## Simple Astable and Monostable Multivibrator Circuits Using IC 555 Explained

written by: Swagatam • edited by: Lamar Stonecypher • updated: 7/25/2011

The article presents some very useful, assorted IC 555 application circuits, which depict the two basic operating modes of the IC- the astable and the monostable modes- and how they may be exploited for various applications.

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Before indulging in some interesting IC 555 hobby circuits, it would be important for all to know about the two fundamental modes of operation of this device, which will hugely influence the included circuits explained later in the article.

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Monostable Mode

Here (see fig) the IC works in a monostable or "single-shot" mode. Initially the attached external capacitor stays discharged internally (through a transistor). A negative trigger less than 1/3 VCC applied at pin #2 initiates a flip-flop action, quickly discharging the capacitor and subsequently driving the output high. This results in an exponential increase in the voltage across the capacitor. The time period of this change can be expressed through the formula, t = 1.1Ra.C. The capacitor is fully charged after this period when the voltage across it finally reaches 2/3 VCC.

The flip-flop is then reset back by the internal comparator stage of the IC, resulting in an immediate discharge of the capacitor and ultimately the output shifts to logic low state.

Astable Mode

Another important configuration is the astable mode, which is basically formed by joining pin #2 and 6 of the IC together. This attributes the circuit with the property of running freely as a multivibrator through self-triggering. The charging of the external timing capacitor takes place with the help of the resistors Ra/Rb, and the discharging process completes through Rb. Therefore the duty cycle of the circuit can be accurately confirmed by adjusting the ratio of resistors Ra/Rb.

Here the capacitor undergoes charging and discharging between 1/3 and 2/3 VCC; these periods are well regulated and are not disturbed by supply voltage variations.

The above operational mode of the IC 555 identifies the chip to be basically a time interval generator with many variations. By varying the time interval ranges, circuits using this chip can be implemented for many different applications: we’ll study some of them through the following simple 555 timer circuit examples.

Images - Courtesy National Semiconductor Datasheet

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### Simple Egg Timer with Buzzer

The figure shows a simple circuit wiring layout for making an egg timer (1 to 15 minutes) and includes a buzzer for indicating the lapsed time. Here, two IC 555 are incorporated, one is used for generating the required time delay while the other stage simply activates a buzzer by inducing the set frequency into the buzzer once the time interval lapses.

A careful inspection shows the lower timer stage is configured as a monostable multivibrator which can be set and reset manually through push buttons. Once set via S1, the stage initiates counting, and depending upon the values of VR1, R3 and C, continues counting for the set time interval until C completely discharges to make the output go high.

This high pulse switches T1 ON which in turn provides the reset ground continuity to the upper oscillator stage which is rigged in the astable mode. Once triggered the oscillator stage sounds the buzzer through its generated frequency and raises the alarm indication. The circuit is reset by pressing S2, when the cycle begins afresh.

Parts List

R1, R2 = To be evaluated from online IC 555 calculator software.

R4, R5 = 100 K,

R3 = 1K,

VR1 = 1 M,

R6, R7 = 10 K,

Buzzer = TOKO, Piezo Buzzer.

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### Simple Touch Operated Call Bell

The figure on the left shows two very interesting applications of the IC 555 in the form of manually operated call bell circuits. Both the configurations depict the IC in the monostable mode and this particular mode of operation becomes absolutely desirable for the discussed application of a doorbell.

With this mode it becomes possible to set the “ringing" of the buzzer for an externally set interval of time, irrespective of the period for which the external manual trigger may be held. That simply means that if the trigger switch is depressed or activated even for a fraction of a second, it will be enough for the circuit to sense and hold its output ON until the time setting through R1 and C1 lapses.

One of the circuits uses a bell-push button while the other circuit employs a Darlington paired transistor making a touch trigger operation compatible with the circuit.

Parts List

R1 and C1 = From online 555 Calculator,

R2 = 10 K,

T1 and T2 = BC 547 B,

C2 = 0.1/50V, Ceramic,

Buzzer = Musical buzzer

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### Simple Day Night Street Lamp Controller

The IC 555 can be also wired as an accurate comparator and finds important applications in this mode of operation. One classic example is a light threshold detector circuit which may be suitably used for automatically switching street or porch lights when dusk falls and switch them OFF at daybreak.

Referring to the circuit, an LDR basically acts as the light sensor whose varying resistance is compared by the ICs internal comparator with R3 and the output changes state once the set threshold is crossed. The output triggers a relay whose contacts switch the integrated lights which are required to be controlled with the relevant specifications.

Parts List

R1 = 100 K,

R2 = Variable resistor, 1 M Pot,

R3 = 1 K,

R4 = 2M2,

C2 = 10 uF / 25 V,

LDR = 10 K in daylight under shade.

Relay = 12 V, 400 Ohm, SPDT.

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### Simple Bedroom Lamp Timer

The figure depicts yet another IC 555 circuit in the monostable mode which has been appropriately configured as an automatic bedroom night lamp. The circuit switches off a table lamp after a short time delay set externally by R1 and C1. Once S1 is pressed, the monostable starts counting and shuts off the relay and the lamp immediately after the set time interval has lapsed.

Parts List

R1 and C1 = Calculate from online 555 calculator.

R2 = 10 K,

Relay = As specified for the previous circuit.

S1 = Push-to-On Switch.

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### Simple Wig-Wag Lamp Flasher

As explained earlier, in the astable mode, the IC 555 runs freely, setting and resetting itself or simply starts oscillating at a fixed frequency, set externally through R1 and C1. In this design the circuit exploits this feature and switches an external load (LEDs) alternately to produce a flashing or a wig-wag effect. If a 12 volt supply is used, then each channel can be connected with 4 LEDs each, making the illuminations more interesting and dazzling.

Parts List

R1, R2, C1 = Calculate from online 555 calculator.

R3, R4 = 1 K.

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### Simple AC Lamp Flasher

Another simple configuration working on same principles is shown in the diagram. Instead of LEDs here we incorporate lamps operated by AC mains, which may become more suitable in places where high intensity flashings are required, like in discotheques, party halls, etc. Again the IC 555 runs in the astable mode, creating flashing pulses at frequencies set by R1 and C1. Since mains AC is involved, a triac becomes imperative for switching the mains operated load exactly in response to the received signals from the IC.

R1, R2, C1= Calculate from Online 555 calculator,

R3 = 1 K,

Triac = BT 136.

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### References

Authors own experience

Component datasheets

All circuits drawn by Swagatam