For beginner radio amateurs, the transition from creating the simplest circuits using resistors, capacitors, diodes to creating printed circuit boards with various microcircuits means a transition to new level skill. However, the circuits are based on the simplest microcircuits, one of which is the NE555 integrated timer microcircuit.
The study of any microcircuit should begin with proprietary documentation - DATA SHEET. To begin with, you should pay attention to the location of the pins and their purpose for the NE555 timer (Figure 1). Foreign companies generally do not provide circuit diagrams their devices. However, the NE555 timer chip is quite popular and has its own domestic analogue KR1006VI1, the circuit of which is shown in Figure 2.
Picture 1
1. Single vibrator based on NE555 (Figure 3).
Figure 3
Circuit operation: a low-level pulse is applied to pin 2 of the microcircuit. At the output 3 of the microcircuit, a rectangular pulse is obtained, the duration of which is determined by the timing RC chain (ΔT = 1.1 * R * C). A high-level signal at pin 3 is formed until the time-setting capacitor C is charged to a voltage of 2/3Upit. Diagrams of the operation of a single vibrator are shown in Figure 4. To generate a pulse to start the operation of the microcircuit, you can use a mechanical button (Figure 5) or a semiconductor element.
Figure 4
Figure 5
The purpose of the single vibrator circuit based on the NE555 integrated timer chip is to create time delays from several milliseconds to several hours.
2 Generators based on integral timer NE555
The generator based on the NE555 is capable of generating pulses with a maximum frequency of several kilohertz for rectangular pulses and with a frequency of several megahertz for non-rectangular pulses. The frequency, as in the case of a single vibrator, will be determined by the parameters of the timing circuit.
2.1 NE555 square wave pulse generator
The scheme of such a generator is shown in Figure 6, and the timing diagrams of the generator in Figure 7. Distinctive feature square wave pulse generator is that the pulse time and pause time are equal to each other.
Figure 6
Figure 7
The principle of operation of the circuit is similar to that of a single vibrator. The only exception is the missing pulse to start the operation of the timer chip at pin 2. The frequency of the generated pulses is determined by the expression f = 0.722 / (R1 * C1).
2.2 Variable duty cycle pulse generator based on NE555
Regulation of the duty cycle of the generated pulses makes it possible to build pulse-width generators based on the NE555. The duty cycle is determined by the ratio of the pulse time to the pulse duration. The reciprocal of the duty cycle is the duty cycle. A circuit of a pulse generator with an adjustable duty cycle based on the NE555 is shown in Figure 8.
Figure 8
The principle of operation of the circuit: the pulse time and the pause time are determined by the charge time of the capacitor C1. A high level signal is generated when C1 is charged along the R1-RP1-VD1 circuit. When the voltage reaches 2 / 3Upit, the timer switches and the capacitor C1 is discharged through the VD2-RP1-R1 circuit. Upon reaching 1/3Upit, the timer switches again and the cycle repeats.
The adjustment of the charge and discharge time of the capacitor C1 is carried out by a variable resistor RP1. In this case, the duty cycle of the output pulses changes at a constant pulse repetition period.
For NE555 integrated timer chip performance test you can assemble the circuit shown in Figure 9 (circuit in the Multisim simulator).
Figure 9
The output voltage is regulated by a variable resistor R1. In the above diagram, it is quite easy to understand the algorithm of the timer. With a supply voltage of 12V, the reference voltage for switching the microcircuit is 4V and 8V. At a voltage of 7.8V (Figure 10), the timer output has a high signal level (LED1 is off). When 8V is reached (Figure 11), the microcircuit will switch - LED1 lights up. A further increase in voltage will not cause any changes in the operation of the timer.
Somehow they asked me to make a simple flasher to control a relay or a low-power light bulb to flash. Assembling the simplest multivibrator, whether symmetrical or not, is somehow banal, and the circuit is unstable and not entirely reliable, despite the fact that it should work at a voltage of 24 volts in a truck, and even the dimensions should not be too large.
