555 Timer as an Astable Multivibrator

An astable multivibrator, often called a free-running multivibrator, is a rectan­gular-wave generating cir­cuit. Unlike the monostable multivibrator, this circuit does not require any ex­ternal trigger to change the state of the output, hence the name free-running. Before going to make the circuit, make sure your 555 IC is working. For that go through the article: How to test a 555 IC for working An astable multivibrator can be produced by adding resistors and a capacitor to the basic timer IC, as illustrated in figure. The timing during which the output is either high or low is determined by the externally connected two resistors and a capacitor. The details of the astable multivibrator circuit are given below.

555-Astable-Multivibrator

Take a look @ 555 Ic Pin configuration and 555 block diagram before reading further.

Pin 1 is grounded; pins 4 and 8 are shorted and then tied to supply +Vcc, output (V_OUT is taken form pin 3; pin 2 and 6 are shorted and the connected to ground through capacitor C, pin 7 is connected to supply + V_CCthrough a resistor R_A; and between pin 6 and 7 a resistor R_B is connected. At pin 5 either a bypass capacitor of 0.01  F is connected or modulation input is applied.

Astable Multivibrator Operation

For explaining the operation of the timer 555 as an astable multivibrator, necessary internal circuitry with external connections are shown in figure.

Astable-Multivibrator-Operation

In figure, when Q is low or output V_OUT is high, the discharging transistor is cut-­off and the capacitor C begins charging toward V_CC through resistances R_A and R_B. Because of this, the charging time constant is (R_A + R_B) C. Eventually, the threshold voltage exceeds +2/3 V_CC, the comparator 1 has a high output and triggers the flip-flop so that its Q is high and the timer output is low. With Q high, the discharge transistor saturates and pin 7 grounds so that the capacitor C discharges through resistance R_B with a discharging time constant R_B C. With the discharging of capacitor, trigger voltage at inverting input of comparator 2 decreases. When it drops below 1/3V_CC, the output of comparator 2 goes high and this reset the flip-flop so that Q is low and the timer output is high. This proves the auto-transition in output from low to high and then to low as, illustrated in fig ures. Thus the cycle repeats.

Astable Multivibrator using 555 IC -Design method

The time during which the capacitor C charges from 1/3 V_CC to 2/3 V_CC is equal to the time the output is high and is given as t_c or T_HIGH = 0.693 (R_A + R_B) C, which is proved below.

Voltage across the capacitor at any instant during charging period is given as,v_c=V_CC(1-e^t/RC)

The time taken by the capacitor to charge from 0 to +1/3 V_CC

_{1/3 VCC ⁼ VCC (1-e^t/RC)}

The time taken by the capacitor to charge from 0 to +2/3 V_CC

or t₂ = RC log_e 3 = 1.0986 RC

So the time taken by the capacitor to charge from +1/3 V_CC to +2/3 V_CC

t_c = (t₂ – t₁) =  (10986 – 0.405) RC = 0.693 RC

Substituting R = (R_A + R_B) in above equation we have

T_HIGH = t_c = 0.693 (R_A + R_B) C

where R_A and R_B are in ohms and C is in farads.

The time during which the capacitor discharges from +2/3 V_CC to +1/3 V_CC is equal to

the time the output is low and is given as

t_d or  T_L0W = 0.693 R_B C where R_B is in ohms and C is in farads The above equation is worked out as follows: Voltage across the capacitor at any instant during discharging period is given as

v_c = 2/3 V_CC e^- t_d/ R_BC

Substituting v_c = 1/3 V_CC and t = t_d in above equation we have

+1/3 V_CC = +2/3 V_CC e^- t_d/ R_BC

Or  t_d = 0.693 R_BC

Overall period of oscillations, T = T_HIGH + T_LOW = 0.693 (R_A+ 2R_B) C , The frequency of oscillations being the reciprocal of the overall period  of oscillations T is given as

f = 1/T = 1.44/ (R_A+ 2R_B)C

Equation indicates that the frequency of oscillation / is independent of the collector supply voltage +V_CC.

