Bouncing often sets a false alarm the many devices (timers, relays, etc.). Means this problem you must learn to fight various diagrams.
So can we fix a bouncing a switch or a button, by using a trigger? And because some chips in the series 74 NShh may have a trigger, can we use it?
The answers are "Yes" and "Yes", although it's not so easy to do as it seems.
You will need:
- Logic chip 74 NS02 containing 4 logical element or dvuhvhodovyh. Logic chip 74 NS00 containing 4 dvuhvhodovyh logical element and-not. — On 1 PC. each chip.
- Single-pole on/off switch. Quantity is 1 piece.
- LEDs with low power consumption. Quantity is 2 PCs.
- Resistors with resistance 10 kΩ and a 1 KOhm. Number on 2 PCs. each denomination.
Mount the components on your Breadboard according to the schema shown in Figure 3. 1. When you apply supply voltage (using a stabilized power supply voltage 5 v), one of the LEDs should light up.
Figure. 1. Simple scheme to test the behavior of two elements or NOT connected as a rudimentary trigger saves its state after the end of the input signal
Now I want to do something strange. Please disconnect single pole on/off switch, separated by a wire that connects the power supply to the plus pole of the switch, as well as pull the wire end of the layout of the Board. When you do this, you'll be a little surprised that the led will remain lit.
Plug the wire back into the layout Board, toggle and first led should turn off, while another will illuminate. Now remove the wire again, and the led will continue to burn.
This is followed by questions that are designed for independent study.
- Trigger only requires an initial impetus.
- Then trigger ignores the status of the input.
How it works
Two logical element or NOT-or two elements and cannot operate at the trigger.
- Use elements or NOT, when you have a pole switch high logic level signal.
- Use the and-not when you have a low signal switch pole the logical level.
In any case, you must use the on/off switch.
I mentioned three times on/off switch (actually four times when you consider an offer!), because for some strange reason most books that relate to the introduction into the field of electronics, forget to highlight this point. When I started studying electronics, I almost lost my mind trying to understand how the two logical element or NOT-or could eliminate bouncing simple unipolar two-step button — until the realized that they could not. The reason is that when the supply voltage is fed to the schema, elements or NOT (or NOT), you specify what should be their initial state. They require a source of certainty, which is set by the switch, which, in turn, may be in the same or in a different State. Therefore, the switch should be on/off.
I applied another simplified multi-pronged scheme, rice. 2 to show the changes that occur in two elements or NOT, when the switch switch it to one, then to another position. To refresh your memory, I added a truth table showing the output signals of the element or not for each combination of signals on the inputs.
Figure. 2. use of two elements or NOT, together with the on/off switch, double pole high logic level signal. Shows the sequence of the four schemes that illustrates a simple trigger response to various input signals
Suppose that the switch is set to the left position of the scheme. In this case, it transmits a signal to the left side of the diagram, suppressing low logic level signal generated using a podtjagivajushhego resistor. So we can be sure that the inlet left element or is not received at least one high logic level signal. Because any signal the high logic level on the input element or-won't make it to issue low-level signal at the output (which follows from the truth table), which, according to the scheme, is passed to the input element or right-never. Thus, in this case the inputs are two elements will signal low logical level that will lead to the formation of the high-level signal. This signal is sent to the entrance of the left element, or-NOT. Thus, this configuration element is always stable.
Now move on to the next, more "zaumnomu" version of the schema. Suppose that the sliding contact switch is moved so that the pole switch is not connected with any of his contacts. (Or it can be assumed that the switch contacts are able to "bounce" and there is no good quality contact, or that we are completely disconnected the switch). In the absence of the high logic level signal at the left entrance gate or left-no signal at its input goes from a high level to low, because it will set the pull-up resistor, connected with the General conclusion of the power source. But the right of the entrance of this element is still a high logic level, and this is quite enough for continuing formation at the output of the left element or no signal low logic level is hence able to scheme no change . In other words, the scheme remained in the previous condition (she "remembers").
However, if you now switch to translate in the extreme right position, i.e. apply the high logic level signal (power supply) on the right entrance right on the diagram, or element, then this element very quickly recognizes that one of its entrance There is a high logic level signal, and therefore he will change its output signal from a high to a low logic level. This signal goes to the input of another element or left-not, which will now have two input signal low level, so its output appears high-level signal back to the input element or right-never.
Thus, the output signals two Boolean elements or NOT swapped. These conditions have changed first, and then remained unchanged, even after sliding contact switch breaks contact. The second group of schemes in Fig. 3 shows a similar scheme works performed on two elements and and when a pole switch signal low logic level. For schema validation, you can use the chip, which NS00 74 listed necessary purchases for this experiment.
Figure. 3. In this scheme, unlike the one that is shown in Figure 1. 2, uses logical elements and and changed to the opposite connection power supply
Both options schemes are examples of triggers (flip-flop), which remain in the reversed order; they are so called precisely because under the influence of input signals can instantly switch and stay in brand new condition (remember him). You can use these charts every time you need to fix a bouncing switch (if the on/off switch).
A more complex version of the trigger is a trigger with synchronizing (clock) input for which you must specify certain signals at each entrance, and then, to change the State of a trigger, you need to apply for an appropriate entry clock (clock). Momentum must be clear and specific, which means that if you apply it from the switch, you must first remove its chattering contacts, possibly through the use of another trigger!
What you need, when you need to fix a bouncing off the regular button or switch? Well, let's say you have a problem! One solution is to buy a special chip, for example, 4490, called "bounce" liquidator and contains digital scheme of suppression. The special part number is MC14490PG from the company's Semiconductor. It contains six circuits for six separate entries, each of which contains an internal load resistor. It is quite expensive, is more than 10 times compared with the price of microchips 74HC02, that contains the items or NOT. Actually you can do this easier using rocker switches, which you can easily eliminate bouncing as described earlier.