Scheme
After searching the network for circuits, I decided to turn on the popular NE555N chip using the datasheet. A precision timer, the cost of which is very low - about 10 rubles per microcircuit in a dip package! But since our load is not quite weak, and it may be necessary high currents regarding the power supply of the timer, then we need some kind of key, which the timer itself will control.
You can take an ordinary transistor, but it will heat up due to large losses due to large drops at the junctions - so I took a high-voltage field-effect transistor for several amperes of current, such a key with a current of even 2 amperes does not need a radiator at all.
The 555 timer itself has limitations in the supply voltage - about 18 volts, although even at 15 it can safely fly out, so we assemble a chain of a limiting resistor and a zener diode with a filtering capacitor at the power input!
A regulator is introduced into the circuit so that by rotating the regulator knob it is possible to change the frequency of the flash pulses of the light bulb or the operation of the relay. If adjustment is not required, you can adjust the frequency to the desired ones, measure the resistance and then solder the finished one. On the above, there are 2 regulators at once, which change the duty cycle (the ratio of the on state of the output to the off state). If a 1:1 ratio is required, we remove everything except one variable resistor.
Video
Some of the elements are made in dip cases, some in smd - for compactness and better layout in general. The pulse generator circuit worked after turning on almost immediately, it remains only to adjust to the desired frequency. It is advisable to fill the board with hot glue or put it in a plastic case so that car owners do not guess to screw it directly to the case or put it on something metal.
I needed to make a speed controller for the propeller. To blow off the smoke from the soldering iron, and ventilate the face of the face. Well, for fun, put everything at the minimum cost. The simplest low-power motor direct current, of course, to regulate with a variable resistor, but to find a sum for such a small denomination, and even the required power, you need to try hard, and it will obviously cost more than ten rubles. Therefore, our choice is PWM + MOSFET.
I took the key IRF630. Why this one MOSFET? Yes, I just got about ten of them from somewhere. So I use it, so you can put something less overall and low-power. Because the current here is unlikely to be more than an ampere, and IRF630 able to drag through itself under 9A. But it will be possible to make a whole cascade of fans by connecting them to one twist - enough power :)
Now it's time to think about what we'll do PWM. The thought immediately suggests itself - a microcontroller. Take some Tiny12 and do it on it. I dismissed this thought instantly.
- Spending such a valuable and expensive part on some kind of fan is disgusting to me. I will find a more interesting task for the microcontroller
- Another software for this to write, doubly zapadlo.
- The supply voltage is 12 volts there, lowering it to power the MK to 5 volts is generally already lazy
- IRF630 will not open from 5 volts, so here you would also have to install a transistor so that it supplies a high potential to the gate of the field worker. Nafig nafig.
Operational amplifiers can be discarded immediately. The point is that the OS general purpose already after 8-10 kHz, as a rule, limiting output voltage
begins to collapse sharply, and we need to jerk the field worker. Yes, even at a supersonic frequency, so as not to squeak.
Op-amps devoid of such a drawback cost so much that you can buy a dozen of the coolest microcontrollers with this money. Into the fire!
Comparators remain, they do not have the ability of the opamp to smoothly change the output voltage, they can only compare two voltages and close the output transistor based on the results of the comparison, but they do it quickly and without blocking the characteristic. I rummaged through the barrels and did not find any comparators. Ambush! More precisely was LM339, but it was in a large case, and religion does not allow me to solder a microcircuit for more than 8 legs for such a simple task. It was also too much to drag into the storehouse. What to do?
And then I remembered such a wonderful thing as analog timer - NE555. It is a kind of generator, where you can set the frequency, as well as the duration of the pulse and pause, with a combination of resistors and a capacitor. How much different crap has been done on this timer, over its more than thirty-year history ... Until now, this microcircuit, despite its venerable age, is stamped in millions of copies and is available in almost every store at a price of a few rubles. With us, for example, it costs about 5 rubles. Rummaged through the bottom of the barrel and found a couple of pieces. O! Right now and stir up.
