In this article, we are going to talk about a really simple but super effective intruder alarm circuit that is very sensitive.
These kinds of circuits are really important for security systems because they can detect even the tiniest intrusions.
The circuit we will look at uses an operational amplifier IC 741 and a timer IC 555 to work.
It also has a Light Dependent Resistor LDR as the main sensor, which makes it super sensitive to changes in light levels.
Circuit Working:
Parts List:
| Component | Value/Type | Quantity |
|---|---|---|
| Resistors (1/4 watt) | ||
| 1k | 4 | |
| 1M | 1 | |
| 10k | 2 | |
| LDR | 1 | |
| Potentiometer | 47k | 1 |
| Capacitors | ||
| Ceramic | 0.01µF | 1 |
| Electrolytic | 10µF/25V | 1 |
| Semiconductors | ||
| IC | 741, 555 | 1 each |
| Transistor | BC557 PNP | 1 |
| 2N2222 NPN | 1 | |
| Buzzer | 1 | |
| LED | Any 5mm, 20mA | 1 |
In this post an intruder walking just a few meters away from the circuit can set off the alarm.
The LDR and the potentiometer R9 work together as a potential divider.
The resistance of the LDR changes based on how much light hits it.
When the light gets brighter, the resistance of the LDR goes down, which causes the voltage across R9 to change.
The operational amplifier IC2 741 checks the voltage from the LDR and potentiometer setup non-inverting input against a reference voltage that is set by another resistor network inverting input.
If the voltage from the LDR goes higher than the reference voltage, the output of IC2 becomes high, which turns on the transistor Q1 BC557.
When Q1 is on, it sends a low pulse to the trigger pin 2 of IC1 555.
The 555 timer is set up in monostable mode, which means it will produce a high output for a specific amount of time based on the values of R4 and C2.
This high output from the IC 555 activates the transistor Q2 2N2222 which then turns on the buzzer.
At the same time, an LED1 lights up to show that an intrusion has been detected.
Formulas with Calculations:
Below mentioned are the calculations with formulas for Super Sensitive Intruder Alarm Circuit
Voltage Divider Output
V_LDR = (R9 / (R9 + R8)) * V_supply
where,
- R8 is the resistance of the LDR, which varies based on light intensity.
Monostable Output Pulse Width
The output pulse width of the IC 555 timer is given by:
T = 1.1 * R4 * C2
where,
- R4 is 1Meg
- C2 is 10µF
T = 1.1 * 1,000,000 * 0.00001 = 11 seconds
How to Build:
To build a Super Sensitive Intruder Alarm Circuit follow the below mentioned steps:
- Gather all the components as shown in the above circuit diagram
- Connect pin 1 of IC1 555 to GND.
- Connect pin 2 of IC1 555 to emitter of transistor Q1.
- Connect pin 3 of IC1 555 to the base of transistor Q2 through resistor R5.
- Connect pin 4 and pin 8 of IC1 555 to +9V of positive supply.
- Connect pin 5 IC1 555 to GND through capacitor C1.
- Connect pin 6 to pin 7 of IC1 555 through resistor R4 to positive supply and capacitor C2 to GND.
- Connect pin 2 of IC2 741 between resistors R6 and positive supply and R7 and GND.
- Connect pin 3 of IC2 741 between LDR R8 and positive supply, and pot R9 to first leg and center leg to GND.
- Connect pin 4 of IC2 741 to GND.
- Connect pin 6 of IC2 741 to base of transistor Q1 through resistor R1.
- Connect pin 7 of IC2 741 to +9V positive supply.
- Connect resistor R3 and LED1 in series from pin 7 of IC 741 and pin 2 of IC1 555.
- Connect a resistor R2 from pin 7 of IC2 and pin 2 of IC1 555
- Connect transistor Q1 collector to GND, base to pin 6 of IC2, and emitter to pin 2 of IC1 555.
- Connect transistor Q2 collector to one leg of buzzer and other end of buzzer to positive supply, base of transistor Q2 to pin 3 of IC1 555, and emitter of transistor Q2 to GND.
Conclusion:
This sensitive intruder alarm circuit is simple to create and very dependable.
It uses an LDR, which helps it react fast to changes in light making it perfect for protecting windows, doors or other important spots.
You can adjust the sensitivity of the circuit by changing the potentiometer to fit different places.
This project shows how basic electronic concepts work and can be a great part of any security setup.