Often the term duty cycle is used in conjunction with the astable multivibrator.

The duty cycle, the ratio of the time t_c during which the output is high to the total time period T is given as

% duty cycle, D = t_c / T * 100 = (R_A + R_B) / (R_A + 2R_B) * 100

From the above equation it is obvious that square wave (50 % duty cycle) output can not be obtained unless R_A is made zero. However, there is a danger in shorting resistance R_A to zero. With R_A = 0 ohm, terminal 7 is directly connected to + V_CC. During the discharging of capacitor through R_B and transistor, an extra current will be supplied to the transistor from V_CC through a short between pin 7 and +V_CC. It may damage the transistor and hence the timer.

However, a symmetrical square wave can be obtained if a diode is connected across resistor R_B, as illustrated in dotted lines in figure. The capacitor C charges through R_A and diode D to approximately + 2/3V_CC and discharges through resistor R_B and terminal 7 (transistor) until the capacitor voltage drops to 1/3 V_CC. Then the cycle is repeated. To obtain a square wave output, R_A must be a combination of a fixed resistor R and a pot, so that the pot can be adjusted to give the exact square wave.

Although the timer 555 has been used in a wide variety of often unique applications it is very hard on its power supply lines, requiring quite a bit of current, and injecting many noise transients. This noise will often be coupled into adjacent ICs falsely triggering them. The 7555 is a CMOS version of the 555. Its quiescent current requirements are considerably lower than that of 555, and the 7555 does not contaminate the power supply lines. It is pin compatible with the 555. So this CMOS version of the 555 should be the first choice when a 555 timer IC is to be used.

555 Timer as Monostable Multivibrator

A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci­tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accom­plished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure.

555-timer-monostable-multivibrator

Monostable Multivibrator Circuit details

Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is kept at + V_CC. To obtain transition of output from stable state to quasi-stable state, a negative-going pulse of narrow width (a width smaller than expected pulse width of output waveform)  and  amplitude of greater than + 2/3 V_CC is applied to pin 2. Output is taken from pin 3. Pin 4 is usually connected to + V_CC to avoid accidental reset. Pin 5 is grounded through a 0.01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor R_A is connected between pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply V_CC.

555 IC Monostable Multivibrator Operation.

555 monostable-multivibrator-operation

For explain­ing the operation of timer 555 as a monostable multivibrator, necessary in­ternal circuitry with external connections are shown in figure.

The operation of the circuit is ex­plained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 V_CC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cut­off, the capacitor C begins charging toward +V_CC through resistance R_A with a time constant equal to R_AC. When the increasing capacitor voltage becomes slightly greater than +2/3 V_CC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low, as illustrated in figure.

Thus the output returns back to stable state from quasi-stable state.

The output of the Monostable Multivibrator remains low until a trigger pulse is again applied. Then the cycle repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in figure.

Monostable Multivibrator Design Using 555 timer IC

The capacitor C has to charge through resistance R_A. The larger the time constant R_AC, the longer it takes for the capacitor voltage to reach +2/3V_CC.

In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as

t_p = 1.0986 R_AC
where R_A is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as

v_c = V_CC (1- e^-t/R_AC)

Substituting v_c = 2/3 V_CC in above equation we get the time taken by the capacitor to charge from 0 to +2/3V_CC.

So +2/3V_CC. = V_CC. (1 – e^-t/RAC)   or   t – R_AC log_e 3 = 1.0986 R_AC

So pulse width, t_P = 1.0986 R_AC s 1.1 R_AC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is widely used in industry for many different timing applications.

555 Timer Oscillator

555-timer-voltage-controlled-oscillator

The circuit is sometimes called a voltage-to-frequency converter because the output frequency can be changed by changing the input voltage.