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How it works
If you do not delve deeply into the structure of the 555 timer, then it is not difficult. Roughly speaking, the timer monitors the voltage on the capacitor C1, which removes from the output THR(THRESHOLD - threshold). As soon as it reaches the maximum (the conder is charged), the internal transistor opens. which closes the output DIS(DISCHARGE - discharge) to the ground. At the same time, at the output OUT a logical zero appears. The capacitor begins to discharge after DIS and when the voltage on it becomes equal to zero (full discharge), the system will switch to the opposite state - at output 1, the transistor is closed. The capacitor starts to charge again and everything repeats again.
The charge of the capacitor C1 follows the path: " R4->upper arm R1 ->D2", and the discharge along the way: D1 -> lower arm R1 -> DIS. When we turn the variable resistor R1, then we change the ratio of the resistances of the upper and lower arms. Which, accordingly, changes the ratio of the pulse length to the pause.
The frequency is set mainly by the capacitor C1 and also depends a little on the value of the resistance R1.
Resistor R3 provides a pull-up output to a high level - so there is an open collector output. Which is not able to set a high level on its own.
Diodes can be installed completely, conders of about the same value, deviations within one order of magnitude do not particularly affect the quality of work. At 4.7 nanofarads set in C1, for example, the frequency drops to 18 kHz, but it is almost inaudible, it seems that my hearing is no longer perfect :(
I dug into the bins, which itself calculates the operating parameters of the NE555 timer and assembled the circuit from there, for an astable mode with a duty cycle of less than 50%, but instead of R1 and R2 I screwed in a variable resistor, which changed the duty cycle of the output signal. It is only necessary to pay attention to the fact that the output DIS (DISCHARGE) through the internal key of the timer connected to ground, so it was impossible to plant it directly to the potentiometer, because when the regulator is turned to the extreme position, this output would sit on Vcc. And when the transistor opens, there will be a natural short circuit and a timer with a beautiful puff will emit magic smoke, on which, as you know, all electronics work. As soon as the smoke leaves the microcircuit, it stops working. That's how it is. Therefore, we take and add another resistor per kilo-ohm. It will not make the weather in regulation, but it will protect it from burnout.
No sooner said than done. Etched the board, soldered the components:
Everything is simple below.
Here I am attaching a signet, in my dear Sprint Layout -
And this is the voltage on the engine. You can see a small transition process. It is necessary to put the conder in parallel on the floor of the microfarad and smooth it out.
As you can see, the frequency floats - it’s understandable, because our operating frequency depends on the resistors and the capacitor, and since they change, the frequency floats, but it doesn’t matter. In the entire range of regulation, it never fits into the audible range. And the whole construction cost 35 rubles, not counting the body. So - Profit!
And finally, they got their hands on it. After assembling small coils, I decided to take a swing at a new scheme, more serious and difficult to set up and operate. Let's move from words to deeds. The complete schema looks like this:
Works on the principle of an autogenerator. Breaker kicks the driver UCC27425 and the process begins. The driver sends a pulse to the GDT (Gate Drive Transformator - literally: a transformer that controls the gates) with the GDT there are 2 secondary windings connected in antiphase. This inclusion provides an alternate opening of the transistors. During opening, the transistor pumps current through itself and the 4.7 microfarad capacitor. At this moment, a discharge is formed on the coil, and the signal goes through the OS to the driver. The driver changes the direction of the current in the GDT and the transistors change (the one that was open - closes, and the second one opens). And this process is repeated as long as there is a signal from the interrupter.
GDT is best wound on an imported ring - Epcos N80. Windings are wound in the ratio 1:1:1 or 1:2:2. On average, about 7-8 turns, if desired, you can calculate. Consider an RD circuit in the gates of power transistors. This chain provides Dead Time (dead time). This is the time when both transistors are off. That is, one transistor has already closed, and the second has not yet had time to open. The principle is this: the transistor opens smoothly through the resistor and quickly discharges through the diode. On an oscilloscope it looks like this:
If you do not provide dead time, then it may turn out that both transistors will be open and then a power explosion is provided.
