Electronic alarm for cash box, lockers

Summary

The project presented can be used as a smart circuit for the cash box that thwarts any theft attempt by activating an emergency alarm. This can also be used to trigger any external burglar alarm by connecting a relay to the output of the circuit.
 
The circuit is built around a NAND gate IC in which all gates are used as an inverter. The circuit is based on the concept of Light Dependant Register (LDR) whose resistance varies with the intensity of light falling on it. This small circuit can also be installed in a locker system where if somebody tries to open the locker, the light will fall on the LDR and the circuit will start sounding with the help of buzzer connected to the relay. A switch is also provided so that the circuit can be switched off when not in use to save the battery power.

Description

This locker security alarm project is built around CD4011 with some additional components like LDR, variable resistor, capacitor, transistor and relay. It can be used to protect the valuables from burglary by sounding the buzzer. It generates a warning beep when somebody attempts to open the locker.
To understand the working you should know about the truth table of NAND gate which is as follows-
 
INPUT
INPUT
OUTPUT
A
B
Y
0
0
1
0
1
1
1
0
1
1
1
0
 
The truth table of NAND gate shows that the output will remain HIGH until any of the signals is LOW. If both the signals go HIGH then in that case the output will be LOW.
The output of the circuit depends on pin P10. Every time when it goes HIGH, the relay energizes and every time when it goes LOW, the relay de-energizes. The state of pin 10 is controlled by the voltage on pin 5 and 6 and the voltage on pin 5 and 6 is controlled by the intensity of light falling on the LDR.
 
When the circuit is switched ON, the loop resistance of LDR goes HIGH causing the voltage on pin 5 and 6 of gate 2 to go HIGH and the voltage at gate 1 or on pin 1 and 2 to remain LOW. As a result the voltage on output pin 4 of gate 2 also goes LOW. This causes the voltage on pin 3 to go high which makes pin 13 and 12 of gate 4 to go high and as a result the output pin 11 remain low. This LOW is supplied to pin 8 and 9 of gate 3 and due to this the output of pin 10 goes HIGH and the HIGH on pin 10 means that the relay will remain in de-energized mode and alarm will not sound.
 
When the circuit is active and somebody tries to open the locker, light falls on LDR and its resistance starts decreasing and the voltage on pin 5 and 6 reduces to zero. As a result the voltage on pin 4 becomes high hence making pins 1 and 2 to go HIGH. Voltage on pin 3, 13 and 12 drops to zero while it goes HIGH on pin 11, 8 and 9.  Thus on pin 10, voltage becomes low making the relay energize to sound an alarm indicating unauthorized access.
 
In other words we can say that when pins 5 & 6 are LOW, the relay is energized and when they are HIGH the relay is de-energized.
The voltage on pin 5 and 6 is controlled with the help of voltage divider rule. The resistors R1 & R2 form a potential divider circuit with the LDR. The sensitivity of the circuit can be set with the help of resistor R1. Resistor R2 can be removed but if resistor R1 is accidentally adjusted to zero ohms – there is a chance that the excess current may damage the LDR. A diode is used with the relay to avoid reverse-voltage spike can short-circuit at source and prevent any damage to the circuit.

Circuit Diagram

Components
Resistor Image
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 Resistor

Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits.

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Capacitor image
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 Capacitor

 

 

Capacitor is a passive component used to store charge. The charge (q) stored in a capacitor is the product of its capacitance (C) value and the voltage (V) applied to it. Capacitors offer infinite reactance to zero frequency so they are used for blocking DC components or bypassing the AC signals. The capacitor undergoes through a recursive cycl...

LDR | Light Dependent Resistor
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 LDR

An LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained...

BC548 | Transistor BC548
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 Transistor BC548

BC548 is general purpose silicon, NPN, bipolar junction transistor. It is used for amplification and switching purposes. The current gain may vary between 110 and 800. The maximum DC current gain is 800.

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Relay Switch Image
  • 21,679-Reads
 Relay Switch

Relay is an electromagnetic device which is used to isolate two circuits electrically and connect them magnetically. They are very useful devices and allow one circuit to switch another one while they are completely separate....

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