Pulse generator with independent frequency and duty cycle control. Scheme, description

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Pulse generator with independent frequency and duty cycle control. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Not so long ago, I needed to assemble a rectangular pulse generator with a relatively powerful output and smooth manual frequency and duty cycle control. Having some experience, I immediately decided that the NE555 timer chip (KR1006VI1) should become the basis of the generator. It has been produced for decades, it is cheap, reliable, has excellent characteristics and is easily consistent with CMOS and TTL logic microcircuits. The timer supply voltage can range from 5 to 15 V, and the output can withstand a load current of up to 200 mA.

Unfortunately, an Internet search for a suitable oscillator circuit did not turn up any results. All those found suffered from the same drawback - when the frequency changed, the duty cycle of the output pulses also changed. Or the duty cycle adjustment is smooth, and the frequency is stepped, using a switch. As a result, the desired generator was developed independently.

As you know, the NE555 timer has two voltage comparators. The response threshold of one of them (conditionally upper) without connecting additional resistors is 2/3 of the supply voltage, and the second (lower) is half as much. The voltage on the time-setting capacitor fluctuates between these thresholds during generator operation. To change the duty cycle, a classic trick is known - to apply voltage from the output of the microcircuit through multidirectional diodes to the extreme terminals of a variable resistor that regulates the duty cycle, and connect its engine to a time-setting capacitor. With such an adjustment, the pulse frequency does not change, since the sum of the resistances of the resistors through which the capacitor is charged and discharged remains constant.

But how to smoothly adjust the frequency without changing the duty cycle? I decided to do this by controlling the difference in the thresholds of the comparators. The smaller it is, the less, other things being equal, it takes time to recharge the capacitor from one threshold to another and back, the higher the pulse frequency becomes.

Having some experience, I immediately decided that the NE555 timer chip (KR1006VI1) should become the basis of the generator.

Pulse generator with independent frequency and duty cycle control
Fig. 1

The problem can be solved by assembling the generator according to the scheme shown in the figure. Here, the internal lower comparator of the DA2 timer is replaced by an external one, assembled on a separate DA1 chip. Its non-inverting input is connected to the time-setting capacitor C1, and a voltage divider from resistors R2, R3, R6-R8 is connected to the inverting input, which sets the response threshold. With an open circuit of the variable resistor R7 or with its very high resistance, the response threshold of the comparator DA1 is exactly the same as that of the disabled internal timer comparator DA2 - 1/3 of the supply voltage. This equality is achieved by a tuned resistor R3. By reducing the resistance of the variable resistor R7, symmetrically with respect to half the supply voltage, the thresholds of the upper comparator of the DA2 timer and the external comparator DA1 are brought closer. As a result, the pulse frequency increases, and their duty cycle, set by the variable resistor R4, remains unchanged.

I must say that in the first version of the generator, the scheme of which I published on the forum of the Internet portal KAZUS.RU kazus.ru/forums/showthread.php?t=94852, there is no resistor R6. But, as it turned out, without it it is not possible to achieve complete symmetry of the thresholds, the voltage divider inside the timer connected to its output 5, which forms the lower one from the upper threshold, interferes. Resistor R6, whose resistance is equal to the sum of the resistances of the resistors of this divider, compensates for its influence, making the complete thresholding circuit symmetrical.

Subjectively, the quality of balancing can be assessed by connecting a DC voltmeter between the output of 3 timers and the common wire. Its readings should depend only on the position of the variable resistor R4. When adjusting the frequency with a variable resistor R7, they should not change. This is achieved using a tuned resistor R3. If the pulse frequency is so low that the voltmeter needle fluctuates in time with it, you should connect the voltmeter to the timer through an integrating RC circuit with a sufficiently large time constant or temporarily increase the pulse frequency by installing a smaller capacitor C1.

With the values of the elements indicated in the diagram and a supply voltage of 15 V, the variable resistor R7 regulates the pulse frequency from approximately 50 to 830 Hz. However, reducing the supply voltage to 5 V leads to a decrease in frequency by almost a factor of two. In this regard, it is desirable to feed the generator with a stabilized voltage.

The load capacity of the NE555 timer output allows you to directly control quite powerful actuators and key elements. This circumstance, as well as the possibility of independent frequency and duty cycle control, can determine a wide range of generator applications.

Author: P. Galashevsky

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