Go ahead. OS ( Feedback) is made in this case in the form of a CT (current transformer). The CT is wound on an Epcos N80 ferrite ring with at least 50 turns. The lower end of the secondary winding is pulled through the ring, which is grounded. In this way, the high current from the secondary turns into a sufficient potential at the CT. Next, the current from the CT goes to the capacitor (smoothes interference), Schottky diodes (pass only one half-cycle) and LED (acts as a zener diode and visualizes generation). In order for the generation to be, it is also necessary to observe the phrasing of the transformer. If there is no generation or very weak, you just need to turn the TT.
Consider separately the interrupter. With a breaker, of course, I sweated. I collected 5 different pieces ... Some are puffy from the HF current, others do not work as they should. Next, I'll tell you about all the breakers that I did. Let me start from the very first TL494. The scheme is standard. Independent adjustment of frequency and duty cycle is possible. The circuit below can generate from 0 to 800-900 Hz if you put a 4.7 uF capacitor instead of 1uF. Duty cycle from 0 to 50. What you need! However, there is one BUT. This PWM controller is very sensitive to RF current and various fields from the coil. In general, when connected to the coil, the breaker simply did not work, either everything was on 0 or CW mode. Shielding partially helped, but did not completely solve the problem.
The next breaker was assembled on UC3843 very common in IIP, especially ATX, from there, in fact, he took it. The scheme is also good and does not concede TL494 by parameters. Here you can adjust the frequency from 0 to 1 kHz and the duty cycle from 0 to 100%. It suited me too. But again, these pickups from the coil ruined everything. Here, even shielding did not help at all. I had to refuse, although I assembled it soundly on the board ...
Decided to return to the oak and reliable, but little functional 555 . Decided to start with a burst interrupter. The essence of the interrupter is that it interrupts itself. One chip (U1) sets the frequency, the other (2) the duration, and the third (U3) the operating time of the first two. Everything would be fine if it were not for the short pulse duration from U2. This interrupter is designed for DRSSTC and can work with SSTC, but I didn’t like it - the discharges are thin, but fluffy. Then there were several attempts to increase the duration, but they were unsuccessful.
Alternator circuits for 555
Then I decided to change the fundamental circuit and make an independent duration on the capacitor, diode and resistor. Perhaps many will consider this scheme absurd and stupid, but it works. The principle is this: the signal goes to the driver until the capacitor is charged (I think no one will argue with this). NE555 generates a signal, it goes through a resistor and a capacitor, while if the resistance of the resistor is 0 Ohm, then it goes only through the capacitor and the duration is maximum (how long the capacity lasts) regardless of the duty cycle of the generator. The resistor limits the charge time, i.e. the greater the resistance, the shorter the time the impulse will go. A signal with a shorter duration, but also frequencies, goes to the driver. The capacitor is discharged quickly through a resistor (which goes to ground 1k) and a diode.
Advantages and disadvantages
pros: frequency independent duty cycle control, SSTC will never go into CW mode if the breaker burns out.
Minuses: duty cycle cannot be increased "infinitely", as for example on UC3843, it is limited by the capacitance of the capacitor and the duty cycle of the generator itself (it cannot be greater than the duty cycle of the generator). Current flows through the capacitor smoothly.
I don’t know how the driver reacts to the latter (smooth charging). On the one hand, the driver can also smoothly open transistors and they will heat up more. On the other side UCC27425- digital microcircuit. For her, there is only a log. 0 and log. 1. So while the voltage is above the threshold - UCC works, as soon as it drops below the minimum - it does not work. In this case, everything works normally, and the transistors open completely.
Let's move from theory to practice
I assembled a Tesla generator in an ATX case. Power supply capacitor 1000uF 400v. Diode bridge from the same ATX for 8A 600V. I put a 10 W 4.7 Ohm resistor in front of the bridge. This ensures a smooth charge of the capacitor. To power the driver, I installed a 220-12V transformer and another stabilizer with a 1800 microfarad capacitor.
I screwed the diode bridges onto the radiator for convenience and for heat dissipation, although they almost do not heat up.
