A electronic combination lock is a device which is designed to activate an electrical solenoid locking system whenever the correct combination of numbers are entered into an attached keypad.
To build this type of electronic combination lock, normally an electronic circuit with a keypad is used.
In the design explained in this article, however, no complex electronic circuits or ICs or keypads are used, rather the system uses a few mechanical rotary switches, and a cheap solenoid, and that's all is needed to build this cheap yet very effective simplest combination lock circuit.
Simple Combination Lock
The basic structure exhibited in most combination locks is based on the simple design as shown in the following figure.
This requires just three ten-way rotary switches arranged in such a manner that if the right combination is entered on them, they will connect electricity to the relay or solenoid (the code being 6-4-5 in this case).
This simple circuit is rarely used in reality since adjusting the switches through all 1,000 potential combinations (0-0-0 to 9-9-9 inclusive) in a rational manner takes just a few seconds.
Combination Lock with Delay
One of the simplest and most effective solutions is to incorporate a delay circuit into the device as shown in the following figure, which prevents power from being delivered to the solenoid or relay until the proper combination has been engaged for a few seconds.
Working through all of the available combinations hastily becomes ineffective at "cracking" the system because the delay circuit prevents the device from reacting even when the proper combination is held for a short instant.
Whoever attempting to "hack" the device is unlikely to succeed unless they are aware of the delay circuit and are willing to spend a significant amount of time figuring out the proper combination.
A time delay circuit dependent on FET semiconductor Q1 is included in the following circuit. C1 will be discharged when the right combination of SW1 to SW3 is adjusted and power is applied to the circuit, providing Q1 zero gate bias. As a result, Q1 is turned off, and no considerable current is sent to the relay or solenoid that serves as its drain load.
C1 charges gradually through R1, after around 4 seconds, the charge voltage on C1 is high enough to bias Q1 into conduction and turn on the relay or solenoid.
When the circuit is reset, R2 discharges C1, allowing it to immediately resume normal operation. The highest gate voltage of Q1 is limited by R2 to around half the supply voltage, however this is more than enough to bias the device hard into conduction and is of no practical significance.
D1 prevents probable harm to Q1 by suppressing the strong back EMF produced across the relay or solenoid when it goes off.
The solenoid can be any spring loaded type, with a shaft that would stretch outward in the absence of power supply, and pull inside as soon as power is supplied to it.
One popular type with matching specifications is shown in the following image.
The 3 rotary switches shown in the above diagrams can be any ordinary 11 position rotary switch, such as the one shown in the image below:
The simple combination lock explained can be effectively used as a fail-proof security lock in various applications such as, table drawers, jewelry box, home doors, safe doors etc.
The code can be changed anytime simply by changing the wire positions across the 3 rotary switch contacts.