The post explains a simple infrared intruder alarm circuit using an infrared transmitter and an infrared receiver module. The transmitter and the receiver photo diodes used in the two modules are aligned in line at a distance of around 2 meters within the restricted area. When an intruder tries to trespass the restricted area, the intruder unknowingly crosses the IR beam cutting of the transmission link between the transmitter and the receiver, which activates the attached relay alarm sound.
The circuit is intended to be fitted inside an existing burglar security system. For distances of around 2 metres or less, the system is quite simple and does not require any extra lenses or filters. Such small range is generally enough to cover room doors, corridors, and other areas.
The device is made up of two circuits: one that generates an infrared beam and another that detects it and sounds an alert if the signal is broken. A pulsed infrared beam is employed, as is the case with many of these technologies of this kind. A modulated beam may easily be determined against ambient infrared radiation, facilitating the use of a low-power beam.
The following image shows the infrared transmitter/receiver link's block diagram. Any entity that comes between the two devices will disrupt the IR beam, causing the relay and the alarm system attached to it to switch ON.
Intruder Alarm IR (infrared) Transmitter Circuit
The transmitter is constructed around the well-known 555 timer IC and operates in astable mode. The timing elements are R1, R2, and C2, which provide a 5.25kHz operational frequency.
The output turns high while C2 charges up through the relatively high resistance of R1+R2, and turns low while C2 discharges through the lower resistance of R2 and an internal transistor of IC1. Due to this operation, a conventional 555 oscillator doesn't really generate a genuine squarewave output because the output is in the high state for a much longer duration than the low state.
The parameters utilized in this example result in an output that is is able to stay in the low state only around 10% of the time period. Q1 is switched on through the base current it gets via R3 throughout these short negative output bursts.
By means of the current limiting resistor R4, it then transfers a current of around 500 mA to infrared LED1. However, the net current passing through LED1 is hardly around 50 mA. Thus, this system produces quite powerful infrared pulses while consuming a relatively modest overall current. LED1 does not use the visible light spectrum for generating the intended infrared rays.
Intruder Alarm IR(infrared) Receiver Circuit
Infrared pulses are intercepted by photo-diode LED1 at the receiver end. This is applied through the supply rails via load resistor R1. The leakage current through LED1 increases temporarily as a result of the infrared pulses, producing a sequence of tiny voltage pulses at the intersection of R1 and LED1.
C2 feeds these pulses to the input of a basic high gain amplifier, which employs Q1 and Q2 in a two-stage directly coupled configuration. C2 and C4 are intentionally set to low levels to ensure that the circuit gets an inferior low-frequency response.
This ensures that 50 Hertz signals generated by LED1 due to the infrared radiation leakage from the mains-powered lights are effectively rejected. However, at the significantly higher working frequency of the transmitter circuit, the circuit seems to provide a larger gain.
C5 connects the amplifier's output to a rectifier and smoothing circuit made up of D1, D2, C6, and R6. This circuit's positive bias is supplied into one of the operational amplifier IC1's inputs.
R7 and R8 provide a bias voltage to the other input. In most cases, the inverting input's fixed bias would be larger than the bias supplied to the non-inverting input. The IC1 is configured like a comparator, and its output switches high while under situations, activating the relay coil.
The separate buffer stage Q3 is employed to supply the relatively high driving current necessary.
If an intruder briefly disrupts the beam, the charge on C6 rapidly decays, dropping IC1's non-inverting input potential below the inverting input. The output of IC1 is therefore turned low, causing the relay to turn off, which opens the contacts of relay RLA1, and the central security system is activated.
How to Set Up the Intruder Alarm
To set up the above explained infrared intruder alarm circuit, you simply have to align the transmitter infrared beam with the receiver photodiode, such that the receiver photodiode is able to remain activated with the transmitter infrared signal.
The above set up can be installed in front of doors, windows, gardens or any place that needs to be monitored against an intrusion.
In this situation, if an intruder tried to cross the path, it momentarily breaks the infrared beam, which in turn causes the receiver to deactivate for a moment.
This instantly causes the receiver relay to activate and sound the connected alarm.
The next design of an IR intruder alarm circuit can e seen in the following images. The idea was contributed by one of the avid readers of this blog Mr. Amit.
Just like the above concept this design also has a transmitter and receiver units which simply needs to be aligned in line so that the IR waves from the transmitter circuit reaches the photodiode of the receiver and keeps its relay activated.
As soon as a potential intruder tries to cross the path, it ends up breaking the IR beam which causes the receiver unit to deactivate for a moment and sound the connected alarm.
The following depicts the transmitter's real circuit diagram. It's built on a NE555 timer that's been set up as an astable multivibrator. This indicates that the infrared emitter, LED1, is turned on and off several times per second - the frequency of oscillation is roughly 5KHz for the values given.
There are several benefits of using a pulsed beam versus a continuous emission. To begin, it encodes the IR pulse in a way that the transmitter can understand and distinguish the signal from the impacts of other emissions, for example from LED, CFL lights in the close surroundings.
Furthermore, by selecting a high frequency, it becomes possible to AC couple the transmitter circuit and enable it to be resistant to gradual changes in ambient light levels, such as transitioning from day to night.
Thirdly, pulsing the transmitter saves power, which is beneficial when the transmitter is powered by batteries.
The transmission frequency may be changed using the RV1 preset potentiometer, allowing for some tuning up based on the practical positions of the transmitter and receiver.
The next figure below depicts the receiver's circuit. LED 1 is an infrared receiver diode that has been reverse biassed by R1. When infrared light strikes a diode, it causes an increase in minority carriers and a commensurate boost in diode conductivity.
This results in a voltage drop at the junction of R1 and LED1. C1 then AC couples the output to the op-amp. This serves to provide the receiver with a high level of tolerance to fluctuations in ambient light levels as well as the impacts of incandescent lights, CFL bulbs and fluorescent lights. IC1 is hooked up as an a non -inverting AC amplifier, while R2 and R3 bias the input to 50% power supply range.
The combination of R4, R5, and C2 determines the amplifier's gain. The gain of the amplifier is near to unity at lower frequency, but as the frequency of the input signal increases, the impedance of C2 decreases, raising the average gain of the amplifier, such that at 5KHz or so of the transmitter signal the amplifier is fixed with a gain of many thousands.
The output of IC1 will become an AC signal of roughly 5KHz whenever the transmitted signal is detected by the receiver. C3 subsequently AC couples this output signal to the voltage doubler circuitry of D1, D2, adequately charging the filter capacitor C4 to switch ON the transistor Q1 and the relay.
C4 discharges through R7 and the relay switches off when the transmission is cut-off or disturbed. R6 and C5 offer further power supply decoupling to the amplifier, eliminating any spikes generated by the relay switching on and off. Back EMF shielding is provided by Diode D3.
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