The breaker assembled almost a canopy, took a piece of textolite and cut out the tracks with a clerical knife.
The power unit was assembled on a small radiator with a fan, later it turned out that this radiator was enough for cooling. The driver is mounted over the powertrain via a thick piece of cardboard. Below is a photo of the almost assembled design of the Tesla generator, but being tested, measured the temperature of the power at various modes(you can see an ordinary room thermometer, stuck to the power one on the thermoplastic).
The coil toroid is assembled from a corrugated plastic pipe with a diameter of 50 mm and glued with aluminum tape. The secondary winding itself is wound on a 110 mm pipe 20 cm high with a wire of 0.22 mm about 1000 turns. The primary winding contains as many as 12 turns, made with a margin in order to reduce the current through the power section. I did it with 6 turns at the beginning, the result is almost the same, but I think it’s not worth risking transistors for a couple of extra centimeters of discharge. The frame of the primary is an ordinary flower pot. From the beginning I thought that it would not pierce if the secondary was wrapped with tape, and the primary over the tape. But alas, it pierced ... In the pot, of course, it also pierced, but here the adhesive tape helped solve the problem. In general, the finished design looks like this:
Well, a few pictures with a discharge
Now everything seems to be.
A few more tips: do not try to immediately plug a coil into the network, it is not a fact that it will work right away. Constantly monitor the power temperature, when overheated, it can bang. Do not wind too high-frequency secondary, transistors 50b60 can operate at a maximum of 150 kHz according to the datasheet, in fact a little more. Check the breakers, the life of the coil depends on them. Find the maximum frequency and duty cycle at which the power temperature is stable long time. Too large a toroid can also disable the power.
Video of SSTC
P.S. Power transistors used IRGP50B60PD1PBF. Project files. good luck with you [)eNiS!
Discuss the article TESLA GENERATOR
The path to amateur radio begins, as a rule, with an attempt to assemble simple circuits. If, immediately after assembly, the circuit begins to show signs of life - blinking, beeping, clicking or talking, then the path to amateur radio is almost open. As for “talking”, most likely it will not work right away, for this you will have to read a lot of books, solder and adjust a number of circuits, maybe burn a large or small pile of parts (preferably a small one).
But flashing lights and tweeters are obtained by almost everyone at once. And the best element than to find for these experiments, simply will not succeed. To begin with, let's look at the generator circuits, but before that, let's turn to the proprietary documentation - DATA SHEET. First of all, let's pay attention to the graphic outline of the timer, which is shown in Figure 1.
And figure 2 shows an image of a timer from a domestic reference book. Here it is given simply for the possibility of comparing the designations of the signals with them and with us, besides “ours” functional diagram shown in greater detail and clarity.
Picture 1.
Figure 2.
Single vibrator based on 555
Figure 3 shows a single vibrator circuit. No, this is not a half of a multivibrator, although it cannot generate vibrations on its own. He needs outside help, no matter how small.
Figure 3. Scheme of a single vibrator
The logic of the single vibrator is quite simple. Trigger input 2 receives a momentary low pulse as shown in the figure. As a result, output 3 produces a rectangular pulse with a duration of ΔT = 1.1*R*C. If we substitute R in ohms and C in farads into the formula, then the time T will be in seconds. Accordingly, with kiloohms and microfarads, the result will be in milliseconds.
And Figure 4 shows how to generate a trigger pulse using a simple mechanical button, although it may well be a semiconductor element - a microcircuit or a transistor.
Figure 4
In general, a single vibrator (sometimes called a monovibrator, and the brave military used the word kipp-relay) works as follows. When the button is pressed, a low-level pulse at pin 2 causes the timer 3 output to go high. It is not for nothing that this signal (pin 2) is called a launch in domestic reference books.
The transistor connected to pin 7 (DISCHARGE) is closed in this state. Therefore, nothing prevents the time-setting capacitor C from being charged. At the time of the kipp relay, of course, there were no 555, everything was done on lamps, at best on discrete transistors, but the operation algorithm was the same.
