Code switch. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Code switch. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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The proposed scheme can be used in any devices where it is required to restrict outsiders' access to switching modes. Depending on what is connected at the output of the circuit (electromagnet, relay, alarm, etc.), the purpose can be very different, for example, disabling the alarm mode.

(click to enlarge)

In the simplest version, together with an electromagnet, the circuit can be used as a combination lock. Its opening is carried out by a set of a code known to a limited circle of people. The code consists of 4 digits (out of 10 possible). Buttons with certain numbers must be pressed in the specified sequence. This allows for at least 5040 possible code choices.

The code can be easily and quickly changed by rearranging the wire clamps with buttons in any order. When setting the code, it is undesirable to occupy the numbers of the consecutive row (1, 2, 3, 4). It is better if the code consists of random numbers, for example: 9, 3, 5, 0.

The code device circuit is assembled on two CMOS chips of the 561 TM2 series (it is possible to replace it with 564TM2). which ensures high reliability and cost-effectiveness of operation. Consumption by the microcurrent circuit makes it easy to perform, if necessary, autonomous power supply. Any, not even a stabilized DC voltage source of 4 ... 15 V, will reach.

The electrical circuit works as follows. At the initial moment, when power is applied, the circuit of capacitor C1 and resistor R1 generates a zero trigger pulse (log "1" will be at outputs 13 and 0 of the microcircuit).

When you press the button of the first digit of the code (in the diagram - SB4), at the moment it is released, the trigger D1.1 will switch, i.e., a log will appear at the output D1 / 1. "1", since there is a log at the input D1 / 5. "1".

When you press the next button, if the input D of the corresponding trigger has a log. "1", i.e. the previous one worked, then the log. "1" will appear in its output as well.

The last trigger D2.2 fires, and so that the circuit does not remain in this state for a long time, transistor VT1 is used. It provides a delay in resetting triggers. The delay is made due to the charge circuit of the capacitor C2 through the resistor R6. For this reason, the output of D2 / 13 is a log signal. "1" will be present for a maximum of 1 second. This time is quite enough for the relay K1 or the electromagnet to operate. Time, if desired, can easily be made much larger by using a larger capacitor C2.

In the process of dialing the code, pressing any erroneous digit resets all triggers. If the control signal for transistor VT1 is removed from the output of not the last trigger (for example, from output D2 / 12), then the required time to press the code numbers will be limited. In this case, even if the code is entered correctly, but slowly, the output signal will not appear.

The circuit is placed near the keypad.

All parts used, with the exception of the VT2 transistor, can be of any type. The VT2 transistor is used with a high gain, and if it is used as a load instead of an electromagnet relay, it must be replaced with a more powerful one from the KT827 series.

To open the latch of the door lock, it is better to use not an electromagnet, but an electric motor with a gearbox. Such nodes are used as part of car alarms to automatically lock doors (they can be purchased at the store). They consume a small current (60...150 mA from 12 V) compared to an electromagnet and allow you to have a low-power power source, which is especially important for autonomous power supply.

See other articles Section Clocks, timers, relays, load switches.

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