This tutorial teaches how to construct a simple alarm system that detects intruders using invisible light beams.
Because it uses infrared light this kind of alarm is known as an IR alarm.
It generates an infrared light beam that travels across a room using an special chip known as an IC 555.
The light is disrupted and the alarm turns off if someone walks through the beam.
Although learning about electronics through this project is enjoyable
What is a Infrared (IR) Sensor Alarm Circuit:
When few conditions occur an electrical system called an Infrared IR Sensor Alarm Circuit is designed to detect the presence of infrared radiation and sound an alarm or other alert device.
IR sensors including infrared motion detectors or IR receivers are commonly used in these circuits to detect variations in the amount of infrared light within their detecting limit.
Often used in security applications the circuit detects movement or intruders in a specific area and sounds an alarm to alert users or security staff.
Circuit Working:
Parts List:
| Component Type | Component Details | Quantity |
|---|---|---|
| Resistors | 10k 1/4 W CFR | 4 |
| 1k 1/4 W CFR | 3 | |
| 100Ω 1/4 W CFR | 1 | |
| 2.2k 1/4 W CFR | 1 | |
| 100k 1/4 W CFR | 1 | |
| Capacitors | Ceramic 0.01µF | 1 |
| Electrolytic 100µF 25V | 1 | |
| Electrolytic 1µF 25V | 1 | |
| Semiconductors | Transistor BC547 | 2 |
| Transistor BC557 | 1 | |
| IC 555 | 1 | |
| IC 7805 | 1 | |
| IC TSOP1738 | 1 | |
| LED Red 20mA 5mm | 1 | |
| Buzzer 12V | 1 |
The circuit is built on the IC 555 monostable multivibrator which remains stable until it is activated.
The values of the resistor R and capacitor C connected to the IC which are explained below describe the formula for the output pulse duration at pin 3.
Through a capacitor a BC547 transistor functions as a switch to just trigger pin 2 and activate the IC 555 monostable.
The capacitor is charged and discharged during this process creating an accurate trigger.
The second BC547 transistor which keeps the first transistor off is an essential component for maintaining standby mode.
This keeps the IC 555 monostable in an inactive condition until something breaks it.
Another IC 555 set up in astable mode generates the continuous 38 kHz infrared signal.
Capacitors C1 and C2 and resistors R1 and R2 control the square wave output frequency.
The 38 kHz signal is released by an infrared LED that is connected to the astable IC 555 output.
The second BC547 transistor is kept active by the continuous signal because of the TSOP1738 IC which is designed to detect this frequency.
The second BC547 transistor turns off in reaction to any disruption in the 38 kHz infrared signal which activates the IC 555 monostable.
The alarm is triggered by the output relay that connects to pin 3.
Formulas and Calculations:
Below are the formulas for Infrared (IR) Sensor Alarm Circuit using TSOP1738:
- The formula for pulse duration:
1.1 * R * C
where,
- The estimated factor which is 1.1 accounts for the 555 IC’s inadequate behavior as well as other circuit components that can differ based on the specific circuit design and component tolerances.
- R represents the resistance value in ohms Ω.
- C represents the capacitance value in farads F.
Restrictions:
Particular Circuit Design:
Only circuits built around a 555 IC set up as a monostable multivibrator can use this formula.
It will not cover IR sensor alarms of different designs.
Accuracy:
The 1.1 factor is only an estimate and component limits and circuit modifications may cause some variation in the pulse duration.
Example,
R = 10k and C = 1uF and assuming a factor of 1.1 the formula would estimate a pulse duration of:
Pulse Duration = 1.1 * 10,000 Ω * 1 µF ≈ 0.011 seconds (or 11 milliseconds)
(τ) = R * C
where,
- (τ) is the greek letter tau represents the time constant (τ) which is an important variable in resistor capacitor RC circuits
- R represents the resistance value in ohms Ω.
- C represents the capacitance value in farads F.
3. IR sensor alarms that use a 555 timer integrated circuit often use this method to find the frequency of an astable multivibrator circuit.
Frequency = 1.44 / ((R1 + 2 * R2) * C1)
where,
- The frequency of a circuit is the number of cycles per second that its output voltage fluctuates between high and low, it is measured in Hertz (Hz).
- The resistance values in ohms and Ω of the two resistors used in the 555 IC timing circuit are represented by R1 and R2.
- The capacitance value of a capacitor connected to the 555 IC timing circuit is represented by C1 in farads F.
- The features of the IC 555 circuit give a theoretical constant of 1.44.
