Password lock using Transistors

  

At first we need to know about LATCH circuit 

 

This is a simple transistor latch circuit made using one NPN and one PNP transistor. The idea is — once the output turns ON, it stays ON even if the input switch is released.

Here, when I press the switch, current flows to the base of the NPN transistor Q1. Q1 turns ON and pulls down the base of the PNP transistor Q2, which turns Q2 ON as well. Now, Q2 supplies base current back to Q1 through resistor R5 — creating a feedback loop.

Because of this feedback, both transistors keep each other ON — even after I release the switch. The circuit ‘remembers’ the ON state just like a simple memory cell. The output will stay high until the power supply is disconnected.

 

Cascaded latch circuit  

 

 

 Overall Function

1. Press Input1 → Stage 1 turns ON and stays ON.

2. Press Input2 → Stage 2 turns ON (only after Stage 1 is ON).

3. Press Input3 → Stage 3 turns ON and produces the final Output.

Each stage depends on the previous one, forming a sequence

 

The Reset Circuit

Once the ouput is high , the input will have no effect on it , to reset the output , the base of transistors need to be grounded

 

The following added has a reset button , when pressed it pull down the base of transistor to ground , which makes the output to zero . 

 

 Analog locker

TO make locker , use 3*3 keypad and connect 4 keys to those push button  and at ouput connect relay with solenoid locker  then , to unlock one has to press the correct  keys sequentially. 

 

One has to press the correct push button trigger the relay.