R₂)

{\mbox{High time}}=0.69R_{1}\times C

{\mbox{High time}}=0.69R_{2}\times C

{\mbox{Mark-space ratio}}=R_{ms}={\frac {R_{1}}{R_{2}}}

The values below give a frequency of about 2 Hz, with a mark-space ratio of about 1:50.

The stripboard layout is as shown below with a component list.

Ref.	Component
IC1	555 Timer IC
R1	10K Resistor
R2	330K Resistor
C1	2.2μF Capacitor
D1	1N4148 Signal Diode

Variations

Enable pin

To give this plugin an enable pin, replace the wire link from Pin 4 to +V with a 10K resistor going from Pin 4 to ground (Pin 1). Then add a header pin to Pin 4. A high signal on this pin will cause the astable to function, a low will disable it, locking the output low.

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