While the capacitor is charging, the output is held high. If at this time another pulse is applied to input 2, the output state will not change, the duration of the output pulse cannot be reduced or increased in this way, and the single vibrator will not restart.
Another thing is if you apply a reset pulse (low level) to pin 4. Output 3 will immediately go low. The "reset" signal has the highest priority and can therefore be given at any time.
As the charge increases, the voltage on the capacitor increases, and, in the end, reaches the level of 2/3U. As described in the previous article, this is the trigger level, the threshold, of the upper comparator, which leads to the reset of the timer, which is the end of the output pulse.
At pin 3, a low level appears and at the same moment the transistor VT3 opens, which discharges the capacitor C. This completes the pulse formation. If, after the end of the output pulse, but not before, another trigger pulse is applied, then an output pulse will be formed at the output, the same as the first one.
Of course, for the normal operation of the one-shot, the trigger pulse must be shorter than the pulse generated at the output.
Figure 5 shows a graph of the operation of a single vibrator.
Figure 5. Single vibrator operation schedule
How can a single vibrator be used?
Or, as the cat Matroskin used to say: “And what will be the use of this one-vibrator?” The answer is that it is big enough. The fact is that the range of time delays that can be obtained from this single vibrator can reach not only a few milliseconds, but also reach several hours. It all depends on the parameters of the timing RC chain.
Here you are, an almost ready-made solution for lighting a long corridor. It is enough to supplement the timer with an executive relay or a simple thyristor circuit, and put a couple of buttons at the ends of the corridor! He pressed the button, passed the corridor, and no need to worry about turning off the light bulb. Everything will happen automatically at the end of the time delay. Well, this is just food for thought. Lighting in a long corridor, of course, is not the only option for using a single vibrator.
How to check 555?
The easiest way is to solder a simple circuit, for this almost no attachments are needed, except for a single variable resistor and an LED to indicate the output status.
At the microcircuit, pins 2 and 6 should be connected and a voltage changed by a variable resistor should be applied to them. A voltmeter or LED can be connected to the timer output, of course, with a limiting resistor.
But you can not solder anything, moreover, conduct experiments even if there is a "lack" of the microcircuit itself. Similar studies can be done using the Multisim simulator program. Of course, such a study is very primitive, but, nevertheless, it allows you to get acquainted with the logic of the 555 timer. The results of the “laboratory work” are shown in Figures 6, 7 and 8.
Figure 6
In this figure, you can see that the input voltage is regulated by a variable resistor R1. Near it, you can see the inscription “Key = A”, which indicates that the value of the resistor can be changed by pressing the A key. ".
In this figure, the resistor is “taken away” to the “ground” itself, the voltage on its engine is close to zero (for clarity, it is measured with a multimeter). With this position of the slider, the output of the timer is high, so the output transistor is closed, and LED1 does not light up, as indicated by its white arrows.
The following figure shows that the voltage has increased slightly.
Figure 7
But the increase took place not just like that, but with the observance of certain boundaries, namely, the thresholds for comparators. The fact is that 1/3 and 2/3, if expressed in decimal fractions as a percentage, will be 33.33 ... and 66.66 ... respectively. It is in percentage that the entered part of the variable resistor is shown in the Multisim program. With a supply voltage of 12V, this will turn out to be 4 and 8 volts, which is quite convenient for research.
So, Figure 6 shows that the resistor is inserted at 65%, and the voltage across it is 7.8V, which is slightly less than the calculated 8 volts. In this case, the output LED is off, i.e. the output of the timer is still high.
Figure 8
A further slight increase in voltage at inputs 2 and 6, by only 1 percent (the program does not allow less) leads to the ignition of the LED1 LED, which is shown in Figure 8 - the arrows near the LED have acquired a red tint. This behavior of the circuit indicates that the Multisim simulator works quite accurately.
If you continue to increase the voltage at pins 2 and 6, then no change will occur at the output of the timer.
Generators on the timer 555
The frequency range generated by the timer is quite wide: from the lowest frequency, the period of which can reach several hours, to frequencies of several tens of kilohertz. It all depends on the elements of the timing chain.
