Suppressor of impulses of a bounce of contacts - a shaper of impulses. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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If digital technology is in the field of interest of a radio amateur, he most likely knows about the purpose of such common nodes as a pulse suppressor for "bounce" of contacts and a pulse shaper. The author of this article offers a description of the original node that combines these functions.

In devices controlled by buttons or switches, radio amateurs usually use protection units against contact "bounce" pulses, described, for example, in [1]. Often, various shapers of rectangular pulses from sinusoidal or arbitrary waveforms are also used [2].

In nodes for suppressing "bounce" pulses, made on the simplest logic elements, when controlled from one pair of contacts, it is not possible to avoid delaying either the front or the fall of the output pulse (see Fig. 1 and 2 in [1]) for a time slightly longer than the expected the duration of the "bounce". In some nodes, RC circuits reduce the input impedance of the device, as well as its speed.

A Schmitt trigger is often used as a pulse shaper, containing a non-inverting element (a non-inverting amplifier or two inverters) and two resistors. Resistors here also reduce the input impedance of the driver, speed and bandwidth. The "hysteresis" inherent in the Schmitt trigger limits the sensitivity of the shaper and causes a delay in the generated voltage drops with respect to the hypothetical points of coincidence of the input signal levels and the threshold Uthr for a time depending on the "hysteresis" value and the input signal slew rate. In other words, during the formation of pulses, the Schmitt trigger introduces a phase shift that depends on the frequency.

The node described below is capable of simultaneously performing the functions of a pulse suppressor for "bounce" of contacts and a pulse shaper, while the difference can only be in the value of the time constant of the RC circuit.

When the input voltage increases from a low level to a high level, the node generates a steep positive drop at the output for the first time the input signal exceeds the threshold level. When the input voltage decreases from a high level to a low level, a steep negative drop appears at the output as soon as the input becomes less than the threshold level.

The node is made on an elegant RS-flip-flop, the scheme of which is shown in fig. 1 (see also Fig. 6 in [3]). In a trigger implemented on a three-input non-inverting majority element DD1, the positive OS circuit connects its output to one of the inputs (all three inputs of the element are equal). The remaining two perform the functions of the inputs of the RS flip-flop: one of them is direct S, the other is inverse R.

These inputs are also equal; any of them in any order can be assigned the indicated designations, which is determined by the mode of storage.

The arrangement (name) of the inputs of the considered RS flip-flop determines the storage mode - on which of these inputs in the storage mode the high level is R, and the other is S, respectively.

What has been said can be formulated in another way. If the output of the majority element is connected to the first input and a high level is applied to the third input, then the second will be input S, the trigger responds only to a positive input voltage drop, and if a low level is applied to the third input, then the second input performs the R functions and the trigger reacts only to negative input voltage drop.

This is the basis for the principle of operation of the proposed unit, the schematic diagram of which is shown in Fig. 2, and the timing diagrams of its operation are shown in Fig. 3. If the trigger DD1.1 is in the zero state (diagram 2 until the moment t1, Fig. 3), then there is a high level at the output of the inverter DD2.1 and on the capacitor C1. The input of the device serves as input S, the node responds to the first positive edge at time t1 and switches to a single state. The R1C1 circuit creates some delay, maintaining for some time a high level at the lower input of the DD1.1 element according to the circuit (Fig. 4), so that the node does not reacts.

By the time t2, fluctuations in the input instantaneous voltage (due to the "bounce" of contacts or other reasons) near the threshold level Upor end, the voltage across the capacitor decreases and a low level appears at the lower input of the element DD1.1. The flip-flop is now ready to receive a negative input voltage drop. Until the moment t3, the element DD1.1 is maintained in state 1 by a high level from the Input of the node and from its Output. Upon the arrival of the first negative drop at time t3, the trigger switches to state 0, and, similarly to what was said above, in the interval t4-t3 does not respond to any drops in the input voltage.

The time constant of the RC circuit in the "bounce" pulse suppression node is chosen to be slightly larger than the expected "bounce" duration, and in the pulse shaper - less than a quarter of the period of the maximum frequency of the input voltage.

The pulse generated by the node is taken from Output 1. At Output 2, there is an inverse signal with respect to Output 1.

The described node has high values ​​of the main characteristics - sensitivity, input impedance, speed, bandwidth - since they are entirely determined by the parameters of the majority element.

As an inverter, in addition to those indicated in the diagram, elements of the K561LA7, K561LE5, K561LP2 microcircuits and the like can be used.

Since there are no circuits in the described node that provide "hysteresis", in the first approximation it should be considered a Schmitt trigger with zero "hysteresis" that does not worsen sensitivity. In reality, however, due to a change in the logic level at the lower input of the majority element according to the scheme (see Fig. 2), the Unop threshold may change.

The values ​​​​of the resistor R1 and capacitor C1, depending on the required value of the time constant, can be varied over an extremely wide range: the resistance of the resistor is from O (jumper) to 10 MΩ, the capacitance of the capacitor is from 0 (absent) to tens and hundreds of microfarads. If the resistance is zero (jumper), the capacitance of the capacitor should not be more than 1000 pF. In the case when the capacitor is absent, its role is played by the input capacitance of the element DD1.1 (12 ... 15 pF). Instead of an RC circuit, any delay element can be used, including one or more non-inverting logic elements.

Literature

1. Biryukov S. Contact bounce suppression devices. - Radio, 1996, No. 8, p. 47, 51.
2. Biryukov S. Generators and pulse shapers on CMOS microcircuits. - Radio, 1995, No. 7, p. 36, 37; No. 9, p. 54, 55.
3. Alekseev S. The use of microcircuits of the KR1533 series. - Radio, 1991 No. 2, p. 64. 65.

Author: A.Samoilenko, Klin, Moscow Region

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