A variety of simple circuits on the NE555. Chip timer NE555 amateur radio designs

The history of the creation of a very popular microcircuit and a description of its internal structure

One of the legends of electronics is integrated timer chip NE555. It was developed back in 1972. Not every microcircuit and not even every transistor can be proud of such longevity. So what is so special about this microcircuit, which has three fives in its marking?

Serial production of the NE555 chip started by Signetics exactly one year after designed by Hans R. Camenzind. The most surprising thing in this story was that at that time Kamenzind was practically unemployed: he quit PR Mallory, but did not have time to get anywhere. In fact, it was "homemade".

The microcircuit saw the light and gained such great fame and popularity thanks to the efforts of the manager of Signetics, Art Fury, who, of course, was a friend of Camenzind. Previously, he worked at General Electric, so he knew the electronics market, what was required there, and how to attract the attention of a potential buyer.

According to the memoirs of Camenzind A. Fury was a true enthusiast and lover of his craft. At home, he had a whole laboratory filled with radio components, where he conducted various studies and experiments. This made it possible to accumulate vast practical experience and deepen theoretical knowledge.

At that time, Signetics products were referred to as "5 **", and A. Fury, who had an uncanny sense of the electronics market, decided that marking 555 (three fives) would be most welcome for the new microcircuit. And he was not mistaken: the microcircuit went like hot cakes, it became, perhaps, the most massive in the entire history of the creation of microcircuits. The most interesting thing is that the microcircuit has not lost its relevance to this day.

A little later, two letters appeared in the marking of the microcircuit, it became known as NE555. But since at that time there was complete confusion in the patenting system, the integrated timer rushed to release all and sundry, naturally, putting other (read your) letters in front of the three fives. Later, based on the 555 timer, dual (IN556N) and quad (IN558N) timers were developed, naturally in more multi-output packages. But the same NE555 was taken as a basis.

Rice. 1. NE555 integral timer

555 in the USSR

The first description of 555 in the domestic radio engineering literature appeared already in 1975 in the journal Electronics. The authors of the article noted the fact that this microcircuit will be no less popular than the widely known operational amplifiers at that time. And they weren't wrong at all. The microcircuit made it possible to create very simple designs, and, moreover, almost all of them began to work immediately, without painful adjustment. But it is known that the repeatability of the design at home increases in proportion to the square of its "simplicity".

In the Soviet Union in the late 80s, a complete analogue of 555 was developed, called KR1006VI1. The first industrial application of the domestic analogue was in the Electronics VM12 video recorder.

The internals of the NE555 chip

Before we grab the soldering iron and start assembling the structure on the integral timer, let's first understand what's inside and how it all works. After that, it will be much easier to understand how a particular practical circuit works.

Inside the integral timer contains more than twenty, the connection of which is shown in the figure -

As you can see, the circuit diagram is quite complicated, and is given here only for general information. After all, you still can’t fit into it with a soldering iron, you won’t be able to repair it. As a matter of fact, this is exactly what all other microcircuits, both digital and analog, look like from the inside (see -). Such is the technology for the production of integrated circuits. It will also not be possible to understand the logic of the device as a whole according to such a scheme, therefore, a functional diagram is shown below and its description is given.

Technical details

But, before you deal with the logic of the microcircuit, you should probably give its electrical parameters. The supply voltage range is quite wide 4.5 ... 18V, and the output current can reach 200mA, which makes it possible to use even low-power relays as a load. The microcircuit itself consumes very little: only 3 ... 6 mA is added to the load current. At the same time, the accuracy of the timer itself practically does not depend on the supply voltage - only 1 percent of the calculated value. Drift is only 0.1%/volt. The temperature drift is also small - only 0.005%/°C. As you can see, everything is quite stable.

Functional diagram of NE555 (KR1006VI1)

As mentioned above, in the USSR they made an analogue of the bourgeois NE555 and called it KR1006VI1. The analogue turned out to be very successful, no worse than the original, so you can use it without any fear or doubt. Figure 3 shows the functional diagram of the integrated timer KR1006VI1. It is fully consistent with the NE555 chip.

Figure 3. Functional diagram of the integrated timer KR1006VI1

The microcircuit itself is not so big - it is available in an eight-pin DIP8 package, as well as in a small-sized SOIC8. The latter suggests that 555 can be used for SMD mounting, in other words, the developers have retained interest in it to this day.

