Triggers in key and counting devices. Scheme, description

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Triggers in key and counting devices. Radio - for beginners

Radio - for beginners

When designing instruments and devices of digital technology, for example, automata of various purposes, switches of electrical circuits, in remote control equipment for models, radio amateurs very widely use D- and JK-flip-flops operating in the pulse counting mode. To do this, high-level pulses are applied to the counting input of the trigger, switching the trigger from one logical state to another, and it, in turn, switches other electrical circuits with its output signals.

In general, such a switch can be controlled by any mechanical switch, for example, a push-button or toggle switch, but always through an additional device that eliminates the result of the so-called "bounce" of contacts, as well as providing other measures to prevent false triggers from various electrical interference.

First of all, what is the "bounce" of contacts? This is the name of the parasitic electrical effect that manifests itself at the moment of contact of the contact surfaces of a mechanical switch. The essence of this phenomenon lies in the fact that at this moment, as a result of repeated collision of contacts in the circuit in which they are included, a series of pulses with a duration of about a millisecond occurs. They lead to false positives of the trigger and, consequently, disruption of its operation.

To eliminate contact bounce, an additional, already familiar to you, RS-trigger is usually introduced. On fig. 1 such RS flip-flop is formed by elements DD1.1 and DD1.2 of the K155LAZ microcircuit. In the initial state of the trigger, its direct output (pin 3) has a high level voltage, and the inverse one has a low voltage. Counting D-flip-flop DD2.1 at this time saves the state in which it was at the time the power source was turned on.

When you press the SB1 button, its movable contact repeatedly touches another, fixed contact, causing a series of "bounce" pulses. The first pulse of the series switches the RS flip-flop to the zero state and no other pulses will change it. At this moment, a positive voltage drop occurs at its inverted output, under the influence of which the counting D-flip-flop DD2.1 changes its logical state to the opposite. When the button is released, a low voltage level is again applied to input 1 of the DD1.1 element and the RS flip-flop switches to its original state. The counting D-trigger can return to its original state only by pressing the SB1 button again. LEDs HL1 and HL2 allow you to visually monitor the status and operation of triggers and draw appropriate conclusions.

Triggers in key and counting devices
Rice. 1 Eliminate contact bounce

The SB2 button allows you to set the D-trigger to the zero state, and control signals can be removed from any of the trigger outputs (pins 5 and 6). In such a device, of course, a JK flip-flop can also work. Why do we need capacitor C1, blocking the power circuit of triggers? The fact is that flip-flops, as, indeed, many other K155 series microcircuits, are very sensitive to various electrical interferences. If, for example, you touch a mounting conductor with a metal object, impulse noise will appear in the device circuits that can change the state of the triggers. From the operation of one trigger in the power supply circuit of the device, impulse noise also occurs, which can switch another trigger. A capacitor that blocks the power circuit protects the flip-flops from such mutual interference.

Remember for the future: for reliable operation of digital devices on their boards between the conductors of the power circuit, it is necessary to install one blocking capacitor with a capacity of 0,033 ... 0,047 uF for every two or three microcircuits, placing them evenly among the microcircuits.

The unused input pin of the microcircuit can also become a source of false alarm, since parasitic electrical impulses can also be induced on it. Unused J inputs of JK flip-flops can be connected to their inverted outputs, and K inputs to direct outputs. You can also connect unused inputs to the output terminal of an unused AND-NOT logic element by connecting its inputs to a common wire. In addition, unused microcircuit inputs can be combined AND connected to the positive conductor of the power source through a resistor with a resistance of 1 ... 10 kOhm.

It is absolutely unacceptable to connect a conductor to the input of the microcircuit, which, during the operation of the device, may not be connected to the output of the control signal source, for example, in the case of controlling the device using a toggle switch or pushbutton switch. To prevent interference, such conductors must be connected to the positive conductor of the power circuit through a resistor with a resistance of 1 ... 10 kOhm. We turn to the description of several simple designs that use microcircuits familiar to you.

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