Simple electronic switch. Scheme, description

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Simple electronic switch. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In this article, the author introduces readers to the original circuit version of a multi-position electronic switch with non-fixed buttons.

The publication [1] presented a diagram and description of an electronic switch with dependent fixation, in which eight buttons with normally open contacts are used, which are not fixed in the pressed position. The switch is assembled on three microcircuits, and the ROM in it acts as a priority encoder. It was shown in [2] that ROM allows designing not only combinational devices (i.e., those in which all combinations of input states uniquely correspond to certain combinations of output states), but also asynchronous potential automata, in which, due to feedback and, as a consequence, there is no such one-to-one correspondence to the appearance of the memory property. The well-known RS flip-flop is suitable as the simplest example of such an automaton.

Using ROM with feedback circuits, it is possible to simplify the switch described in [1] by excluding the memory register from it and assigning its function to ROM. It is also possible to exclude the decoder. If any device under development requires a similar switch with no more than five buttons, it is convenient to perform it on the K155REZ PROM.

A diagram of a switch option assembled on this chip is shown in fig. 1. The node generates two output codes. One of them (code - "1 of 5", active level - low) is output through five parallel lines - information outputs of the ROM DS1 - combined with five address inputs of the ROM. This code is suitable, in particular, for selecting the operating mode of the device in which the switch will be built.

Simple electronic switch

It should be noted, by the way, that turning on the LEDs through a common resistor (as in [1]) can reduce the logic unit voltage at the decoder outputs below 2,4 V. Therefore, additional resistors are provided here that reliably provide a normal unit voltage.

The second code, if needed, is output through the remaining three bits of the ROM. This code (of any kind, such as binary) can be used to control the switching of digital or analog signals.

The switch works as follows. In five ROM cells in accordance with Table. 1 information is recorded in such a way that five of its output lines "support" five input lines, i.e., a low level comes from the output to the input that corresponds to the pressed button, and a high level goes to the other four. Thus, the switch is in a steady state and remains there after the button is released.

Simple electronic switch

For the remaining 27 ROM addresses, units are written to all information bits (FF numbers). Therefore, when you press another button, at first there is a low level at the address inputs from both the first pressed button and the second one. At any ROM address containing such a "double" low level, the number FF is written, which replaces zero with one at the input that "remembered" the low level from pressing the first button. As a result, an address with one zero will appear at the input - from the second pressed button, which will immediately be "supported" by the corresponding information from the ROM output, and the switch will go into another stable state.

Thus, we are talking about a device with six stable states. Five of them correspond to one of the five pressed buttons each, and the sixth corresponds to five units on all ROM inputs. For practice, this position is idle, since it cannot be set by pressing the buttons. Thanks to the "support", the switch is not afraid of the "bounce" of the contacts.

Using additional elements, it is not difficult to make a six-state switch with six buttons. To do this, it is required to form a high level at the input of the CS ROM when the sixth button is pressed. Inverter DD1.1 can serve as such a shaper (Fig. 2). Diode VD1 is necessary for the correct formation of output codes and the glow of the sixth LED while pressing the button SB6.

Simple electronic switch

Eight outputs of the ROM is no longer enough to form the codes "1 of 6" and binary, therefore, if both of them are needed, the missing ninth output is obtained using the AND-NOT DD2.1 element. The order of programming the ROM for this version of the switch is presented in Table. 2.

Simple electronic switch

If you want the switch to always be set to a certain state each time the power is turned on (any one of 5 or 6 can be selected). in parallel with the button with the corresponding number, an oxide capacitor with a capacity of 10 ... 47 microfarads is soldered, which, when charging, simulates pressing this button for a short time immediately after power is applied.

It is permissible to use not only one group of five (six) buttons, but also two groups or more, if the task is to make several control panels for the switch. In this case, all buttons of additional groups are connected in parallel to the corresponding buttons of the main group. There is no priority in this case. The switch will go to a stable state corresponding to the last button released from any group.

The choice of the order of connecting the output lines is arbitrary, but for each option there will be a new ROM programming table. In the described embodiment, this connection order is chosen to facilitate tracing conductors on a printed circuit board - another advantage of ROM over hard logic. The pins of the microcircuit are connected in pairs, which are located one opposite the other in the case. To write information to the ROM, you can use any suitable programmer, for example, described in [3].

Author: A.Brazhnikov, Penza

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