The circuit described here is an electronic combination Security Code Lock. It opens only when you press four keys in the correct sequence (referred to as a code) out of a total of eight keys installed at your entrance door. If you press the wrong key sequence, the circuit resets, and the lock will not open. The author’s prototype is shown in Fig. 1.
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Security Code Lock – Circuit and Working
The circuit diagram of the security code lock is shown in Fig. 2. The circuit consists of step-down transformer X1, bridge rectifier BR1, 12V voltage regulator LM7812 (IC1), dual-D flip-flop ICs 4013 (IC2 and IC3), rectifier diode 1N4007 (D1), PNP transistor BC557 (T1), npn transistor 2N2219 (T2), 12V single changeover relay RL1, and a few other components.
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