If a strictly rectangular waveform is not required, then frequencies up to several megahertz can be generated. Sometimes this is quite acceptable - the form is not important, but the impulses are present. Most often, such carelessness about the shape of the pulses is allowed in digital technology. For example, the pulse counter reacts to the rise or fall of the pulse. Agree, in this case, the "rectangularity" of the pulse does not matter.
Square pulse generator
One of options meander pulse generator is shown in Figure 9.
Figure 9. Scheme of meander-shaped pulse generators
Timing diagrams of the generator operation are shown in Figure 10.
Figure 10. Timing diagrams of generator operation
The top graph illustrates the output signal (pin 3) of the timer. And the lower graph shows how the voltage across the timing capacitor changes.
Everything happens in exactly the same way as has already been considered in the one-shot circuit shown in Figure 3, only the triggering single pulse at pin 2 is not used.
The fact is that when the circuit is turned on, the voltage on capacitor C1 is zero, and it is this voltage that will put the timer output in a high level state, as shown in Figure 10. Capacitor C1 starts charging through resistor R1.
The voltage on the capacitor increases exponentially until it reaches the threshold of the upper threshold of operation 2/3*U. As a result, the timer switches to the zero state, so the capacitor C1 begins to discharge to the lower threshold of 1/3*U. When this threshold is reached, the output of the timer is set high and everything starts over. A new period of oscillation is being formed.
Here you should pay attention to the fact that the capacitor C1 is charged and discharged through the same resistor R1. Therefore, the charge and discharge times are equal, and, consequently, the shape of the oscillations at the output of such a generator is close to a meander.
The oscillation frequency of such a generator is described by a very complex formula f = 0.722/(R1*C1). If the resistance of the resistor R1 in the calculations is indicated in Ohms, and the capacitance of the capacitor C1 in Farads, then the frequency will be in Hertz. If in this formula the resistance is expressed in kiloohms (KΩ), and the capacitance of the capacitor is in microfarads (μF), the result will be in kilohertz (KHz). To get a generator with an adjustable frequency, it is enough to replace the resistor R1 with a variable.
Pulse generator with adjustable duty cycle
The meander, of course, is good, but sometimes situations arise that require regulation of the duty cycle of the pulses. This is how the frequency of rotation of DC motors (PWM controllers) is carried out, which are with a permanent magnet.
A meander is a rectangular pulse, in which the pulse time (high level t1) is equal to the pause time (low level t2). This name came to electronics from architecture, where a meander is called a brickwork pattern. The total time of the pulse and pause is called the period of the pulse (T = t1 + t2).
Duty cycle and Duty cycle
The ratio of the pulse period to its duration S = T/t1 is called the duty cycle. This quantity is dimensionless. For a meander, this indicator is 2, since t1 \u003d t2 \u003d 0.5 * T. In the English-language literature, instead of duty cycle, the reciprocal value is more often used, - duty cycle D = 1/S, expressed as a percentage.
If we slightly improve the generator shown in Figure 9, we can get a generator with an adjustable duty cycle. A diagram of such a generator is shown in Figure 11.
Figure 11.
In this circuit, the charge of the capacitor C1 occurs through the circuit R1, RP1, VD1. When the voltage on the capacitor reaches the upper threshold 2/3*U, the timer switches to a low level and the capacitor C1 is discharged through the circuit VD2, RP1, R1 until the voltage on the capacitor drops to the lower threshold 1/3*U, after which repeats the cycle.
Changing the position of the RP1 slider makes it possible to adjust the duration of the charge and discharge: if the duration of the charge increases, then the discharge time decreases. In this case, the pulse repetition period remains unchanged, only the duty cycle, or duty cycle, changes. Well, it's more convenient for anyone.
Based on the 555 timer, you can design not only generators, but also many more useful devices which will be discussed in the next article. By the way, there are programs - calculators for calculating the frequency of generators on the 555 timer, and in the program - the Multisim simulator there is a special tab for this purpose.
Boris Aladyshkin,
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