COMSATS University Islamabad,Wah Campus
Electrical& Computer Engineering Department
Program: BEE
Semester/Section: 4th Semester, 4A
Subject:
Principle of Electrical Machines
Instructor:Eng .Sir Imran
Project Title:
One Transistor Electronic Code Lock System Circuit
Submitted by:
Muhammad Abdullah
Shoaib Ali
Muhammad Ibrahim Atiq
FA20-BEE-075
FA20-BEE-008
FA20-BEE-160
 Objectives
Our objective is to design a one transistor electronic code lock system circuit
in which we are about to use some of the main components like transformer,
capacitor, diodes.
 Summary
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.
 Block Diagram
 Circuit Diagram
 Working
 The circuit consists of a step down transformer whose input is AC supply
of 230V and 50HZ. In the step down 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.
Circuit Components:

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Transformer.
Bridge rectifier.
Resistor.
Capacitor.
Buttons.
Diode.
Transistor.

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Relay.
Motor
Ac source.
Simulations (in any software)
Proteus
 Hardware Implementation
o Problems encountered
o Trouble shooting
 Results and Conclusions
The Conclusion 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.