Locker Guard

Locker Guard

Summary
Most of the theft happens in mid night when we go into the deep sleep. The circuit presented here can be used as a smart circuit for your cash box that thwarts the theft attempt by activating an emergency alarm. This can be used to trigger any external burglar alarm with the help of relay connected in the output of the circuit.
The circuit is built around a popular NAND gate IC and in this all gates are simply working as an inverter. This circuit utilizes the simple phenomenon of LDR whose resistance vary with the intensity of light fall on it with the help of voltage divider circuit. You can put this small circuit in your locker when somebody will open the locker light will fall on LDR and the circuit starts sounding with the help of buzzer connected to the relay. In this we have also provide a switch so that you can switch off the circuit when not in use it will save your battery power.
Description
The circuit is build around CD4011 with the help of some more components like LDR, Variable resistor, Capacitor, transistor, relay and can be used to protect your valuables from burglary using this simple circuit. It generates warning beep when somebody attempts to open the locker.
To understand the working you should know about the truth table of NAND gate are as follows-

Input		Output
A	B
0	0	1
0	1	1
1	0	1
1	1	0

In NAND gate we will get output when any of the input signals go low and if both are high we will not get output in that case.
Output  of the circuit will depend on the pin 10 every time when it go low relay energizes and for every time pin 10 goes low relay de-energizes. And the state of pin 10 is controlled by the voltage on pin 5 and 6 and voltage on pin 5 and 6 is controlled by the intensity of light falling on LDR.
To understand it properly has a look on truth table. When you on the circuit in locker resistance of LDR high as result voltage on pin 5 and 6 on gate 2 is high and voltage at gate 1 or on pin 1 and 2 is low as a result voltage on output pin 4 of gate 2 is also low which causes the voltage on pin 3 to go high which also make pin 13 and 12 of gate 4 to go high as a result output pin 11 will remain low. And this low is also supplied to pin 8 and 9 of gate 3 and output pin 10 goes high and high on pin 10 means relay will remain in de-energize mode and alarm will not sound.
In this we can see that all gates are just working as an inverter if they are receiving logic high output is low and if they are receiving logic low output is high.
When somebody tries to open the locker as light fall on LDR it resistance start decreasing and voltage on pin 5 and 6 reduces to zero as a result voltage on pin 4 become high which will also make pin 1 and 2 high and voltage on pin 3 ,13 and 12 drops to zero  and pin 11,8 and 9 become high and voltage on pin 10 becomes low and relay energizes to sound an alarm that somebody is there.
In other words - while pins 5 & 6 remain low - the relay will remain energized. And the opposite of it also true.
We are controlling the voltage on pin 5 and 6 with the help of voltage divider rule. R1 & R2 form a potential divider with the LDR. We can set the sensitivity of our circuit with the help of R1.Hear you can remove R2 but if R1 were accidentally adjusted to zero ohms - there's a chance that excess current might destroy the LDR. We are using a diode with the relay because any reverse-voltage spikes will be short-circuited at source and they can do any damage.

Components
IC
IC1(CD4011)	1
Resistor
R1(1K)	1
R2(100K)	1
R3(4.7K)	1
Capacitor
C1(.1uF)	1
Miscellaneous
Relay	1
T1(BC547)	1
LDR	1
D1(1N4007)	1