Using the Formula:
Component Values:
If the values for R1, R2 and C1 are given in the circuit this formula is used to figure out the potential oscillation frequency of the circuit.
Fine Tuning Frequency:
In some IR sensor warnings, R1, R2 or C1 are tuned.
By changing these figures and using the method one can figure out the resulting frequency and possibly modify the alarms operating frequency.
Example:
R1 = 10k, R2 = 10k and C1 = 1uF:
- Total Resistance: R1 + 2 * R2 = 10kΩ + 2 * 10kΩ = 30kΩ
- Theoretical Frequency: Frequency = 1.44 / ((30kΩ) * (1µF)) = 48 Hz
RC Network for IC 555 Monostable:
Resistors are represented by R and capacitors by C in the IC 555 monostables RC network.
The output pulse duration a crucial element in the operation of the intruder alarm system is set by the cooperation of these parts.
By achieving the required time delay for alarm activation depends critically on the correct resistor and capacitor values used.
Because of this modification the alarm will react to any possible threat in a timely and efficient manner.
Output Relay and Alarm Control:
A relay is connected to the IC 555 monostables output that comes from pin 3.
When activated the relay acts as a switch that reacts to the high output generated by the IC.
A key component for controlling the alarm circuit is the relay.
When activated it makes it easier for electricity to pass through the alarm circuit generating the preset alarm signal or sound.
The timing elements of the IC 555 monostable circuit are closely related to the alarms duration.
IC 555 IR Transmitter:
Parts List:
| Component Type | Component Details | Quantity |
|---|---|---|
| Resistors | 470Ω 1/4 W CFR | 2 |
| 2.2k 1/4 W CFR | 1 | |
| Preset 10k 1/4 W CFR | 1 | |
| Capacitors | Ceramic 47nF | 1 |
| Electrolytic 10µF 25V | 1 | |
| Semiconductors | IC 555 | 1 |
| IR Emitter LED | 1 |
An extra IC 555 is set up as an astable multivibrator which acts as the continuous 38 kHz signal’s infrared transmitter.
Capacitors C1 and resistors RV1 and R2 have exactly set values that decide the square wave output frequency.
To create 38 kHz from the IC 555 pin 3 and the IR emitter LED the RV1 preset needs to be properly modified.
An infrared LED is easily connected to the astable IC 555s pin 3 square wave output.
This component is essential to the continued functioning of the intruder alarm system since it produces the 38 kHz infrared signal.
How to Build:
Building the IR signal based alarm circuit involves following step for connections.
IC 555 Section:
- Connect pin 2 of the IC1 555 to the collector of the first BC547 transistor through a capacitor C2.
- Connect pin 6 of the IC1 555 to pin 2 through resistors R1 and R2.
- Connect pin 2 of the IC1 555 to the base of the first BC547 transistor.
- Connect pin 7 to pin 6 through a resistor R3.
- Connect pin 7 of the IC1 555 to the collector of the second BC547 transistor.
- Connect pin 6 of the IC1 555 to the collector of the second BC547 transistor.
IR Signal Section:
- Place the IC 555 in astable mode.
- Connect pins 6 and 2 to pin 7 through resistors R1 and R2 and connect pin 6 to pin 2 through capacitors C1 and C2.
- Connect pin 3 of the astable IC 555 to the anode of the IR LED.
- Connect the cathode of the IR LED to the positive terminal of the power source.
IR Signal Detection Section:
- Connect the TSOP1738 IR Sensor IC to the circuit with its output connected to the base of the second BC547 transistor.
- Connect the collector of the second BC547 transistor to the base of the first BC547 transistor.
Testing:
- Apply power to the circuit.
- Ensure the continuous 38 kHz IR signal is emitted from the IR LED.
- To test interruption detection block the infrared transmission.
- Verify that the alarm is activated when an interruption occurs.
Important Notes:
- Based on the required time delays and IR signal frequency change the values of the resistor and capacitor.
- Check that the components are connected and polarized correctly.
- Before installation test the circuit in a controlled setting for security reasons.
Safety:
- When working with electronic components always remember to take proper safety measures and if you lack experience think about getting help from someone who knows electronics.
Conclusion:
To ensure precise control over alarm activation the complex IR signal based alarm circuit uses an IC 555 monostable configuration that is triggered by a BC547 transistor.
The standby mode is maintained by the constant 38 kHz infrared signal generated by the astable IC 555 functioning.
When the infrared signal is disrupted the alarm is activated instantly resulting in a strong security system.
Leave a Reply