There are also few elements inside the microcircuit. The main one is DD1. When a logical one is applied to the R input, the flip-flop is reset to zero, and when a logical one is applied to the S input, of course, it is set to one. To form control signals at the RS inputs, it serves, which will be discussed a little later.

The physical levels of a logical unit depend, of course, on the supply voltage used and practically range from Upit/2 to almost full Upit. Approximately the same ratio is observed in CMOS logic circuits. The logical zero is, as usual, within 0 ... 0.4V. But these levels are inside the microcircuit, you can only guess about them, but you can’t feel them with your hands, you can’t see them with your eyes.

Output stage

To increase the load capacity of the microcircuit, a powerful output stage on transistors VT1, VT2 is connected to the trigger output.

If the RS - flip-flop is reset, then the output (pin 3) has a logic zero voltage, i.e. open transistor VT2. In the case when the flip-flop is set, the output also has a logical one level.

The output stage is made according to a push-pull scheme, which allows you to connect the load between the output and the common wire (pins 3.1) or the power bus (pins 3.8).

A small note on the output stage. When repairing and adjusting devices on digital microcircuits, one of the methods for checking the circuit is to apply a low-level signal to the inputs and outputs of the microcircuits. As a rule, this is done by shorting these same inputs and outputs to a common wire using a sewing needle, while doing no harm to the microcircuits.

In some circuits, the power supply of the NE555 is 5V, so it seems that this is also digital logic and can also be done quite freely. But actually it is not. In the case of the 555 microcircuit, more precisely with its push-pull output, such “experiments” cannot be done: if the output transistor VT1 is in the open state at that moment, then a short circuit will occur and the transistor will simply burn out. And if the supply voltage is close to the maximum, then the deplorable ending is simply inevitable.

Additional transistor (pin 7)

In addition to the transistors mentioned, there is also a transistor VT3. The collector of this transistor is connected to the output of the microcircuit 7 "Discharging". Its purpose is to discharge the timing capacitor when using the microcircuit as a pulse generator. The discharge of the capacitor occurs at the time of reset trigger DD1. If we recall the description of the trigger, then at the inverse output (indicated by a circle in the diagram) at this moment there is a logical unit, leading to the opening of the transistor VT3.

About reset signal (pin 4)

You can reset the trigger at any time - the "reset" signal has a high priority. For this, there is a special input R (pin 4), indicated in the figure as Usbr. As can be understood from the figure, a reset will occur if a low-level pulse is applied to output 4, not more than 0.7V. In this case, a low-level voltage will appear at the output of the microcircuit (pin 3).

In cases where this input is not used, it is driven to a logic one level to get rid of impulse noise. The easiest way to do this is to connect pin 4 directly to the power rail. In no case should you leave it, as they say, in the "air". Then you will have to wonder and think for a long time, why does the circuit work so unstable?

Notes on trigger "in general"

In order not to completely get confused about the state of the trigger, it should be recalled that in reasoning about the trigger, the state of its direct exit is always taken into account. Well, if it is said that the trigger is “set”, then the state of the logical unit is on the direct output. If they say that the trigger is “reset”, then the direct output will certainly have a state of logical zero.

At the inverse output (marked with a small circle), everything will be exactly the opposite, therefore, the trigger output is often called paraphase. In order not to confuse everything again, we will not talk about this anymore.

Anyone who has carefully read up to this point may ask: “Excuse me, because this is just a trigger with a powerful transistor cascade at the output. Where is the actual timer? And he will be right, since the matter has not yet reached the timer. To get the timer, his father, the creator of Hans R. Kamenzind, invented an original way to control this trigger. The whole trick of this method lies in the formation of control signals.

Formation of signals on the RS - inputs of the trigger

So what did we get? The trigger DD1 runs the whole thing inside the timer: if it is set to one, the output of the microcircuit is high, and if it is reset, then pin 3 is low and, in addition, the transistor VT3 is open. The purpose of this transistor is to discharge a time-setting capacitor in a circuit, for example, a pulse generator.

The trigger DD1 is controlled by comparators DA1 and DA2. In order to control the operation of the trigger at the outputs of the comparators, you need to receive high-level R and S signals. A reference voltage is applied to one of the inputs of each comparator, which is formed by a precision divider across resistors R1…R3. The resistance of the resistors is the same, so the voltage applied to them is divided into 3 equal parts.

