This code lock is useful for appliancesrequiring exclusive or authorised useby those who know the preset code.If desired, the code can be changed. The circuit doesn’t require additionalAND or NOT gate operations atthe outputs. It uses two pairs of 4-wayDIP switches. The code is set using DIPswitches DIP3 and DIP4. Then these twoswitches are hidden inside the assembly.With DIP3 and DIP4, up to 256 code combinationsare possible. The unlocking codeis set by the user using DIP switches DIP1and DIP2, which is compared with thepreset code entered earlier via DIP3 andDIP4. If the two codes match, transistorT1 conducts.The codes are compared using twocascaded 4-bit magnitude comparator ICs(IC1 and IC2). If the input nibble presentat DIP1 matches with preset DIP3nibble, output pin 6 of IC1 (connected toinput pin 3 of IC2)goes high. Now ifnibble present atDIP2 matches withthe preset nibbleat DIP4, pin 6 ofIC2 also goes high.This high outputdrives transistor T1and the applianceturns on via relaycontacts.After use, disturbthe positionsof DIP1 and DIP2 sothat the appliancecan’t be operated byunauthorised persons.This will alsoswitch the applianceoff.The circuit worksoff a 5V DC powersupply. Hidden switchS0 can be used tomanually turn on/offthe appliance if youhave forgotten thepreset code.
Caution. Youmay use this codelock at your ownrisk. After all, a clever intruder will try all256 possible combinations one after theother to break the secret code.


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