Simple code lock. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling
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The proposed scheme of the code lock (Fig. 1) is simple and reliable in operation.
Fig. 1
The device consists of three time relays connected in series, assembled on transistors VT1, VT2 and VT3, a blocking time relay on VT4 and an amplifying transistor VT5, which amplifies the weakened signal from VT3.
The principle of operation of the device is as follows. In the initial state, the current consumed is units of microamperes. By successively pressing the keys SB1, SB2 and SB3, the capacitors C1, C2, C3 are charged, which hold the transistors in the open state, and the transistors, in turn, turn on the relay. With the parameters of capacitors and resistors indicated in the diagram, the holding time of the relay is several seconds (depending on the supply voltage). If at least one key that is not related to the code set is pressed, the blocking time relay on VT4 will work, which will reliably close the VT3 transistor and, therefore, de-energize the relay. Capacitor C4 is specially selected with a larger capacitance compared to capacitors C1, C2, C3 in order to hold VT3 longer in the closed state.
If desired, you can increase the number of digits of the code to four by adding another transistor link.
To complicate the code set, you can introduce two additional diodes VD1 and VD2, as shown in Fig. 2.
Fig. 2
In this case, to open the lock, you need to press the keys in the following sequence: SB1, SB2, SB1 It should be noted that when turned on according to this scheme, the number of wires connected to the keyboard is reduced by one. If you plan to power the device from batteries or an accumulator, then to limit the current flowing through the relay coil, you can put an RC circuit in series with it, which acts as an economical relay. The selection of resistors achieves the minimum current of the relay operation.
With a small current, the transistor VT5 can be replaced by KT361. Otherwise - to a more powerful one. Diodes will fit any.
Author: A.Khamratov, Mogilev
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