Simple keyboard. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Many electronic devices include a control unit - a keyboard that is used to enter various information. It should have protection against "bounce" of button contacts, as well as automatically repeat the code of the pressed key when it is held for more than a certain time. The article describes a simple keyboard that has the listed features.

The keyboard can be organized as an MxN matrix or a set of buttons with one common pin. In the first case, it allows you to combine many keys with a small number of connecting lines. However, its implementation is very laborious and requires the use of an encoder, for example, a diode one containing a large number of elements, or a microprocessor (microcontroller). These devices are not only complex. They form a code that is not easy to decipher. A keyboard with a common output is much easier to manufacture. It is advisable to use it when a large number of keys is not required. The encoder for such a keyboard is applicable to a simple diode, although it can give incorrect information when several keys are pressed at the same time.

Devices with sequential polling of each button are more efficient. A similar keyboard is assembled on counters with a decoder or multiplexers. Polling is done continuously or at the push of a button. In the second case, when several buttons are pressed at the same time, a key code with a higher priority is issued, which prevents conflict situations. When a button is pressed, a key code and a strobe signal are generated at the keyboard output to write this code. For some time, the keyboard should not respond to other key presses and releases, which is necessary to suppress the "bounce" of contacts, which will disrupt the operation of the entire device. The delay can be formed using an integrating RC circuit or a single vibrator.

The ability to repeat the code when a key is pressed for a long time is easy to provide by using a single vibrator in the delay element. In this case, when the delay after pressing the key expires, and the key remains pressed, the one-shot is started again and the whole cycle repeats. A diagram of one of these keyboard options, implemented on TTL microcircuits, is shown in the figure. The proposed sixteen-button keyboard performs the functions listed above.

While no button is pressed, the pulses from the clock generator on the elements DD3.1 and DD3.2 are fed to the input of the binary counter DD4. The multiplexer DD1, to the address inputs of which a digital code from the counter DD4 is applied, sequentially iterates over the inputs to which the keyboard contacts are connected. If a button is pressed, the corresponding input of the multiplexer will be low. At the moment when the code corresponding to the pressed button appears at the address inputs of the multiplexer, a high level voltage will appear at its output, which will start the DD2 single vibrator. The single vibrator will give a low level signal to the output "Strobe" and prohibit further operation of the generator. At the output of the counter DD4 there will be a code corresponding to the pressed button.

During the time determined by the values ​​of the elements R1, R2 and C1, pressing any buttons will not cause a change in the digital code at the output of the counter DD4. After this time, a high level voltage will appear at the output of the single vibrator DD2, allowing the operation of the generator. If the button is not released during this time, the cycle will repeat.

If several buttons are pressed at the same time, the code of one of them will be generated, which should not cause malfunctions in the system.

Establishing the device consists in setting the trimming resistor R2 to the required time interval, during which the code of the pressed button is present at the output of the counter DD4.

The described keyboard can be used in combination locks, security systems for entering a password, in electronic switches and in other equipment.

The reliability of the device will increase somewhat if each input of the multiplexer DD1 (D0-D15) is connected through resistors with a resistance of 10 kOhm to the positive power wire.

Author: S.Kuleshov, Kurgan

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