One Transistor Electronic Code Lock System

This article describes one transistor code lock circuit which is a key coded door lock system. Earlier, we have already seen how a password based door lock system works using 8051 microcontroller. For that circuit, we require programming. But, here interesting thing about this circuit is that it can be done without any programming. This circuit is simple and made up of simple transistor and buttons.

One Transistor Electronic Code Lock System Circuit Principle:

The main principle of this circuit is that the door lock opens only when the buttons in series are pressed at a time. If any button which is in parallel is pressed door is locked. When all these buttons are pressed transistor switches the relay and the door is opened. In real time these buttons can be arranged in an order  to open the door.

One Transistor Electronic Code Lock System Circuit Diagram:

Circuit Diagram of One Transistor Electronic Code Lock System

Circuit Components:

• Transformer.
• Bridge rectifier.
• Resistor.
• Capacitor.
• Buttons.
• Diode.
• Transistor.
• Relay.
• Motor
• Ac source.

One Transistor Electronic Code Lock System Circuit Design:

The circuit consists of a stepdown transformer whose input is AC supply of 230V and 50HZ. In the stepdown transformer, number of windings in the primary coil is less than the number of windings in the secondary coil.

It is connected to the bridge rectifier. The bridge rectifier rectifies the full wave coming from the transformer .It has 4 pins in which two pins are connected to the transformer. In the other two pins, one is connected to ground and the other pin is connected to the resistor and capacitor. Capacitor other end is grounded. From the resistor, four buttons S1, S2, S3 and S4 are connected in series. Now from the next button S5, they are connected in parallel. Thus S5, S6, S7 are connected in parallel.

A resistor of 100k is connected in parallel to the buttons. Base of the transistor is connected to the buttons in parallel. Emitter pin of the transistor is connected to the ground. Collector pin is connected to the 1N4007 diode, relay. Other end of the diode is connected to the one end of the relay. Here diode is used for protecting transistor, as relay produces  back emf. Relay has five pins. One pin is connected to the positive terminal of the diode. Other pin is connected to the ground. Thus relay output is connected to the motor through its AC source.

The relay used here is a magnetic relay. It has five pins i.e. NC, NO, COM, A, B. A, B are the inputs of the coil .When no voltage is applied to these pins, COM pin is connected to the normally closed pin i.e. NC pin. When a DC voltage is applied to A, B pins of the relay, the COM pin is connected to the normally open pin i.e. NO pin of the relay.

How One Transistor Electronic Code Lock System Circuit Works?

1. Initially, apply the input voltage of 230V and 50 HZ to the transformer.
2. Thus, step down transformer reduces this voltage to 6V.
3. This reduced AC voltage is applied to the bridge rectifier. This outputs a pulsating DC.
4. The output voltage is applied to the buttons through a resistor of 2.2K ohms.
5. Now initially press the switch S1 and then press switch S3.
6. Now press the switches S2 and S4.
7. Now current flows through these switches to the transistor base as the parallel buttons are opened.
8. The relay starts switching and supply is connected to the motor.
9. Now, you can see the motor rotating, indicating that its door lock is open.

Electronic Code Lock System Circuit Advantages and Applications:

• This circuit is very simple, reliable and low cost.
• We can use it in security applications.
• We can also use it in door lock systems to open the door.

Limitations of this Circuit:

• If the user forgets the password i.e. the order of the buttons to be pressed it is difficult to open the lock.

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