Short-Range Light Barrier for Intruder Alarm
This setup serves as a brief-distance light barrier designed for applications like intruder alarms in doorposts and similar locations. In the transmitter circuit (Figure 1), the 555 timer oscillates at approximately 4.5 kHz, generating pulses with a duty cycle of roughly 13% to maintain reasonable power consumption. Virtually any infrared LED (IRED) can be utilized in this configuration. Commonly available options include LD271 and SFH485. The precise pulse frequency is fine-tuned using the preset P1.
Peak Current Control for LEDs in Transmitter
In the transmitter circuit, the LEDs undergo pulsing with a peak current of approximately 100 mA, regulated by the 47 Ω series resistor. Moving to the receiver configuration (Figure 2), the photodiode D2’s peak sensitivity should align with the wavelength of the infrared LEDs employed in the transmitter. Suitable options include SFH205F, BPW34, or BP104. It’s vital to ensure correct polarity: the photodiode must be reverse-biased. If a measurement reveals around 0.45 V across the device, it’s highly likely that it has been incorrectly installed.
The received pulses first undergo amplification via T1 and T2. Subsequently, a PLL (phase lock loop) circuit incorporating the revered NE567 (or LM567) is employed. The PLL chip sets its output, pin 8, Low when it locks onto the 4.5 kHz ‘tone’ received from the transmitter. In situations where the normally invisible light beam is obstructed, such as someone entering the room, the received signal vanishes, causing IC1 to pull its output pin High.
Oscillator Activation and Audible Alarm in Receiver
Upon activation, oscillator IC2 in the receiver triggers, generating an audible alarm. The two-transistor amplifier in the receiver is intentionally slightly overdriven to maintain an output pulse duty cycle of approximately 50%. However, if the transmitter moves too far from the receiver, overdriving might not be consistently guaranteed. Consequently, an alarm condition might fail to enable IC1. To optimize the circuit’s coverage distance, start by adjusting the value of R2 until the amplifier output signal once again achieves a 50% duty cycle. The adjustment process is straightforward.
Calibration and Testing Procedure
Initiate the receiver, and the buzzer should become audible. Subsequently, activate the transmitter and direct its LEDs toward the receiver input from a relatively short distance, around 30 cm. Adjust P1 on the transmitter until the buzzer ceases. Toggle the receiver’s power off and on a few times to ensure it consistently locks onto the transmitter carrier signal under various conditions. If necessary, slowly increase the distance between the transmitter and receiver while re-adjusting P1 as needed.