The circuit 'electronic code lock' described here uses bistable multivibrators to build a 3-digit electronic code lock. This circuit can be adapted to any electronic gadget which needs to be protected from unauthorised persons/access. A keyboard with small metal plates and a contact probe is used to operate the lock. For opening the lock, specific metal plates forming the code combination are touched with a grounded probe.
In this case, the code combination used is '9EJ'. The number can be easily changed by changing the wiring at the keyboard. The order of touch is unimportant as long as only the combination letters/figures (9EJ) are touched. When the power is switched on, the whole circuit will be in reset state. On reset, the collectors of transistors T2, T4 and T6 will be at low potential whereas collector of transistor T8 and base of transistor T12 will be at high potential. When the correct number is operated, the collectors of transistors T2, T4 and T6 changeover to high potential. As a result transistors T9, T10 and T11 are driven into conduction (while transistor T12 had been already conducting),which in turn activate the relay and the lock opens. It should be remembered that in reset state, base of transistor T12 is at high potential. So if a wrong number is touched with the probe, the base of transistor T12 will go to low potential. Now even if all the points are touched, the relay will not get activated since transistor T12 is in non-conducting state. The use of capacitors in the base of one of the two transistors of each multi-vibrator ensures that after a reset, the circuit returns to the same initial conditions. Number of active digits can be increased by simply increasing the number of multivibrators and output transistors. Input to base of T7 can be extended to any number of points on the keyboard panel and any kind of confusing symbols or letters can be used to mark the metallic contact plates on the keyboard. Lastly, it should be noted that numbers or letters should not be repeated in the code, as it decreases the number of available digits.