As discussed in previous blog posts, pin 5 terminal is voltage control terminal and its function is  to control the threshold and trigger levels. Normally, the control voltage is ++2/3V_CC because of the internal voltage divider. However, an external voltage can be applied to this terminal directly or through a pot, as illustrated in figure, and by adjusting the pot, control voltage can be varied. Voltage across the timing capacitor is depicted in figure, which varies between +V_control and ½ V_control. If control voltage is increased, the capacitor takes a longer to charge and discharge; the frequency, therefore, decreases. Thus the fre­quency can be changed by changing the control volt­age. Incidentally, the control voltage may be made available through a pot, or it may be output of a transistor circuit, op-amp, or some other device.

Ramp Generator Circuit-using 555 Timer IC

We know that if a capacitor is charged from a voltage source through a resistor, an exponential waveform is produced while charging of a capaci­tor from a constant current source produces a ramp. This is the idea behind the circuit. The circuit of a ramp generator using timer 555 is shown in figure. Here the resistor of previ­ous circuits is replaced by a PNP transistor that produces a constant charging current.

Ramp Generator Circuit

Charging current produced by PNP constant current source is

i_C = Vcc-V_E / R_E

where V_E = R₂ / (R1 + R2) * V_CC + V_BE

When a trigger starts the monostable multivibrator timer 555 as shown in figure, the PNP current source forces a constant charging into the capacitor C. The voltage across the capacitor is, therefore, a ramp as illustrated in the figure. The slope of the ramp is given as

Slope, s = I/C

Police siren using NE555

Description.

A lot of electronic circuits using NE555 timer IC are already published here and this is just another one.Here is the circuit diagram of a police siren based on NE55 timer IC. The circuit uses two NE555 timers ICs and each of them are wired as astable multivibrators.The circuit can be powered from anything between 6 to 15V DC and is fairly loud.By connecting an additional power amplifier at the output you can further increase the loudness.

IC1 is wired as a slow astable multivibrator operating at around 20Hz @ 50% duty cycle and IC2 is wired as fast astable multivibrator operating at around 600Hz.The output of first astable mutivibrator is connected to the control voltage input (pin5) of IC2. This makes the output of IC2 modulated by the output frequency of IC1, giving a siren effect. In simple words, the output frequency of IC2 is controlled by the output of IC1.

Circuit diagram.

Notes.

• The circuit can be assembled on a Perf board.
• I used 12V DC for powering the circuit.
• Instead of using two NE55 timer ICs, you can also use a single NE556 timer.
• NE556 is nothing but two NE555 ICs in one package.
• Refer the datasheets of NE555 and NE556 to have a clear idea.
• Speaker can be a 64ohm, 500mW one.

Voltage doubler circuit using NE555

Description.
The circuit diagram of a very simple voltage doubler using NE555 timer is shown here. Here IC NE555 is wired as an astable mutivibrator operating at around 9KHz. The base of the two transistors (Q1 and Q2) is shorted and output of the astable multivibrator (pin 3) is connected to it. When the output of astable multivibrator is low, Q1 will be OFF and Q2 will be ON. The negative terminal of the capacitor C3 will be shorted to ground through T2 and it will be charged to the input supply voltage. When the output of the astable multi vibrator is high, transistor Q1 will be ON and transistor Q2 will be OFF. The capacitor C4 will be charged to the voltage across capacitor C3 plus the input supply voltage (that is double the input voltage). This is how the circuit works.

This voltage doubler circuit can deliver only up to 50mA output current and above that current limit the output voltage will be dramatically reduced. The actual output voltage will be around 19V for a 12V DC input and also the output voltage will be a bit unstable. Anyway, for low current applications this circuit is well enough.

Circuit diagram.

• Notes.
• The circuit can be assembled on a vero board.
• The output current should not be allowed to exceed 70mA.
• IC1 must be mounted on a holder.

