Three-dial combination lock

Description: This is an efficient combination lock that can be assembled in a single evening. To unlock it, the user must input the correct combination using three rotary or thumbwheel switches. When the correct combination is entered, current flows through resistor R1 to the gate terminal of Q1, which causes the silicon-controlled rectifier (SCR) to enter a conductive state. This activation allows current to flow through relay K1, which closes its contacts and powers the connected load. After unlocking, the dials of switches S1 through S3 should be turned away from the correct combination to maintain security.

The lock remains in an open state, keeping the load activated, due to the SCR being latched. To secure the lock, it is necessary to interrupt the anode current flowing through the SCR by pressing pushbutton S4.

The combination lock circuit is designed to provide enhanced security through the use of a silicon-controlled rectifier (SCR) and a relay. The three rotary or thumbwheel switches (S1, S2, S3) serve as the primary input interface, allowing the user to set a specific combination. The resistive element R1 is crucial as it limits the current flowing into the gate terminal of the SCR (Q1), ensuring that the SCR only activates when the correct combination is dialed in.

Upon entering the correct combination, the gate of the SCR receives sufficient current, transitioning it into a latched state. This state is maintained even after the combination is no longer present, allowing the connected relay (K1) to remain engaged. The relay's contacts close, enabling the load, which could be any device or system requiring activation, to operate continuously until the lock is reset.

The security feature of this design is enhanced by the requirement to rotate the dials away from the correct combination after unlocking. This action prevents unauthorized users from easily discerning the combination by observing the positions of the dials.

To reset the lock and deactivate the load, the user must press pushbutton S4, which interrupts the anode current to the SCR. This action effectively turns off the SCR, releasing the relay K1 and opening its contacts, thus disconnecting the load.

This combination lock circuit offers a practical solution for applications where secure access control is necessary, combining simplicity in assembly with effective operational security.Here's an effective little combination lock that you can put together in one evening's time. To open the lock, simply dial in the correct combination on the three rotary or thumbwheel switches. With the correct combination entered, current flows through Rl into Ql's gate terminal, causing the SCR to latch in a conductive state. This sends a current through relay Kl, which responds by closing its contacts and actuating whatever load is attached.

After opening the lock, twirl the dials of SI through S3 away from the correct combination so that nobody gets a look at it. The lock will remain open and your load will remain on because the SCR is latched on. To lock things up, it's only necessary to interrupt the flow of anode current through the SCR by pressing pushbutton S4.