Generation of trigger control signals

Start timer

A reference voltage of 1 / 3U was applied to the direct input of the comparator DA2, and the external voltage for starting the timer Uzap through pin 2 was applied to the inverse input of the comparator. In order to act on the input S of the trigger DD1 at the output of this comparator, you must get a high level. This is possible if the voltage Uzap will be within 0 ... 1 / 3U.

Even a short-term pulse of such a voltage will cause the trigger DD1 to fire and a high voltage level will appear at the output of the timer. If the input Uzap is affected by a voltage above 1 / 3U and up to the supply voltage, then no changes will occur at the output of the microcircuit.

Stop timer

To stop the timer, you just need to reset the internal trigger DD1, and for this, a high-level signal R is generated at the output of the comparator DA1. Comparator DA1 is included a little differently than DA2. The reference voltage of 2/3U is applied to the inverting input, and the control signal "Threshold" Upor is applied to the direct input.

With this inclusion, a high level at the output of the comparator DA1 will occur only when the voltage Upor at the direct input exceeds the reference voltage 2/3U at the inverting one. In this case, trigger DD1 will be reset, and a low level signal will be set at the output of the microcircuit (pin 3). The “discharge” transistor VT3 will also open, which will discharge the time-setting capacitor.

If the input voltage is within 1/3U…2/3U, none of the comparators will work, the timer output will not change state. In digital technology, this voltage is called the "gray level". If you simply connect pins 2 and 6, you get a comparator with trigger levels of 1/3U and 2/3U. And without even a single additional detail!

Reference voltage change

Conclusion 5, indicated in the figure as Uobr, is designed to control the reference voltage or change it using additional resistors. It is also possible to supply a control voltage to this input, due to which it is possible to obtain a frequency or phase modulated signal. But more often this output is not used, and to reduce the effect of interference, it is connected to a common wire through a small capacitor.

The microcircuit is powered through pins 1 - GND, 2 + U.

Here is the actual description of the NE555 integral timer. On the timer, a lot of all sorts of schemes have been assembled, which will be discussed in the following articles.

Boris Aladyshkin

Article continued:

Every radio amateur has met with the NE555 chip more than once. This little eight-legged timer has gained enormous popularity for its functionality, practicality and ease of use. On the 555 timer, you can assemble circuits of various levels of complexity: from a simple Schmitt trigger, with a body kit of just a couple of elements, to a multi-stage combination lock using a large number of additional components.

In this article, we will take a closer look at the NE555 chip, which, despite its advanced age, is still in demand. It should be noted that, first of all, this demand is due to the use of ICs in circuitry using LEDs.

Description and scope

NE555 is the development of the American company Signetics, whose specialists did not give up in the conditions of the economic crisis and were able to bring to life the works of Hans Camenzind. It was he who in 1970 managed to prove the importance of his invention, which at that time had no analogues. The NE555 IC had a high mounting density at a low cost, which earned it a special status.

Subsequently, competing manufacturers from around the world began to copy it. This is how the domestic KR1006VI1 appeared, which remained unique in this family. The fact is that in KR1006VI1 the stop input (6) has priority over the start input (2). In imported analogues of other firms, this feature is absent. This fact should be taken into account when developing circuits with the active use of two inputs.

However, in most cases, priorities do not affect the operation of the device. In order to reduce power consumption, back in the 70s of the last century, the production of a CMOS timer was launched. In Russia, the field-effect transistor microcircuit was named KR1441VI1.

The 555 timer found its greatest application in the construction of generator circuits and time relays with the possibility of delays from microseconds to several hours. In more complex devices, it performs the functions of eliminating contact bounce, PWM, restoring a digital signal, and so on.

Features and disadvantages

A feature of the timer is an internal voltage divider that sets a fixed upper and lower threshold for two comparators. Since the voltage divider cannot be eliminated and the threshold voltage cannot be controlled, the scope of the NE555 is narrowed.

Timers assembled on CMOS transistors do not have these disadvantages and do not need to install external capacitors.

The main parameters of the IC series 555

The internal structure of the NE555 includes five functional nodes, which can be seen in the logic diagram. A resistive voltage divider is located at the input, which forms two reference voltages for precision comparators. The output contacts of the comparators go to the next block - an RS flip-flop with an external reset pin, and then to the power amplifier. The last node is an open collector transistor, which can perform several functions, depending on the task.

