Long pulse shaper. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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When building generators, timers and other similar devices, timing RC circuits are usually used, the operation of which is based on charging and discharging a capacitor through a resistor from a DC source. These RC circuits are connected, as a rule, to various active devices - transistors, op-amps, comparators.

When forming pulses of relatively long duration, one has to choose either a large capacitor or a high-resistance resistor. In the first case, the choice is limited by the leakage current and an increase in the dimensions of the capacitor, in the second, by a decrease in the operating charging current, which leads to an increase in the dependence of the time delay on external destabilizing factors and the effect of the capacitor leakage current on the mode of connected active elements.

Fig. 1

One of the options for overcoming these contradictions is the use of a relaxation system consisting of an integrator and a comparator. A schematic diagram of a long pulse shaper built according to this principle is shown in fig. 1, and voltage diagrams explaining its operation are shown in Fig. 2.

Fig. 2

An integrator is assembled on the op-amp DA1. The rate of rise and fall of the voltage at its output depends on the voltage at its non-inverting input. The integrator should use an op-amp with field-effect transistors at the input, providing a high input impedance.

OU DA2 - voltage comparator The threshold of its operation depends on the ratio of the resistance values ​​of the resistors R6 and R7. The variable resistor R1 allows you to adjust the duration of the output pulses of the shaper over a wide range.

At the initial moment, the voltage Uc at the output of the comparator DA2 is approximately + Upit, and at the output of the integrator DA1 UB = 0. Since the positive voltage Uc through the divider R3R1R2 is applied to the non-inverting input of the integrator, its output voltage increases.

At the moment t1, it will reach the upper threshold of the comparator:

and it will switch to another state. The polarity of the voltage at its output will change to the opposite, and the voltage at the output of the integrator will begin to decrease.

At the moment t2, it will reach the lower threshold of the comparator and the cycle will repeat. The frequency of generated oscillations can be determined by the formula

The output voltage Uc (Fig. 2) of the shaper is a sequence of bipolar pulses with a duty cycle of 2. With the values ​​\u73b\u17bof the elements indicated in the diagram and the use of the time-setting capacitor KXNUMX-XNUMX, the duration of the output pulses and pauses between them can be changed from one to thirty minutes.

If the driver load requires a single supply, a transistor output stage can be used, as shown in fig. 1. Here, the load of the transistor VT1 is the sound annunciator PKI-1, used in fire and security alarm systems. Its supply voltage is 12±3 V; current consumption -20...40 mA. The sound pressure provided by the siren reaches 105 dB. I used this device to scare away birds during the ripening period.

The described long pulse shaper can also be used in other devices where precise time intervals are not required, for example, for automating the maintenance of aquariums, incubators (with an appropriate design of the actuating units).

Author: S. Semikhatsky, Yeysk, Krasnodar Territory; Publication: radioradar.net

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