PWM lamp dimmer using NE555

A simple and efficient PWM lamp dimmer using timer IC NE555 is discussed in this article. Yesterdays linear regulator based dimmers can only attain a maximum efficiency  of 50% and are far inferior when compared to the PWM based dimmers which can hit well over 90% efficiency. Since less amount of power is wasted as heat, the switching elements of PWM dimmers require a smaller heat sink and this saves a lot of size and weight. In simple words, the most outstanding features of the PWM based lamp dimmers are high efficiency and low physical size. The circuit diagram of a 12V PWM lamp dimmer is shown below.

Fig 1 : PWM lamp dimmer using NE555

As you can see, NE555 timer IC which is wired as an astable multivibrator operating at 2.8KHz forms the heart of this circuit. Resistors R1,R2, POT R3 and capacitor C1 are the timing components. Duty cycle of the IC’s output can be adjusted using the POT R3. higher the duty cycle means higher the lamp brightness and lower the duty cycle means lower the lamp brightness. Diode D1 by-passes the lower half of the POT R3 during the charging cycle of the astable multivibrator. This is done in order to keep the output frequency constant irrespective of the duty cycle. Transistors Q1 and Q2 forms a darlington driver stage for the 12V lamp. Resistor R4 limits the base current of transistor Q1.

Understanding the variable duty cycle astable multivibrator.

As I have said earlier, the variable duty cycle astable multi vibrator based on NE555 forms the foundation of this circuit and a good knowledge on it is essential for designing projects like this. For the ease of explanation the timing side of the astable multivibrator is redrawn in the figure below.

Fig 2: Astable multivibrator with variable duty cycle

Upper and lower halves of the POT R3 are denoted as Rx and Ry respectively. Consider the output of the astable multivibrator to be high at the starting instant. Now the capacitor C1 charges through the path R1, Rx, and R2. The lower half of POT R3 ie; Ry is out of the scene because the diode D1 by-passes it. When the voltage across the capacitor reaches 2/3 Vcc, the internal upper comparator flips its output which makes the internal flip flop to toggle its output. As a result the output of the astable multivibrator goes low. In simple words, the output of the astable multivibrator remains high until the charge across C1 becomes equal to 2/3 Vcc and here it is according to the equation Ton =0.67(R1+Rx+R2)C1.

Since the internal flip flop is set now, the capacitor starts discharging through the path R2,Ry into the discharge pin. When the voltage across the capacitor C1 becomes 1/3 Vcc, the lower comparator flips its output and this in turn makes the internal flip flop to toggle its output again. This makes the output of the astable multivibrator high. To be simple, the output of the astable multivibrator remains low until the voltage across the capacitor C1 becomes 1/3 Vcc and it is according to the equation Toff = 0.67(R2+Ry)C1. Have a look at the internal block diagram of NE555 timer shown below for better understanding.

Fig3: NE555 internal block diagram

How does the frequency remain constant irrespective of the position of POT3 knob?.

What ever may be the position of  POT3 knob, the total resistance across it remains the same (50K here). If anything decreases in the upper side (Rx) the same amount will be increased in the lower (Ry) and the same thing gets applied to the higher(Ton) and lower(Toff) time periods. The derivation shown below will help you to grasp the matter easily. 

With reference to Fig 2, we have:

Ton = 0.67(R1+Rx+R2)C1

Toff= 0.67(R2+Ry)C1

Total time period of the output waveform “T” is according to the equation :

T = Ton + Toff

There fore, T = 0.67(R1+Rx+R2+R2+Ry)C1

                        T= 0.67(R1+2R2+Rx+Ry)C1

We know that Rx+Ry = R3

There fore T = 0.67(R1+2R2+R3)C1

Therefore frequency F = 1/(0.67(R1+2R2+R3)C1) 

From the above equation its is clear that the frequency depends only on the value of the components C1, R1, R2  and the over all value of R3 and it has nothing to do with the position of R3 knob.