The post explains a simple infrared intruder alarm circuit using an IR transmitter and an IR 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.
Intruder Alarm IR 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 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.