The recommended supply voltage for IC types NA, NE, SA is in the range from 4.5 to 16 volts, and for SE it can reach 18V. In this case, the current consumption at the minimum Upit is 2–5 mA, at the maximum Upit it is 10–15 mA. Some 555 CMOS ICs draw as little as 1 mA. The largest output current of the imported microcircuit can reach 200 mA. For KR1006VI1, it is not higher than 100 mA.

Build quality and manufacturer greatly affect the operating conditions of the timer. For example, the operating temperature range of NE555 is 0 to 70°C and SE555 is -55 to +125°C, which is important to know when designing devices for outdoor environments. You can get acquainted with the electrical parameters in more detail, find out the typical values ​​​​of voltage and current at the CONT, RESET, THRES, and TRIG inputs in the datasheet on the XX555 series ICs.

Location and purpose of pins

The NE555 and its counterparts are predominantly available in 8-pin PDIP8, TSSOP, or SOIC packages. The layout of the pins, regardless of the case, is standard. The conventional graphic designation of the timer is a rectangle labeled G1 (for a single pulse generator) and GN (for multivibrators).

1. Common (GND). The first conclusion is regarding the key. Connects to the negative power of the device.
2. Trigger (TRIG). Applying a low-level pulse to the input of the second comparator leads to the launch and the appearance of a high-level signal at the output, the duration of which depends on the value of the external elements R and C. Possible variations of the input signal are described in the "Single vibrator" section.
3. Output (OUT). The high level of the output signal is (Upit-1.5V), and the low level is about 0.25V. Switching takes about 0.1 µs.
4. Reset (RESET). This input has the highest priority and is able to control the operation of the timer regardless of the voltage on the other outputs. To enable the launch, it is necessary that a potential of more than 0.7 volts be present on it. For this reason, it is connected through a resistor to the power supply of the circuit. The appearance of a pulse less than 0.7 volts disables the operation of the NE555.
5. Control (CTRL). As can be seen from the internal structure of the IC, it is directly connected to the voltage divider and, in the absence of external influence, gives out 2/3 Upit. By applying a control signal to CTRL, you can get a modulated signal at the output. In simple circuits, it is connected to an external capacitor.
6. Stop (THR). It is the input of the first comparator, the appearance on which a voltage of more than 2/3Upit stops the trigger and sets the timer output to a low level. In this case, there should be no trigger signal at pin 2, since TRIG has priority over THR (except for KR1006VI1).
7. Discharge (DIS). Connected directly to the internal transistor, which is connected in a common collector circuit. Typically, a timing capacitor is connected to the collector-emitter junction, which discharges while the transistor is in the on state. Less commonly used to increase the load capacity of the timer.
8. Power supply (VCC). It is connected to the plus of the 4.5-16V power supply.

NE555 operating modes

The 555 series timer operates in one of three modes, we will consider them in more detail using the NE555 microcircuit as an example.

single vibrator

The circuit diagram of the single vibrator is shown in the figure. To form single pulses, in addition to the NE555 microcircuit, you will need a resistance and a polar capacitor. The scheme works as follows. A single low-level pulse is applied to the input of the timer (2), which leads to the switching of the microcircuit and the appearance of a high signal level at the output (3). Signal duration is calculated in seconds using the formula:

After the specified time (t) has elapsed, a low-level signal is generated at the output (initial state). By default, pin 4 is combined with pin 8, that is, it has a high potential.

During the development of schemes, you need to take into account 2 nuances:

1. The power supply voltage does not affect the duration of the pulses. The higher the supply voltage, the higher the charge rate of the timing capacitor and the greater the amplitude of the output signal.
2. An additional pulse that can be applied to the input after the main one will not affect the operation of the timer until the time t expires.

The operation of the single pulse generator can be influenced from the outside in two ways:

• send a low-level signal to Reset, which will reset the timer to its original state;
• as long as input 2 is low, the output will remain high.

Thus, with the help of single signals at the input and the parameters of the timing chain, it is possible to obtain rectangular pulses with a clearly defined duration at the output.

multivibrator

The multivibrator is a generator of periodic rectangular pulses with a given amplitude, duration or frequency, depending on the task. Its difference from a single vibrator is the absence of an external disturbing influence for the normal functioning of the device. A schematic diagram of a multivibrator based on the NE555 is shown in the figure.

