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Simple Electronic Code Lock Circuit

This circuit teaches how to make a secret code lock!

To open anything like a box or a door it requires a specific code rather than a key.

Circuit Working:

Parts List:

ComponentQuantityNotes
Resistors (1/4 watt) 1k 2
Semiconductors
Transistor BC5471
Diode 1N40071
Relays1
Mini switches (ON/OFF)8

This electronic code lock circuit is unique by its simple design which uses a remarkable code implementation with only one active component and a minimum of eight button switches.

How does the code locks circuit operate?

To activate the relay all four buttons from S1 to S4 must be pushed together.

If any of the S5 to S8 buttons are pressed the relay stays inactive preventing the code from opening.

It is essential that the supply voltage should match the functioning voltage of the relay.

A transistor such as the BC547 can handle currents up to 50 mA but the transistor 2N2222 can only take 0.2A for larger relays.

Formulas:

Relay Coil Current is a tool used to calculate how much resistance a bipolar junction transistor (BJT) requires to operate a relay coil.

Let us look at what is included in the formula below:

R = (Us – 0.6)hFE / Relay Coil Current

How the formula is used:

Voltage Drop:

The phrase (Us – 0.6) takes note of the voltage drop across the BJTs base emitter junction (Vbe).

Silicon BJTs usually have a voltage of around 0.6V.

Subtracting this number from the supply voltage (Us) gives the voltage that can provide driving current through the base resistor R and into the base of the BJT.

Base Current Amplifier:

The word hFE multiplies the base current.

To obtain the required current through the relay coil a greater hFE provides a lower base current.

Ohms Law:

Finally using ohms law formula (R = V / I) can divide the desired relay coil current by the resistor value R.

This ensures that the expected resistor value activates the relay coil limiting the current traveling through the BJTs base to the right amount.

In other words this formula is designed to choose a resistor that when used with the voltage in the circuit and the gain of the BJT it allows enough current to travel through the BJTs base to activate the relay coil.

How to Build:

To build a Simple Electronic Code Lock Circuit following steps are used for connections:

Identify components:

Circuit Design:

Place buttons S1 to S4 in parallel:

Resistor Placement:

Optional LED Connection:

Soldering:

Testing:

Safety:

Conclusion:

Electronic code locks provide several advantages over traditional locks including the ability to easily change codes, monitor access and connect with other security systems.

They are used at homes, workplaces, industrial sites and other control of access cases where electronic security is essential.

References:

Development of a Programmable Electronic Digital Code lock system

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