Resistors R 1, R 2 and capacitor C 1 are involved in the formation of repetitive pulses. Pulse time (t 1), pause time (t 2), period (T) and frequency (f) are calculated using the formulas below: From these formulas, it is easy to see that the pause time cannot exceed the pulse time, that is, it will not be possible to achieve a duty cycle (S \u003d T / t 1) of more than 2 units. To solve the problem, a diode is added to the circuit, the cathode of which is connected to pin 6, and the anode to pin 7.

In the datasheet for microcircuits, they often operate with the reciprocal of the duty cycle - Duty cycle (D \u003d 1 / S), which is displayed as a percentage.

The scheme works as follows. At the time of power-up, capacitor C 1 is discharged, which puts the timer output in a high level state. Then C 1 starts charging, gaining capacity up to the upper threshold value 2/3 U PIT. Having reached the threshold, the IC switches, and a low signal level appears at the output. The process of discharging the capacitor (t 1) begins, which continues until the lower threshold value 1/3 U PIT. Upon reaching it, the reverse switching occurs, and a high signal level is set at the output of the timer. As a result, the circuit goes into self-oscillating mode.

Precision Schmitt Trigger with RS Trigger

Inside the NE555 timer, a two-prog comparator and an RS flip-flop are built-in, which allows you to implement a precision Schmitt trigger with an RS flip-flop in hardware. The input voltage is divided by the comparator into three parts, upon reaching each of which the next switching occurs. In this case, the value of hysteresis (reverse switching) is equal to 1/3 U PIT. The possibility of using NE555 as a precision trigger is in demand in the construction of automatic control systems.

3 most popular circuits based on NE555

single vibrator

A practical version of the TTL NE555 single vibrator circuit is shown in the figure. The circuit is powered by a unipolar voltage from 5 to 15V. The time-setting elements here are: resistor R 1 - 200 kOhm-0.125 W and electrolytic capacitor C 1 - 4.7 μF-16V. R 2 maintains a high potential at the input until some external device resets it to a low level (for example, a transistor switch). Capacitor C 2 protects the circuit from through currents at the moments of switching.

The activation of a single vibrator occurs at the moment of a short-term short circuit to the ground of the input contact. In this case, a high level is formed at the output with a duration of:

t \u003d 1.1 * R 1 * C 1 \u003d 1.1 * 200000 * 0.0000047 \u003d 1.03 s.

Thus, this circuit generates a delay of the output signal relative to the input signal by 1 second.

Flashing LED on multivibrator

Based on the multivibrator circuit discussed above, you can assemble a simple LED flasher. To do this, an LED is connected in series with the resistor to the output of the timer. The value of the resistor is found by the formula:

R=(U OUT -U LED)/I LED ,

U OUT - the amplitude value of the voltage at pin 3 of the timer.

The number of connected LEDs depends on the type of NE555 chip used, its load capacity (CMOS or TTL). If it is necessary to blink an LED with a power of more than 0.5 W, then the circuit is supplemented with a transistor, the load of which will be the LED.

Time relay

The scheme of the adjustable timer (electronic time relay) is shown in the figure.
With its help, you can manually set the duration of the output signal from 1 to 25 seconds. To do this, in series with a fixed resistor of 10 kΩ, a variable value of 250 kΩ is installed. The capacitance of the timing capacitor is increased to 100 uF.

The scheme works as follows. In the initial state, pin 2 is high (from the power supply), and pin 3 is low. Transistors VT1, VT2 are closed. At the moment a positive pulse is applied to the base VT1, a current flows through the circuit (Vcc-R2-collector-emitter-common wire). VT1 opens and puts the NE555 into timing mode. At the same time, a positive pulse appears at the output of the IC, which opens VT2. As a result, the emitter current VT2 leads to the operation of the relay. The user can interrupt the execution of the task at any time by briefly shorting RESET to ground.

The SS8050 transistors shown in the diagram can be replaced with KT3102.

It is impossible to review all popular circuits based on NE555 in one article. For this, there are entire collections, which contain practical developments for the entire time of the existence of the timer. We hope that the information provided will serve as a guide during the assembly of circuits, including the load of which are LEDs.

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